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We have developed a conceptual design of a next-generation pulsed-power accelerator that is optimized for advanced high-energy-density-physics experiments. The prime power source of the machine consists of 210 impedance-matched Marx generators (IMGs). Each IMG drives a 150-ns-long coaxial-transmission-line impedance transformer. The coaxial lines provide a minimum of 300 ns of transit-time isolation between each pair of IMGs. The lines in turn drive six radial-transmission-line impedance transformers, which transport the power generated by the IMGs to a six-level vacuum-insulator stack. The stack is connected to six conical outer magnetically insulated vacuum transmission lines (MITLs); these are joined in parallel at a 12-cm radius by a triple-post-hole vacuum convolute. The convolute sums the electrical currents at the outputs of the six outer MITLs, and delivers the combined current to a single short inner MITL. The inner MITL transmits the combined current to the accelerator's physics load. Since the accelerator would be the largest and most-powerful pulsed-power machine developed to date, we refer to it as Jupiter. The conceptual design of Jupiter is 72 m in diameter, stores 140 MJ of electrical energy, and generates 960 TW of peak electrical power at the output of the IMG system. The design delivers 2700 TW, 67 MA, and 9.2 MJ in a 110-ns pulse to a 0D magnetized-liner inertial-fusion (MagLIF) target. The principal goal of the design is to achieve high-yield thermonuclear fusion; i.e., a fusion yield that exceeds the energy initially stored by the accelerator's capacitors.

Intense laser beams focused onto thin high-atomic-number targets can generate short intense bursts of MeV X-rays from a small area of the target. Such systems are being developed as short-pulse point-projection X-ray sources for imaging high-density objects. Here, large-scale (400-million macroparticles and 15-million grid cells) 3-D particle-in-cell simulations are described that model the dynamic interaction between the laser beam, a blowoff plasma layer, and the solid-density target. The simulations self-consistently treat the nonlinear interaction between the incident laser pulse and the blowoff plasma layer where a relativistic electron beam is generated. This beam propagates into the solid-density high-atomic-number target where MeV bremsstrahlung is generated. The model tracks the generation, propagation, and self-absorption of radiation in the blowoff plasma, target, and beyond. Radiation production (fluence and energy spectrum) is characterized in the simulations as a function transverse target size, laser-injection angle, and laser energy. The simulated X-ray fluence for the case of a 45 °-angle-of-incidence 100-J 0.5-ps laser pulse with a 6- μm FWHM focus produces a peak dose in excess of 0.2 rad from a 10-μm-thick square gold target, consistent with experimental measurements.

Here, we have developed conceptual designs of two petawatt-class pulsed-power accelerators: Z 300 and Z 800. The designs are based on an accelerator architecture that is founded on two concepts: single-stage electrical-pulse compression and impedance matching [Phys. Rev. ST Accel. Beams 10, 030401 (2007)]. The prime power source of each machine consists of 90 linear-transformer-driver (LTD) modules. Each module comprises LTD cavities connected electrically in series, each of which is powered by 5-GW LTD bricks connected electrically in parallel. (A brick comprises a single switch and two capacitors in series.) Six water-insulated radial-transmission-line impedance transformers transport the power generated by the modules to a six-level vacuum-insulator stack. The stack serves as the accelerator’s water-vacuum interface. The stack is connected to six conical outer magnetically insulated vacuum transmission lines (MITLs), which are joined in parallel at a 10-cm radius by a triple-post-hole vacuum convolute. The convolute sums the electrical currents at the outputs of the six outer MITLs, and delivers the combined current to a single short inner MITL. The inner MITL transmits the combined current to the accelerator’s physics-package load. Z 300 is 35 m in diameter and stores 48 MJ of electrical energy in its LTD capacitors. The acceleratormore » generates 320 TW of electrical power at the output of the LTD system, and delivers 48 MA in 154 ns to a magnetized-liner inertial-fusion (MagLIF) target [Phys. Plasmas 17, 056303 (2010)]. The peak electrical power at the MagLIF target is 870 TW, which is the highest power throughout the accelerator. Power amplification is accomplished by the centrally located vacuum section, which serves as an intermediate inductive-energy-storage device. The principal goal of Z 300 is to achieve thermonuclear ignition; i.e., a fusion yield that exceeds the energy transmitted by the accelerator to the liner. 2D magnetohydrodynamic (MHD) simulations suggest Z 300 will deliver 4.3 MJ to the liner, and achieve a yield on the order of 18 MJ. Z 800 is 52 m in diameter and stores 130 MJ. This accelerator generates 890 TW at the output of its LTD system, and delivers 65 MA in 113 ns to a MagLIF target. The peak electrical power at the MagLIF liner is 2500 TW. The principal goal of Z 800 is to achieve high-yield thermonuclear fusion; i.e., a yield that exceeds the energy initially stored by the accelerator’s capacitors. 2D MHD simulations suggest Z 800 will deliver 8.0 MJ to the liner, and achieve a yield on the order of 440 MJ. Z 300 and Z 800, or variations of these accelerators, will allow the international high-energy-density-physics community to conduct advanced inertial-confinement-fusion, radiation-physics, material-physics, and laboratory-astrophysics experiments over heretofore-inaccessible parameter regimes.« less

We are carrying out a multidisciplinary multi-institutional program to develop the scientific and technical basis for inertial fusion energy (IFE) based on laser drivers and direct-drive targets. The key components are developed as an integrated system, linking the science, technology, and final application of a 1000-MWe pure-fusion power plant. The science and technologies developed here are flexible enough to be applied to other size systems. The scientific justification for this work is a family of target designs (simulations) that show that direct drive has the potential to provide the high gains needed for a pure-fusion power plant. Two competing lasers are under development: the diode-pumped solid-state laser (DPPSL) and the electron-beam-pumped krypton fluoride (KrF) gas laser. This paper will present the current state of the art in the target designs and lasers, as well as the other IFE technologies required for energy, including final optics (grazing incidence and dielectrics), chambers, and target fabrication, injection, and tracking technologies. All of these are applicable to both laser systems and to other laser IFE-based concepts. However, in some of the higher performance target designs, the DPPSL will require more energy to reach the same yield as with the KrF laser.

