Search

To advance the current state-of-the-art of capillary-based laser plasma accelerators (LPAs), the tunability of capillary discharge plasma channels needs to be improved. We present the techniques used to determine critical properties of the plasma density distribution. Independent tailoring of plasma channel width and on-axis density are required to produce higher energy electron beams with existing facilities. A scheme involving an additional, nanosecond laser pulse to locally heat the channel has been proposed previously. We discuss recent progress on the implementation of this scheme, demonstrating a heating effect on the plasma channel as evidenced from nanosecond-resolved spectroscopy on transversely emitted plasma light. PIC simulations indicate the possibility of accelerating high charge beams up to 8.4 GeV average energy if other technique advances are made as well. These include the need for longer plasma channels of 10s of centimeters, low plasma density and an ionization injection scheme to inje...

Beam breakup due to the hose instability for electron transport through hollow plasma channels is examined numerically and compared with analytical theory. Wakefields are simulated for different geometries, including cylindrical hollow channels and the “bubble” regime, and the simulations are carried out with advanced Particle-In-Cell algorithms. Results are compared to previous estimates for beams accelerated in a plasma.

In this work we propose a novel positron driven plasma wakefield acceleration configuration in the non linear regime using tightly focused positron drive beams to create a hollow plasma channel with no background plasma ions, where positrons can accelerate to high energy. A simplified model for the background plasma ion motion in this scenario was analyzed. The proposed scheme was explored numerically resorting to multi-dimensional PIC simulations using the numerical code OSIRS. info:eu-repo/semantics/publishedVersion

The plasma properties of a plasma waveguide are critical to the performance of laser-plasma accelerators (LPAs). By measuring the group velocity in plasma channels through spectral interferometry, the density in the channel is retrieved. In this paper, experimental methods and results are presented for the plasma density in LPA-relevant plasma channels of various lengths.

The propagation of a short and intense laser pulse in a plasma channel is investigated through the nonlinear paraxial wave equation. An analytic condition describing quasi-matched laser propagation (i.e., when the second order normalized-intensity-weighted transverse moment of the laser-pulse remain constant) is obtained. For any arbitrary temporal laser pulse profile and any prescribed transverse density profile in the channel, we determine the laser intensity distribution along the pulse, i.e., the slice-dependent laser waist, ensuring quasi-matched propagation. For the case of a Gaussian laser with an initially uniform spot throughout the pulse, we determine the optimal channel depth that minimizes spot size oscillations. The analytical and semi-analytical results obtained for both cases are validated through comparison with numerical simulations.

Two-dimensional particle-in-cell simulations are presented for a laser-irradiated solid-density target with and without an underdense preplasma. It is shown that an underdense preplasma can generate an energetic electron tail in addition to the warm electrons generated at the critical surface. Preplasma electrons are accelerated in a quasi-static positively charged channel formed by the laser. At ultra-relativistic laser intensities (a0 = 10), the acceleration mechanism is not sensitive to the laser polarization. An energetic tail with energies significantly exceeding the energy expected for a single electron in a vacuum is present in simulations with s and p-polarized beams. This suggests that the mechanism of parametric amplification of laser-driven electron acceleration is a likely explanation for the observed phenomenon.

The generation of GeV-scale electron beams in the plasma channel formed in a gas-filled capillary discharge waveguide is investigated. Electron beams with energies above 900 MeV and with root-mean-square divergence of 3.5 mrad are observed for plasma densities of 2.15 × 1018 cm-3 and a peak input laser power of only 55 TW. The variation of the electron energy with the plasma density is measured and found to exhibit a maximum at plasma densities for which the dephasing length approximately matches the length of the plasma channel. Injection and acceleration of electrons at the relatively low plasma density of 3.2 × 1017 cm-3 is observed. The energy spectra of the generated electron beams are shown to exhibit good shot-to-shot reproducibility, with the observed variations attributable to the measured shot-to-shot jitter of the laser parameters. Two methods for correcting for the effects on the measured energy spectrum of off-axis electron beam propagation are investigated. © 2012 American Institute of Physics.

We demonstrated the production of an optical waveguide in a capillary discharge‐produced plasma using a cylindrical capillary. Plasma parameters of its waveguide were characterized by use of both a Normarski laser interferometer and a hydrogen plasma line spectrum. A space‐averaged maximum temperature of 3.3 eV with electron densities of the order of 1017 cm−3 was observed at a discharge time of 150 ns and a maximum discharge current of 200 A. An ultrashort, intense laser pulse was guided by use of this plasma channel.