This paper presents plans for neutralized drift compression experiments, precursors to future target heating experiments. The target-physics objective is to study warm dense matter (WDM) using short-duration (∼1 ns) ion beams that enter the targets at energies just above that at which dE/dx is maximal. High intensity on target is to be achieved by a combination of longitudinal compression and transverse focusing. This work will build upon recent success in longitudinal compression, where the ion beam was compressed lengthwise by a factor of more than 50 by first applying a linear head-to-tail velocity tilt to the beam, and then allowing the beam to drift through a dense, neutralizing background plasma. Studies on a novel pulse line ion accelerator were also carried out. It is planned to demonstrate simultaneous transverse focusing and longitudinal compression in a series of future experiments, thereby achieving conditions suitable for future WDM target experiments. Future experiments may use solenoids for transverse focusing of un-neutralized ion beams during acceleration. Recent results are reported in the transport of a high-perveance heavy ion beam in a solenoid transport channel. The principal objectives of this solenoid transport experiment are to match and transport a space-charge-dominated ion beam, and to study associated electron-cloud and gas effects that may limit the beam quality in a solenoid transport system. Ideally, the beam will establish a Brillouin-flow condition (rotation at one-half the cyclotron frequency). Other mechanisms that potentially degrade beam quality are being studied, such as focusing-field aberrations, beam halo, and separation of lattice focusing elements.

This paper covers the results of a scoping study to assess the possible application of magnetic diversion to a laser IFE reactor. Its impact on the engineering design and performance of the reactor is discussed, key issues are identified, and the findings from this assessment are summarized.

The need to reach high temperatures in an inertial fusion energy (IFE) target (or a target for the study of High Energy Density Physics, HEDP) requires the ability to focus ion beams down to a small spot. System models indicate that within the accelerator, the beam radius will be of the order of centimeters, whereas at the final focal spot on the target, a beam radius of the order of millimeters is required, so radial compression factors of order ten are required. The IFE target gain (and hence the overall cost of electricity) and the HEDP target temperature are sensitive functions of the final spot radius on target. Because of this sensitivity, careful attention needs to be paid to the spot radius calculation. We review our current understanding of the elements that enter into a systems model (such as emittance growth from chromatic, geometric, and non-linear space charge forces) for the final focus based on a quadrupolar magnet system.

In this paper we report on two Heavy Ion Fusion (HIF) driver point design studies. The Robust Point Design (RPD) was completed over a year ago, and the Modular Point Design (MPD) is still in progress. The goal of any point design study is to construct a detailed design that is self-consistent and integrated from injector to target. This has been the primary theme of both studies.

We report on an ongoing study on modular Heavy Ion Fusion (HIF) drivers. The modular driver is characterized by {approx}20 nearly identical induction linacs, each carrying a single high current beam. In this scheme, one of the full size induction linacs can be tested as an 'integrated Research Experiment' (IRE). Hence this approach offers significant advantages in terms of driver development path. For beam transport, these modules use solenoids, which are capable of carrying high line charge densities, even at low energies. A new injector concept allows compression of the beam to high line densities right after the source. The final drift compression is performed in a plasma in which the large repulsive space charge effects are neutralized. Finally, the beam is transversely compressed onto the target, using either external solenoids or current-carrying channels (in the assisted pinch mode of beam propagation). We report on progress towards a self-consistent point design from injector to target. Considerations of driver architecture, chamber environment as well as the methodology for meeting target requirements of spot size, pulse shape and symmetry are also described. Finally, some near-term experiments to address the key scientific issues are discussed.

The heavy-ion fusion community recently developed a power-plant design that meets the various requirements of accelerators, final focus, chamber transport and targets. The point design is intended to minimize physics risk and is certainly not optimal for the cost of electricity. Recent chamber-transport simulations, however, indicate that changes in the beam ion species, the convergence angle and the emittance might allow more-economical designs.

Krypton fluoride (KrF) lasers produce highly uniform beams at 248?nm, allow the capability of 'zooming' the spot size to follow an imploding pellet, naturally assume a modular architecture and have been developed into a pulsed-power-based industrial technology that readily scales to a fusion power plant sized system. There are two main challenges for the fusion power plant application: to develop a system with an overall efficiency of greater than 6% (based on target gains of 100) and to achieve a durability of greater than 3 ? 108 shots (two years at 5?Hz). These two issues are being addressed with the Electra (700?J, 5?Hz) and Nike (3000?J, single shot) KrF lasers at the Naval Research Laboratory. Based on recent advances in pulsed power, electron beam generation and transport, hibachi (foil support structure) design and KrF physics, wall plug efficiencies of greater than 7% should be achievable. Moreover, recent experiments show that it may be possible to realize long lived electron beam diodes using ceramic honeycomb cathodes and anode foils that are convectively cooled by periodically deflecting the laser gas. This paper is a summary of the progress in the development of the critical KrF technologies for laser fusion energy.

The ARIES-IFE study was an integrated study of inertial fusion energy (IFE) chambers and chamber interfaces with the driver and target systems. Detailed analysis of various subsystems was performed parametrically to uncover key physics/technology uncertainties and to identify constraints imposed by each subsystem. In this paper, these constraints (e.g., target injection and tracking, thermal response of the first wall, and driver propagation and focusing) were combined to understand the trade-offs, to develop operational windows for chamber concepts, and to identify high-leverage research and development directions for IFE research. Some conclusions drawn in this paper are (a) the detailed characterization of the target yield and spectrum has a major impact on the chamber; (b) it is prudent to use a thin armor instead of a monolithic first wall for dry-wall concepts; (c) for dry-wall concepts with direct-drive targets, the most stringent constraint is imposed by target survival during the injection process; (d) for relatively low yield targets (

High-energy electron beam pumped krypton fluoride (KrF) gas lasers are an attractive choice for inertial fusion energy (IFE). Their short wavelength and demonstrated high beam uniformity optimizes the laser-target physics, and their pulsed power technology scales to a large system. This paper presents the principals of this type of laser and the progress toward developing technologies that can meet the IFE requirements for repetition rate (5 Hz), efficiency (>6%), and durability (>3/spl times/10/sup 8/ shots). The Electra laser at the Naval Research Laboratory (NRL) has produced >500 J of laser light in short 5-Hz bursts. Research on Electra and the NRL Nike laser (3000 J, single shot) has shown that the overall efficiency should be greater than 7%. This is based on recent advances in electron beam stabilization and transport, electron beam deposition, KrF laser physics, and pulsed power. The latter includes the development of a new solid-state laser triggered switch that will be the basis for a pulsed power system that can meet the IFE requirements for efficiency, durability, and cost. The major remaining challenge is to develop long-lived hibachi foils (e-beam transmission windows). Based on recent experiments, this may be achievable by periodically deflecting the laser gas.

A coordinated, focused effort is underway to develop Laser Inertial Fusion Energy. The key components are developed in concert with one another and the science and engineering issues are addressed concurrently. Recent advances include: target designs have been evaluated that show it could be possible to achieve the high gains (>100) needed for a practical fusion system.These designs feature a low-density CH foam that is wicked with solid DT and over-coated with a thin high-Z layer. These results have been verified with three independent one-dimensional codes, and are now being evaluated with two- and three-dimensional codes. Two types of lasers are under development: Krypton Fluoride (KrF) gas lasers and Diode Pumped Solid State Lasers (DPSSL). Both have recently achieved repetitive 'first light', and both have made progress in meeting the fusion energy requirements for durability, efficiency, and cost. This paper also presents the advances in development of chamber operating windows (target survival plus no wall erosion), final optics (aluminium at grazing incidence has high reflectivity and exceeds the required laser damage threshold), target fabrication (demonstration of smooth DT ice layers grown over foams, batch production of foam shells, and appropriate high-Z overcoats), and target injection (new facility for target injection and tracking studies).

In heavy-ion inertial confinement fusion, ion beams are transported several meters through the reactor chamber to the target. This standoff distance mitigates damage to the final focus magnets and chamber walls from the target explosion. A promising transport scheme makes use of a preformed discharge channel to confine and guide the beams. In this assisted-pinched transport scheme, many individual beams are merged into two high-current beams for two-sided illumination of the fusion target. The beams are combined and focused outside the chamber before propagating at small radius in the discharge channel to the target. A large beam divergence can be contained by the strong magnetic field resulting from the roughly 50-kA discharge current. Using a hybrid particle-in-cell simulation code, we examine the dynamics of heavy-ion inertial confinement fusion driver-scale beams in this transport mode. Results from detailed two-dimensional simulations of assisted-pinched transport in roughly 1-Torr Xe suggest that th...

This paper studies, experimentally and theoretically, the grad-B drift transport and focusing of an intense electron beam. The Naval Research Laboratory’s Gamble II generator [J. D. Shipman, Jr., IEEE Trans. Nucl. Sci. NS-18, 243 (1971)] was used to drive a 900 kV, 850 kA beam. This beam drifted in the radial magnetic field gradient produced by an on-axis guide wire carrying a current of 40 kA. An additional axial magnetic field gradient generated by focusing spokes was used to radially compress the beam. Experimental diagnostics included radially resolved calorimetry and radiachromic film to study wall losses. Extensive simulations were performed to better understand the experiments. Energy transport efficiencies were slightly below 70%, and the focusing spokes were observed to compress the beam radius by approximately 10%.

In this paper, we describe the development of electron beam pumped KrF lasers for inertial fusion energy. KrF lasers are an attractive driver for fusion, on account of their demonstrated very high beam quality, which is essential for reducing imprint in direct drive targets; their short wavelength (248 nm), which mitigates the growth of plasma instabilities; and their modular architecture, which reduces development costs. In this paper we present a basic overview of KrF laser technology as well as current research and development in three key areas: electron beam stability and transport; KrF kinetics and laser propagation; and pulsed power. The work will be cast in context of the two KrF lasers at the Naval Research Laboratory, The Nike Laser (5 kJ, single shot), and The Electra Laser (400–700 J repetitively pulsed).

We have developed conceptual designs of two next-generation petawatt-class pulsed-power accelerators. The designs are based on the architecture described in Ref. [1]. The prime power source of both designs is a system of lineartransformer drivers (LTDs) [2,3]. Both designs use six water-insulated radial-transmission-line impedance transformers [1,4,5] to transport the power generated by the LTDs to a six-level vacuum-insulator stack. The stack is connected to six radial magnetically insulated transmission lines (MITLs); the MITLs are joined in parallel at small radius by a triple-post-hole vacuum convolute [6-9]. The convolute delivers the combined power of the six MITLs to a single short MITL that transmits the power to the load. The first accelerator will generate a peak electrical power of 300 TW, and deliver an effective peak current of 50 MA to a z pinch that implodes in 130 ns. This accelerator is 35 m in diameter, and will fit within the existing Z-accelerator building. The second, which is 52 m in diameter, will generate 800 TW, and deliver an effective peak current of 66 MA to a pinch that implodes in 120 ns. Both accelerators will allow high-energy-density physics experiments to be conducted over heretofore inaccessible parameter regimes.