Starting from the cases of the current clinical facilities as a source of reference to compare with, final performance of lasers and accelerators and even cost target were discussed. We assume maximum flexibility to enable treatment of small as well as large in‐depth tumor volumes requiring the maximum energy of 250 MeV for protons and 400 MeV/u for carbon. We adopt their reference numbers for the required total number of protons / carbon ions per fraction (5 min) as well as peak numbers (per second). Other parameters (like energy spread and total number of voxels) are adjusted to the particularities of laser acceleration, which include a much higher production energy spread than in the synchrotron case and a laser pulse rate currently suggested by technology. As far as cost, it is assumed that a single laser driver unit is foreseen for one treatment room. It is suggested that the target for 10–20 years of development could be a cost of the laser driver unit not exceeding 1/4 of the conventional synchrotron facility cost, which is 40 M€. Further, design study, proposal and preliminary experimental results for all optical Compton scattering X‐ray source for nuclear material detection at University of Tokyo is introduced. By using the Ti:Sapphire laser at about 7 TW and our original gas‐jet and magnetic plasma channel, we succeeded in generating more than 100 MeV electrons with a reasonable emittance. Compton scattering with beam‐spread Ti:Sapphire laser pulse can yield quasi‐monochromatic X‐rays around 110 keV near the K‐edges of U, Np, Pu. Subtraction imaging across the K‐edge realizes clear recognition and distingushment of those compounds in liquid.

The wavefront of a short laser pulse after interaction in a laser‐plasma accelerator (LPA) was measured to diagnose laser‐guiding quality. Experiments were performed on a 100 TW class laser at the LOASIS facility of LBNL using a hydrogen‐filled capillary discharge waveguide. Laser‐guiding with a pre‐formed plasma channel allows the laser pulse to propagate over many Rayleigh lengths at high intensity and is crucial to accelerate electrons to the highest possible energy. Efficient coupling of laser energy into the plasma is realized when the laser and the channel satisfy a matched guiding condition, in which the wavefront remains flat within the channel. Using a wavefront sensor, the laser‐guiding quality was diagnosed based on the wavefront of the laser pulse exiting the plasma channel. This wavefront diagnostic will contribute to achieving controlled, matched guiding in future experiments.

Plasma Channel Diagnostic Based on Laser Centroid Oscillations A. J. Gonsalves, K. Nakamura, C. Lin, J. Osterhoff, S. Shiraishi, C. B. Schroeder, C. G. R. Geddes, Cs. Toth, E. Esarey, and W. P. Leemans Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA Abstract. A technique has been developed for measuring the properties of discharge-based plasma channels by monitoring the centroid location of a laser beam exiting the channel as a function of input alignment offset between the laser and the channel. The centroid position of low-intensity ( 10 18 Wcm −2 are used for laser plasma acceleration, the intensity was kept low in these experiments so that non-linear effects such as self-focusing [11] could be neglected and the plasma properties measured in a straight- forward way. The technique relies on the fact that a transverse offset of the laser pulse at the entrance of the plasma channel causes the laser beam to undergo transverse oscillations about the channel axis. These oscillations depend on the initial centroid offset and angle with respect to the channel axis, and on the channel properties. As will be shown, measuring the dependence of the exit laser centroid position on the input and on channel formation timing provides direct information on the channel matched spot size and profile. This technique reduces the measurement error compared to interferometry and provides increased precision in channel alignment to minimize pointing errors in LPA applications [12].

An overview is presented of the design of a 10 GeV laser plasma accelerator (LPA) that will be driven by a PW‐class laser system and of the BELLA Project, which has as its primary goal to build and install the required Ti:sapphire laser system for the acceleration experiments. The basic design of the 10 GeV stage aims at operation in the quasi‐linear regime, where the laser excited wakes are largely sinusoidal and offer the possibility of accelerating both electrons and positrons. Simulations show that a 10 GeV electron beam can be generated in a meter scale plasma channel guided LPA operating at a density of about 1017 cm−3 and powered by laser pulses containing 30–40 J of energy in a 50–200 fs duration pulse, focused to a spotsize of 50–100 micron. The lay‐out of the facility and laser system will be presented as well as the progress on building the facility.

A Surface Plasma Source (SPS) with plasma generation by a saddle type antenna is discussed. The following features of the helicon discharge with a saddle type antenna in magnetic field are identified: efficient plasma generation in resonant condition, low gas density, strong separation of plasma from the wall, possibility to control plasma flux distribution by magnetic field configuration. Applications of saddle type antenna in SPS for accelerators for Homeland Security and for Neutral Beam Injectors (NBI) are considered.