The proposed Laboratory Microfusion Facility (LMF) will require /spl ges/10 MJ of 30 MeV lithium ions to be transported and focused onto high-gain, high-yield inertial confinement fusion targets. The light-ion LMF approach uses a multimodular system with individual ion extraction diodes as beam sources. Previous work examined the effect of time-of-flight bunching on energy transport efficiency, /spl eta//sub t/, under realistic constraints on diode operation, beam transport, and packing. Target design considerations suggest that the instantaneous power efficiency, /spl Gamma//sub t/, be maximized near peak power, Because of time-of-flight bunching, peak power occurs at the end of the power pulse for LMF designs. This work examines the effect of power efficiency tuning on /spl eta//sub t/ for an LMF design using ballistic transport with solenoidal lens focusing. Results indicate that tuning the power pulse to maximize /spl Gamma//sub t/ at about three-quarters through the pulse provides high power efficiency at the end of the pulse while still maintaining high /spl eta//sub t/. In addition to power efficiency tuning, effects on /spl eta//sub t/ from variations of the diode impedance model and the diode voltage waveform are also examined. >

A 3D fully electromagnetic (EM) model of the principal pulsed-power components of a high-current linear transformer driver (LTD) has been developed. LTD systems are a relatively new modular and compact pulsed-power technology based on high-energy density capacitors and low-inductance switches located within a linear-induction cavity. We model 1-MA, 100-kV, 100-ns rise-time LTD cavities [A. A. Kim et al., Phys. Rev. ST Accel. Beams 12, 050402 (2009)PRABFM1098-440210.1103/PhysRevSTAB.12.050402] which can be used to drive z-pinch and material dynamics experiments. The model simulates the generation and propagation of electromagnetic power from individual capacitors and triggered gas switches to a radially symmetric output line. Multiple cavities, combined to provide voltage addition, drive a water-filled coaxial transmission line. A 3D fully EM model of a single 1-MA 100-kV LTD cavity driving a simple resistive load is presented and compared to electrical measurements. A new model of the current loss through the ferromagnetic cores is developed for use both in circuit representations of an LTD cavity and in the 3D EM simulations. Good agreement between the measured core current, a simple circuit model, and the 3D simulation model is obtained. A 3D EM model of an idealized ten-cavity LTD accelerator is also developed. The model results demonstrate efficient voltage addition when driving a matched impedance load, in good agreement with an idealized circuit model.

A three-dimensional, fully electromagnetic (EM) model of the principal pulsed-power components of a new high-current linear transformer driver (LTD) z-pinch accelerator has been developed. LTD systems are a relatively new modular compact pulsed-power technology based on high energy density capacitors and low-inductance switches packed around a parallel-plate disk feed. We model 1-MA, 100-kV, 100-ns rise-time LTD cavities developed for driving z-pinch loads1. The simulation model tracks the generation and propagation of electromagnetic power from individual capacitors through triggered gas switches driving a radially symmetric output line. Multiple cavities are combined for power multiplication that drives a water-filled coaxial transmission line. A fully EM 3D model of a single 1-MA 100-kV LTD cavity driving a simple resistive load is presented and compared to electrical measurements. A new model of the current loss through the ferromagnetic cores is developed for use both in circuit representations of an LTD cavity and in the 3D EM simulations. Good agreement between the measured core current, a simple circuit model, and the 3D simulation model is obtained. A 3D EM model of a 10 cavity LTD accelerator, presently under construction at Sandia National Laboratories, is also developed. The model results demonstrate efficient voltage addition when driving a matched impedance load, in good agreement with an idealized circuit model.

The self-magnetic pinch diode is currently fielded on the RITS-6 accelerator at Sandia National Laboratories operating between 7-12 MV and is the leading candidate for future radiographic source development at the Atomic Weapons Establishment. The diode is capable of producing sub 3-mm radiation spot sizes and greater than 350 Rads measured at 1m. Complex physical processes affect the diode operation which in turn may affect its radiographic potential. High-resolution, time-resolved diagnostics have been utilized to help quantify the diode physics which include plasma spectroscopy, gated imaging, X-ray p-i-n diodes, spot size, and diode current measurements. The data from these diagnostics are also used to benchmark particle-in-cell simulations in order to better understand the underlying physics of operation. An overview of these experiments and simulations including future plans is presented.

A fully three‐dimensional electromagnetic model of the major pulsed power components of the 26‐MA ZR accelerator is presented. This large‐scale simulation model tracks the evolution of electromagnetic waves through the intermediate storage capacitors, laser‐triggered gas switches, pulse‐forming lines, water switches, tri‐plate transmission lines, and water convolute to the vacuum insulator stack. The plates at the insulator stack are coupled to a transmission line circuit model of the four‐level magnetically‐insulated transmission line section and post‐hole convolutes. The vacuum section circuit model is terminated by either a short‐circuit load or dynamic models of imploding z‐pinch loads. The simulations results are compared with electrical measurements made throughout the ZR accelerator and good agreement is found, especially for times before and up to peak load power. This modeling effort represents new opportunities for modeling existing and future large‐scale pulsed power systems used in a variety of high energy density physics and radiographic applications.

Longitudinal bunching factors in excess of 70 of a 300-keV, 27-mA K^{+} ion beam have been demonstrated in the neutralized drift compression experiment [P. K. Roy et al., Phys. Rev. Lett. 95, 234801 (2005)PRLTAO0031-900710.1103/PhysRevLett.95.234801] in rough agreement with particle-in-cell source-to-target simulations. A key aspect of these experiments is that a preformed plasma provides charge neutralization of the ion beam in the last one meter drift region where the beam perveance becomes large. The simulations utilize the measured ion source temperature, diode voltage, and induction-bunching-module voltage waveforms in order to determine the initial beam longitudinal phase space which is critical to accurate modeling of the longitudinal compression. To enable simultaneous longitudinal and transverse compression, numerical simulations were used in the design of the solenoidal focusing system that compensated for the impact of the applied velocity tilt on the transverse phase space of the beam. Complete source-to-target simulations, that include detailed modeling of the diode, magnetic transport, induction bunching module, and plasma neutralized transport, were critical to understanding the interplay between the various accelerator components in the experiment. Here, we compare simulation results with the experiment and discuss the contributions to longitudinal and transverse emittance that limit the final compression.

Summary form only given: A series of experiments were conducted at Sandia National Laboratories on the RITS-6 accelerator configured in the low impedance mode (7.5 MV, 180 kA) to investigate electrode plasma formation and propagation in relativistic electron beam diodes used for flash x-ray radiography. In particular the Self- Magnetic Pinch diode (SMP), which employed a hollow metal cathode positioned ~12 mm from a thin aluminum foil anode, in-front of a high atomic number bremsstrahlung x-ray converter, was studied. Anode and cathode plasmas composed of surface contaminants and metals with densities of up to 1017 cm-3 are formed and expand across the gap with velocities of 10's of cm/microsecond. It is believed that the dynamics and interactions of these plasmas are responsible for the observed impedance behavior of the diode. Visible and ultraviolet spectroscopy is used to spatially and temporally measure individual plasma species. Plasma densities and temperatures are determined using collisional-radiative models. Diagnostics include gated, intensified CCD camera imaging and gated/streaked spectroscopy using high resolution 1 meter Czerny-Turner monochromators. Recent results are presented.

Summary form only given as follows. Sandia National Laboratories' Radiographic Integrated Test Stand (RITS-6) is a six-stage inductive voltage adder (IVA) accelerator. It is designed to produce maximum currents of 186 kA at 7.8 MV in 70 ns pulses into electron beam diode loads when configured for low-impedance operation. Measurements of current and X-ray dose rate taken from a variety of diode loads have sometimes shown an oscillation typical of a resonant cavity. The oscillations appear intermittently from shot to shot and occur early in the pulse. The source of these oscillations is being studied with fully relativistic particle-in-cell simulations. Results from 3D simulations of the full RITS-6 transmission line, including the induction cavities, are compared to data. Insight into the nature of these oscillations and the modes supported by the geometry are presented.

Paraxial electron diodes with gas transport cells have been used to focus intense electron beams onto a high-Z target producing bremsstrahlung radiation. This configuration is being fielded on the recently commissioned RITS-6 accelerator producing 35-kA current and 10-MeV energy electron beams. Direct ionization of the beam and avalanche from the electron secondaries drive a break down of the gas that rapidly increases the gas cell conductivity. Non-ideal effects result in an axial sweep of the beam focus position that ultimately limits the radiographic spot. These effects include cathode plasma evolution, stimulated emission of anode ions, and magnetic diffusion in the gas cell. The gas breakdown is studied with hybrid particle-in-cell simulations using several different gas chemistry models including kinetic Monte Carlo interaction. In addition, we test the validity of these models against measured time- dependent beam spot and radiation production rates.

Summary form only given. Development of intense electron beam-driven diodes for flash X-ray radiography is being carried out at 7.5-12 MV on the RITS-6 accelerator at Sandia National Laboratories. One of several diodes under investigation is the paraxial diode, which employs a gas-filled transport cell to focus an electron beam onto a high-atomic-number target to generate X-rays. Three key objectives are to produce a small spot size, 600 rads at 1 m, and efficiently couple this relatively high-impedance diode to the lower-impedance RITS-6 accelerator. Particle-in-cell (PIC) simulations have shown that the primary limitation in spot size is due to the finite decay of the plasma return current which causes the beam focal location to sweep axially during the timescale of the pulse, hence leading to an increasing spot size. Time-resolved measurements of the spot size which convey this trend are reported. Interpretation of these results was aided by PIC simulations in which additional physics was included. Other outstanding issues for the paraxial diode on RITS-6 are also presented which include electron powerflow coupling to the diode, cavity modes which may affect focusing, and gas-breakdown models. Measurements of dose, dose rate, time-integrated (and time-resolved) spot size, and current are reported for experimental results at 7.5 and 10.5 MV.

Summary form only given. Investigations are underway at Sandia National Laboratories' RITS-6 accelerator facility (10 MV and 120 kA) to study plasma formation and propagation in electron beam diodes used for flash X-ray radiography. In particular the Self Magnetic Pinch (SMP) Diode has been studied using time and spatially resolved optical diagnostics to gain an understanding of the role of plasmas in diode performance. The SMP diode employs a hollow metal cathode and a thin aluminum foil anode in front of a high atomic number target to produce bremsstrahlung X-ray radiation. Results show plasma formation beginning during the second half of the (~70 ns) X-ray radiation pulse. While plasma formation is observed on both the cathode and anode, it is predominantly the anode plasma which expands across the A-K gap region (~10 mm). Data shows the presence of hydrocarbon and aluminum ion species with expansion velocities of a few 10's of cm/microsecond. Spectroscopic analysis is performed using time-dependent, collisional-radiative models to provide plasma density and temperature information. Diagnostics include optical imaging using gated, intensified CCD cameras, and gated/streaked spectra obtained using 1 meter Czerny-Turner monochromators. Recent results are presented.