A small plasma focus device was constructed for the EUV radiation experiment. The storage energy of capacitor bank is 3.4 kj, and the maximum discharge current is 140 kA. Although the device was not well optimized for strong pinch, EUV radiation was observed from the focus plasma. The plasma dynamics was affected by electrode material, and the use of Sn electrode was proved to be effective for producing Sn plasma.

The effects of radiation reaction on electron beam dynamics are studied in the context of plasma‐based accelerators. Electrons accelerated in a plasma channel undergo transverse betatron oscillations due to strong focusing forces. These oscillations lead to emission by the electrons of synchrotron radiation, with a corresponding energy loss that affects the beam properties. An analytical model for the single particle orbits and beam moments including the classical radiation reaction force is derived and compared to the results of a particle transport code. It is shown that the radiation could significantly affect the beam properties (e.g., increased relative energy spread) in plasma wakefield accelerators.

The fundamentally new approach to the lightning step and dart leaders structure model is suggested, which shows a possibility of the drift propagation for the electrons in a plasma channel. Appearance of the strong Hall electric field in the current channel by the account of the magnetic field can result in the generation of the relativistic drifting electrons to be held in the channel due to the magnetic self‐insulation effect. The range of the measured x‐ray emission from the lightning channel 30–250 keV, which corresponds to the measured current value 4–11 kA, is in a reasonably good agreement with the estimates made in the framework of presented model.

The experimental and theoretical results show that the anode double layer (DL) is a very sensitive plasma formation suitable for fine optogalvanic studies. The obtained results demonstrate that the parameters of the oscillations sustained by a DL (frequency, amplitude) can be used as optogalvanic detectors.

The working group considered high transformer ration schemes for an afterburner based on the design of a future linear collider. The main linac produces high charge beams of 100 GeV. A multiple stage plasma based accelerator would accelerate a portion of this beam to 500 GeV. The length of each plasma stage is expected to be of the order of a few meters while the isochronous beam transport required for multiple stages would occupy about a kilometer. Discussions in the working group were centered on issues to be addressed: ion motion in the plasma channel, positron side of accelerator … The state of present e‐beam driven plasma and dielectric Wakefield accelerators is very mature and closely resembles parameters of the afterburner for ILC. The main result of this working group is a multistage afterburner scheme of an afterburner for ILC and discussion of the experimental program to address main issues.

In practical terms, those plasmas produced by a DC voltage power supply do not attain densities above the 108 to 109 cm−3 band. Here we present a power supply, controlled in current and voltage, which has been successfully designed and constructed delivering plasma densities in the orders of 109 – 1010 cm−3. Its experimental performance test was conducted within one toroidal and one cylindrical chambers capable of 29 and 35 litres, respectively, using nitrogen gas. The DC plasma was characterized by a double electric probe. Several physical phenomena present in the PIII process have been keenly investigated including plasma sheath dynamics, interaction of plasma and surface, etc. In this paper we analyze the effect of the implantation voltage, plasma density and pulse time in the PIII average heating power and fluence density.

The corrugated plasma channel allows micron‐scale control of the intensity and phase velocity of a guided intense femtosecond laser pulse. Direct electron acceleration by a radially polarized laser pulse can be achieved by quasi‐phase matching in these channels.

We observed a universal phenomenon of the competition among multiple filaments generated during the propagation of intense femtosecond laser pulses in air. We show that the fluorescence signal from the excitation of nitrogen molecules inside the plasma channel contains important information pertaining to the formation and interaction of multiple filaments. The detected backscattered nitrogen fluorescence from inside the filaments yielded irregular changes from shot to shot which cannot be explained by fluctuation arising from the initial laser pulse itself. Numerical simulations reveal a complex dynamics of multiple filament propagation and interaction dynamics that depends strongly on the initial perturbations of the laser beam. The irregular changes of the fluorescence signal are attributed to the interference between adjacent hot spots that evolve into filaments which give rise to new hot spots (filaments) in between, and thus give the appearance of the fusion or branching of filaments.

We report on the results of spectroscopic experiments that were conducted by focusing an intense ultra‐short laser pulse onto a helium gas target. The scattered light from the interaction region was measured spectrally and spatially from various directions as a function of laser intensity and plasma density. The experimental data showed that forward Stimulated Raman Scattering (SRS) was sensitive to the focus position of laser relative to the nozzle. Together with the plasma channel that was imaged by a CCD camera, the measurements indicate that SRS is seeded by the ionization blue‐shifted light. The cross‐phase modulation between the SRS and laser beam was also observed in the experiment.

An ablative capillary discharge is installed into a linac beamline and serves as a plasma source for generating and characterizing wakefields. Simultaneously, the electron beam is used as a tool for plasma diagnostics. A high‐energy picosecond CO2 laser channeled within the same capillary strongly affects a counterpropagating electron beam. These observations, supported with simulations, suggest the possibility of manipulating relativistic electron beams by steep plasma channels ponderomotively produced by a laser.

The decay process of a magnetized plasma having a flow‐shear layer after plasma discharge is observed by using two‐dimensional multi‐probes arranged on a plane perpendicular to magnetic field. The plasma of appropriate potential structure has long lifetime.

A high‐energy CO2 laser is channeled in a capillary discharge. Occurrence of guiding conditions at a relatively low plasma density (

A new diagnostic method, which is named facing double probe (FDP) method, for measuring plasma flow in a magnetized plasma has been developed. The method uses a pair of facing electrodes located along the magnetic field line and connected to each other by a power supply. Its shape is similar to a conventional double probe but the surface of each electrode is open only its facing side. The characteristic properties of the plasma between the facing electrodes with different potentials are investigated using one‐dimensional particle‐in‐cell simulation in order to obtain better understanding of the FDP method.

Recent progress and new developments in Electromagnetic Structure‐Based Acceleration were presented in this Working Group (WG). The presentations made to the WG were separated into two categories: slow‐wave and fast‐wave accelerating structures. The slow‐wave talks included a theoretical study of accelerators at optical frequencies that showed there are important and surprising differences between optical structures and traditional microwave accelerators. Practical issues for dielectric loaded accelerators were also addressed including a novel method for frequency tuning of dielectric structures and, in a joint session with the mm‐wave sources WG, dielectric high power test results were presented. Theory and simulation results were presented for both laser‐driven and beam‐driven (i.e., wakefield) novel structures including cylindrical and planar dielectric devices and photonic band gap (PBG) devices including a scheme to mimic a plasma channel with a PBG. In the fast‐wave talks, progress in developing highly‐tapered inverse free electron lasers (IFEL), staging IFELs for monoenergetic laser acceleration, and generating attosecond electron pulses were presented. New designs and applications of cyclotron‐based devices driven by laser or RF energy were also discussed.

For final beam transport in an IFE reactor three alternatives are mainly discussed. These are neutralized ballistic transport, serf-pinched transport, and plasma channel transport. Discharge plasma channels were investigated in the recent years at GSI Darmstadt and at LBNL Berkeley in a number of experiments. Different initiation mechanisms for gas discharges of up to 60 kA were studied and compared. In the Berkeley experiments laser ionization of organic vapors in a buffer gas was used to initiate and direct the discharge while at GSI laser gas heating and ion beam induced gas ionization were tested as initiation mechanisms. Measurements of temperature, electron density, gas density, and magnetic field distribution in the channels are compared with results of beam transport experiments at the GSI UNILAC accelerator and with MHD simulations of the ID-fluidcode CYCLOPS, which was developed in Berkeley. Good agreement between plasma diagnostics results, measured ion optical properties and MHD simulations was found. Parameters that are required for a reactor application are a discharge current of 50 kA, a channel diameter below 1 cm, a pointing stability better than 500 µm, and MHD stability for more than 10 µs. These parameters have been demonstrated in the recent experiments. The results imply that transport channels work with sufficient stability, reproducibility and ion optical properties in a wide pressure range and for various discharge gases.

When intense and short laser pulses pass through a preformed Z‐pinch discharge plasmas, the wave guiding of the laser lights is created in performing the high density and high temperature plasma phase. The dynamics of the laser propagation is analyzed by the wave equation of the laser electric field coupled with the plasma hydrodynamic equations and the rate equations describing the plasma ion states. The optimum soft x‐ray repetition lasing occurs in the 3s‐3p transition in Ar IX, Ne‐like plasmas, in the irradiated plasma channel. A gain coefficient 4.55 cm−1 without opacity was obtained by numerical computations.

The paper describes the regimes of energy input in the pseudospark discharge in typical conditions of EUV source operation. It is shown that in a mode of superdense glow discharge the electron temperature in the discharge plasma reaches rather high values without self-magnetic compression of the plasma channel.

The spark created in a neck of a dense Z-pinch can ignite a fusion detonation in the adjacent D-T plasma channel. Using an appropriate transition between the ignited D-T plasma and an inertially confined cylinder of highly compressed advanced fuel plasma it is possible to amplify the spark energy to a level adequate for the ignition of a detonation wave in the advanced fuel. An m=0 instability of a Z-pinch carrying a current of the order of 10 MA, with a rise time inferior to 10 ns can generate a spark capable of igniting a fusion detonation in the adjacent D-T plasma channel. Such μZ-pinch may be produced by a fast implosion of a cylindrical liner, while a conical channel properly chosen can amplify the spark energy. In order to derive some general rules for the parameters of the spark, the transition, the cylinder of advanced fuel and the liner different numerical models were used. We present here a review of these results and an outline of a possible experimental arrangement for obtaining such a Z-pinc...