Summary form only given. Designs of pulsed-power driven high-intensity electron beam accelerators continue to strive for higher power for brighter radiographic sources. Power delivery from the voltage drive source to the load is commonly via a magnetically insulated transmission line (MITL) in which a portion of the power flows as free electrons emitted from the cathode. Increasing the drive power creates challenges for modeling particle emission and power flow in MITLs beyond the tvpical field stress emission. In this presentation, we compare results from standard particle emission models in the particle-in-cell code LSP to data from Sandia National Laboratories' Radiographic Integrated Test Stand (RITS-6), a six-stage inductive voltage adder (IVA) designed to produce currents of 120 kA at voltages in excess of 10 MV in 70 ns. The transmission line is modeled for both a blade-load termination and a self-magnetic-pinch diode (SMP). Comparisons between simulation, data, and power-flow theory are shown, and additional plasma emission models are discussed.

Presently, investigations of intense electron beam-driven diodes for flash x-ray radiography are being conducted on the upgraded RITS-6 accelerator, 7-12 MV, at Sandia National Laboratories. One of several diodes under investigation is the paraxial diode, which employs a gas-filled transport cell to focus the beam onto a high atomic number target to generate x-rays. Two key objectives are to produce a small spot size, ≪ 5 mm, and high forward directed dose, ∼ 600-1000+ Rads@1m. LSP simulations have shown that the primary limitation in spot size is due to the finite decay of the plasma return current which causes the beam focal location to drift axially during the timescale of the pulse, hence leading to an increased spot size [1]. Replacing the gas with a preionized plasma has shown potential for stabilizing the beam focus by completely neutralizing the beam current [2]. Plans for the incorporation of a novel plasma-filled version of the gas-filled paraxial are presented. Another outstanding issue for the paraxial diode on RITS-6 concerns the powerflow coupling from the magnetically insulated transmission line to the diode. The radiation pulse is dominated by large amplitude oscillations, ∼125 MHz, which are correlated with the drive pulse. LSP simulations have shown that higher frequency modes, ∼ 220 – 365 MHz, are excited in the diode region as well. Efforts to mitigate these oscillations are presented. Forward-directed dose as well as time-integrated and time-resolved radiation spot measurements are reported.

Summary form only given. The self-magnetic-pinch (SMP) diode is being developed as an intense electron beam source for high-power x-ray radiography. The diode is comprised of a ~1-cm diameter, hollow cathode with a rounded tip from which a high-current electron beam is emitted. The team self-focuses in its own magnetic field as it propagates across a ~1-cm vacuum gap where it deposits its energy onto a planar high-atomic-number target from which bremsstrahlung is produced. The evolution and dynamics of expanding electrode plasmas has long been recognized as a limiting factor in the impedance lifetimes of high-power vacuum diodes. Accurate modeling of such plasmas is being pursued to aid in the understanding of the operating characteristics of these diodes as well as establishing scaling relations for reliable extrapolation to higher voltages. Here, a hybrid particle-in-cell code is used to study the evolution of electrode plasmas in the SMP diode for a nominal 4 MV voltage and different cathode configurations. A low density plasma front is calculated to quickly evolve towards the anode producing an effective 1 mm anode-cathode gap. The simulated plasma motion is consistent with that expected from jxB acceleration. The small sheath enables a high intensity electron spot at and rapid heating of the anode. The interplay of the cathode plasma with the subsequently expanding anode plasma will determine the overall diode impedance.

Summary form only given. Sandia National Laboratories is investigating and developing high-dose, high-brightness flash radiographic sources. One of the diodes we are developing is the immersed-Bz diode. This diode is a foil-less electron-beam diode with a long, thin, needle-like cathode inserted into the bore of a solenoid. The solenoidal magnetic field guides the electron beam emitted from the cathode to the anode while maintaining a small beam radius. The electron beam strikes a thin, high-atomic-number anode and produces bremsstrahlung. Recently, we reported on an extensive series of experiments where an immersed-Bz diode was fielded on the RITS-3 pulsed power accelerator, a 3-cell inductive voltage generator that produced peak voltages between 4 and 5 MV, ~140 kA of total current, and power pulse widths of ~50 ns. The analysis of those results is on-going. A summary of those results and analyses are presented along with plans for future experiments on RITS-6 at 10-12 MV.

The possibility of utilizing magnetic intervention (MI) in a laser-driven inertial fusion energy (IFE) dry wall chamber is being considered to steer away the ions from the chamber wall to more readily accessible and replaceable dump regions at the equator and poles. This paper summarizes the current status of this study, describing the overall MI chamber core configuration and layout and highlighting the key design and analysis results for the different components.

Longitudinal bunching factors in excess of 70 of a 300- keV, 27-mA K+ ion beam have been demonstrated in the Neutralized Drift Compression Experiment in rough agreement with particle-in-cell end-to-end simulations. These simulations include the ion source temperature, experimental diode voltage and induction bunching module voltage waveforms in order to determine the initial beam longitudinal phase space critical to longitudinal compression. To maximize simultaneous longitudinal and transverse compression, we designed a solenoidal focusing system that compensated for the impact of the applied velocity tilt on the transverse phase space of the beam. Here, pre-formed plasma provides beam neutralization in the last one meter drift region where the beam perveance becomes large. We compare simulation results with the experimental measurements and discuss the contributions to longitudinal and transverse emittance that limit compression.