We report guiding of intense (I=1.3±0.7×1017 W/cm2) 80 fs laser pulses with negligible spectral distortion through 1.5-cm-long preformed helium plasma channels. Channels were formed by axicon-focused Nd:YAG laser pulses of either 0.3 J energy, 100 ps duration, after pre-ionizing a 200–700 Torr backfill of He gas to ne∼1016 cm−3 with a pulsed electrical discharge; or 0.6–1.1 J energy, 400 ps duration, which required neither pre-ionization nor intentional impurities for seeding. Transverse interferometry showed that He was fully-ionized on the channel axis in both cases. Identical fs pulses suffered substantial ionization-induced blueshifts after propagating through Ar and Ne channels of similar dimensions.

A 3-D envelope equation for a laser pulse in a tapered plasma channel is derived, which includes wakefields, relativistic and non-paraxial effects such as finite pulse length and group velocity dispersion. It is shown that electron energies of ∼GeV in a channel-guided LWFA can be achieved by using short pulses where the forward Raman and modulation nonlinearities tend to cancel. Further energy gain can be achieved by tapering the plasma density to reduce electron dephasing.

We report two variations on plasma channel generation using the propagation of intense Bessel beams. In the first experiment, the propagation of a high intensity Bessel beam in neutral gas is observed to give rise to resonantly enhanced plasma channel generation, resulting from resonant self-trapping of the beam and enhanced laser-plasma heating. In the second experiment, a high power, hollow Bessel beam (J5) is produced and the optical breakdown of a gas target and the generation of a tubular plasma channel with such a beam is realized for the first time. Hydrodynamic simulations of the laser-plasma interaction of are in good agreement with the results of both experiments.

The electromagnetic wake field response of a hollow plasma channel to a driver (laser or charged particle beam) of arbitrary velocity is derived. The dispersion and loss factors of excited fundamental and higher order azimuthal modes are computed. Growth rates for beam breakup instabilities are calculated and beam transport is studied. External focusing is shown to provide a method of controlling transverse instabilities. For parameters of interest for high gradient plasma-based accelerators, it is shown that the most severe limitation to the interaction length of a single accelerator stage based on the hollow plasma channel structure is the transverse instability of the particle beam.

Raman Forward Scattering (RFS) instability of an intense laser pulse in a single-moded plasma channel is studied in the slab geometry. For a particular class of channels the growth rate is found to be significantly smaller than in the homogeneous plasma. This reduction, appreciable even for sub-relativistic laser intensities and shallow plasma channels, is caused by the radial shear of the plasma frequency and the existence of the collisionlessly damped hybrid (electrostatic/electromagnetic) modes of the transversely inhomogeneous plasma.

In this paper we summarize our recent effort and results in theoretical study of the emittance issues of multistaged Laser Wakefield Accelerator (LWFA) in TeV energy range. In such an energy regime the luminosity and therefore the emittance requirements become very stringent and tantamount to the success or failure of such an accelerator. The system of such a machine is very sensitive to jitters due to misalignment between the beam and the wakefield. In particular, the effect of jitters in the presence of a strong focusing wakefield and initial longitudinal phase space spread of the beam leads to severe transverse emittance degradation of the beam. To improve the emittance we introduce several methods: a mitigated wakefield focusing by working with a plasma channel, an approximately synchronous acceleration in a superunit setup, the “horn” model based on exactly synchronous acceleration achieved through plasma density variation and lastly an algorithm based on minimization of the final beam emittance to actively control the stage displacement of such an accelerator.

The hollow plasma channel is analyzed as an accelerating structure. The loss factors and mode frequencies are calculated for all azimuthal modes. Implications for plasma-based accelerator design are discussed, including beam loading, beam breakup, and tuning tolerances. The asymptotic growth of the transverse beam displacement is computed. Methods for beam breakup reduction are considered.

We report formation of a guiding channel for high intensity laser pulses in pure, preionized helium using an axicon focused laser pulse, and demonstrate a single shot technique to measure the temporal and spatial evolution of the plasma channel.

In this paper probe measurements of plasma parameters in a switch with a full grid control were used for calculations of the current balance in the conductive channel of the grid mesh using the electrical transparency model. Calculation has shown that the current balance agrees with real discharge current when the potential hump (virtual cathode) is assumed in the current channel in the mesh. Analysis shows that the virtual cathode is, probably, an intrinsic property for the condition in plasma switches.