The growth of the resistive hose instability for intense proton beams is examined using three-dimensional particle-in-cell simulations. The simulation results are compared with a time-dependent model of resistive hose growth that uses a spread-mass formulation and a time-dependent conductivity model. Radius tailoring of the beam head is shown to suppress high-frequency instability growth. In addition, the effects of a reduced-density plasma channel on the growth of the resistive hose instability is calculated.

Summary form only given. The self-magnetic-pinch diode is being developed as an intense electron beam source for high-power X-ray radiography. The diode geometry includes a ~5-cm long tapered cathode stalk, ~4-mm radius, with a rounded and hollow tip from which a high-current hollow electron beam is emitted. The beam is accelerated and focuses across a small ( 50 kA, and pulse widths of 10-50 ns. Experimental data suggests that the hollow cathode region acts to inhibit the enhancement of bipolar flow by expanding electrode plasmas, thus extending the impedance lifetime of the diode. Hybrid electromagnetic particle-in-cell simulations are being used to study the dynamical evolution of cathode and anode plasmas in several of the experimentally tested cathode configurations. Computational models of plasma evolution from solid surfaces are presently under development that will allow detailed comparisons with measured data including electron beam spot size on the anode and temporal impedance history

Summary form only given. Paraxial electron diodes with gas transport cells have been used to focus intense electron beams onto a high-Z target producing bremsstrahlung radiation. The quality of the resulting radiographic X-ray source increases linearly with the dose and inversely with the square of the spot size. This configuration is being fielded on the recently commissioned RITS-6 accelerator producing 35-kA current and 10-MeV energy electron beams. Direct ionization of the beam and avalanche from the electron secondaries drive a break down of the gas that rapidly increases the gas cell conductivity. Because of the delay and incompleteness of the gas breakdown, the gas cells typically operate with a small but finite and slowly increasing net current (sum of the plasma and beam currents). These non-ideal effects result in an axial sweep of the beam focus position that ultimately limits the radiographic spot. Other factors limiting the spot include intrinsic beam emittance, foil and gas scattering, and nonlinear magnetic field evolution in the gas cell. The gas breakdown is being studied with hybrid particle-in-cell simulations using several different gas chemistry models. In this paper, we numerically optimize the beam spot for radiographic utility. In addition, we assess the accuracy of these models via comparison with measured time-dependent beam spot and radiation production rates

Summary form only given. Experiments were conducted at SNL on the RITS-3 accelerator (5.5 MV and 120 kA), and are continuing on the next generation RITS-6 accelerator (10 MV and 120 kA) to study the role of plasmas in electron beam driven flash X-ray radiographic diodes. Plasmas formed by interactions with a relativistic electron beam can affect the impedance behavior of the diodes and contribute to variations in beam focusing. Initial studies investigated beam transport through a preionized background hydrogen plasma. Visible spectroscopy was used to spatially and temporally map the plasma conditions in the focusing cell during the time of the electron beam propagation. Results indicate that the electron density, temperature, and charge state distributions change with the presence of the beam, and can thus be used as quantitative measures of beam location and quality. Studies are now underway to look inside the transport region of neutral gas-filled paraxial diodes to gain similar insights. Experiments looking at plasma formation on the vacuum side of the thin metal anode foils used with paraxial diodes were also conducted. The presence of plasmas in the vacuum gap during the pulse can lead to impedance collapse and pulse shortening. Initial studies on RITS-3 showed foil-plasma formation occurring on the timescale of the beam pulse. These experiments are now being repeated at higher voltages on RITS-6. Diagnostics include a gated, intensified multichannel plate camera with a multifiber input, and a multichannel plate intensified streak camera, each combined with a 1 meter Czerny-Turner monochromator. Recent results are presented

Summary form only given. In electron beam driven flash X-ray radiography, an electron beam is focused onto a high atomic number target to generate high dose X-rays while achieving small

Summary form only given. Sandia National Laboratories began commissioning the six-stage inductive voltage adder (IVA) configuration of its Radiographic Integrated Test Stand (RITS-6) in 2006. The lower impedance IVA technology results in excess power-flow from the transmission line to the diode region. Schemes to divert the excess flow include attaching a large-radius field shaper ("knob") to the transmission line immediately upstream from the diode region with a correspondingly larger anode radius ("dustbin"). The RITS-6 commissioning includes a series of shots with a blade-diode load for configurations with and without the dustbin and knob. Numerical modeling of each configuration was conducted prior to commissioning using the particle-in-cell code LSP. Modeling included power flow from the transmission line to a blade-diode load with load impedances ranging from under- to over-matched. Simulation results for transmission line currents and impedances for each blade-diode configuration are presented. These results identified geometries which result in cavity resonances and undesirable voltage fluctuations. Efforts to understand and mitigate these resonances are discussed

Summary form only given, as follows. The rod-pinch diode concentrates an intense electron beam onto the tip of a tungsten rod anode, producing an intense x-ray source with sub-mm spot size? Plasmas evolving from the electrodes can influence electron trajectories, affecting the diode impedance and the x-ray source distribution. Plasma evolution from 1- and 0.5-mm-diam rod-pinches operated at 1.5 MV, 40 kA, and 50 ns on Gamble I1 is investigated using Schlieren photography, shearing interferometry, and high-sensitivity, twocolor interferometry3. The measurements show plasma expansion from the last 5 mm of the rod, with low-density plasma expanding at 10 c d p and high-density plasma expanding at 1-2 c d p. Results are compared with MHD and hybrid simulations of expanding plasma resulting from electron beam deposition. Implications for rod-pinch operation will be discussed.

Summary form only given. In flash X-ray radiography, one of the key goals is to focus an electron beam onto a high-Z-target to generate X-rays. One of several goals is to create a small radiographic spot. The paraxial diode, which utilizes a gas-filled transport cell to focus the beam into a small spot, has been investigated for several decades. It has been shown in simulations using LSP, a particle-in-cell code, that the primary limitation to achieving a small spot size is due to time-dependent net currents in the transport cell which cause the beam's focal plane to shift axially during the timescale of the pulse. This movement, often referred to as ''beam sweep", smears out the radiographic spot, leading to a larger size than is desired. Recently, calculations with LSP, demonstrate that replacing the gas with a pre-ionized plasma with density on the order of 1016 cm-3 significantly slows down the net current evolution, and yields a more stable focal plane and thus decreased time-integrated spot size. Recent experiments utilizing gas and plasma-filled transport cells have been performed on the RITS accelerator (4.5 MV, 20-30 kA beam current 60 ns FWHM) to study these predictions. The most recent results and analysis from these experiments are presented

Summary form only given. Spectroscopic experiments are being conducted on a plasma-filled paraxial X-ray radiography diode on the RITS-3 accelerator at Sandia National Laboratories. Initial data shows line and continuum emission occurring during the time of the electron beam propagation through a partially-ionized background plasma. Additional impurity ion species have also been identified. The purpose of these studies is to understand how a 4-4.5 MeV electron beam interacts with a background plasma as compared to a neutral gas, and the effects this has on beam focusing, instability formation, and X-ray production. By studying selected lines, plasma parameters such as charge states, densities, and temperatures can be obtained. Diagnostics include a gated, intensified microchannel plate camera with a 1 meter Czerny-Turner monochromator, giving a temporal evolution of a few nanoseconds. This is combined with a multi-fiber spectral input, which allows for spatial resolution in two dimensions in front of the target surface. Studies of this type are important for X-ray radiography diode research and help to support modeling efforts and to ensure future predictive capabilities

In order to drive an inertial fusion target or study high energy density physics with heavy ion beams, the beam radius must be focused to < 3 mm and the pulse length must be compressed to < 10 ns. The conventional scheme for temporal pulse compression makes use of an increasing ion velocity to compress the beam as it drifts and beam space charge to stagnate the compression before final focus. Beam compression in a neutralizing plasma does not require stagnation of the compression, enabling a more robust method. The final pulse shape at the target can be programmed by an applied velocity tilt. In this paper, neutralized drift compression is investigated. The sensitivity of the compression and focusing to beam momentum spread, plasma, and magnetic field conditions is studied with realistic driver examples. Using the 3D particle-in-cell code, we examine issues associated with self-field generation, stability, and vacuum-neutralized transport transition and focusing.

Summary form only given. As part of a continuous research effort into the development of high brightness flash radiographic sources, Sandia National Laboratories is developing a suite of advanced diagnostics that will aid in the characterization of these high-intensity electron-beam diodes. There are various time dependent beam transport features that inhibit the stable operation of these sources. A scintillator based time resolved spot size diagnostic (TRSD) was developed to quantify these beam variations. Preliminary results demonstrated beam centroid tracking as well as time dependent beam spot size measurements. Detailed characterization of the TRSD, including an assessment of the temporal and spatial resolution of the system is underway. Results of these tests as well the improvements and modifications being incorporated into a 2nd generation TRSD design are detailed in this paper.

Summary form only given. Traditionally, the paraxial radiographic diode has relied on a gas-filled chamber to focus an electron beam into a small spot for the purpose of generating X-rays. Focusing is provided by the electron beam-produced plasma whose density increases during the course of the beam pulse. Simulations using Lsp, a particle-in-cell code, have shown that the location of the electron beam focus wanders during the timescale of the pulse in this case, resulting in a larger radiographic spot than is desirable. Lsp simulations have also shown that replacing the gas-filled focusing chamber with a fully-ionized plasma of density on the order of 10/sup 16/ cm/sup -3/ will prevent the spot from significantly wandering, hence resulting in a smaller, more stable spot size. Sandia National Laboratories is implementing a plasma prefilled paraxial diode on the RITS-3 accelerator. The plasma-filled cell will be created using a z-discharge in a hydrogen fill. Laser interferometry is used to assess the plasma density on bench-top experiments, and plans are underway to perform the diagnostics during RITS-3 experiments. Details of the plasma source, its characterization in a bench-top setting, and plans for investigation during RITS-3 operation are presented.

Summary form only given. The self-magnetic-pinch diode is being developed as an intense electron beam source for high-power X-ray radiography. The diode uses an 8-mm-diameter hollow-cylindrical cathode with a rounded tip from which a high current electron beam is emitted. The beam self-focuses in its own magnetic field as it propagates across a /spl sim/1-cm vacuum gap where it deposits its energy on a planar high-atomic-number target from which bremsstrahlung radiation is produced. This diode has been experimentally optimized for electrical operating parameter ranges of 1-5 MV,>50 kA, and pulse widths of 20-50 ns. Experimental data suggests that the hollow cathode region significantly extends the impedance lifetime by providing electrode plasmas produced in the vicinity of the pinch a place to expand. The hybrid particle-in-cell code LSP is used to study the evolution of cathode and anode plasmas in several of the experimentally tested cathode configuration. Comparisons with available experimental results are made.

Summary form only given. In this paper we discuss the design of RITS-6 including the dimensions of the MITL, the cathode, anode and vacuum flow currents, and voltage at each stage of the MITL/voltage adder. RITS-6 output operating conditions for diode impedances less than 85.5 ohms where an over-insulation wave propagates back up the MITL from the diode was presented. Particle-in-cell simulations (LSP) of the power flow from the cell to the load was compared with parapotential flow predictions. LSP simulations based on RITS-3 PFL water output switch jitter data was used to estimate the expected range of vacuum insulator voltages on RITS-6 if water PFL switches are employed.