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Plasma-jet-driven magneto-inertial fusion is an alternative approach to controlled nuclear fusion, which aims to utilize a line-replaceable dense plasma liner as a repetitive spherical compression driver. In this experiment, first measurements of the formation of a spherical argon plasma liner formed from 36 discrete pulsed plasma jets are obtained on the Plasma Liner Experiment. Properties including liner uniformity and morphology, plasma density, temperature, and ram pressure are assessed as a function of time throughout the implosion process and indicate an apparent transition from initial kinetic inter-jet interpenetration to collisional regime near stagnation times, in accordance with theoretical expectation. A lack of primary shock structures between adjacent jets during flight implies that arbitrarily smooth liners may be formed by way of corresponding improvements in jet parameters and control. The measurements facilitate the benchmarking of computational models and understanding the scaling of plasma liners toward fusion-relevant energy density. [ABSTRACT FROM AUTHOR]

The infra-red video bolometer (IRVB) is a diagnostic equipped with an infra-red camera that measures the total radiated power in thousands of lines of sight within a large field of view. Recently validated in MAST-U [Fderici et al., Rev. Sci. Instrum. 94, 033502 (2023)], it offers a high spatial resolution map of the radiated power in the divertor region, where large gradients are expected. The IRVB's sensing element comprises a thin layer of high Z absorbing material, typically platinum, usually coated with carbon to reduce reflections [Peterson et al., Rev. Sci. Instrum. 79, 10E301 (2008)].Here, the possibility of using a relatively inert material such as titanium, is explored that can be produced in layers up to 1 μm compared to 2.5 μm for Pt and then coat it with Pt of the desired thickness (0.3 μm per side here) and carbon. This leads to a higher temperature signal (about 3 times) and better spatial resolution (about 4 times), resulting in higher accuracy in the measured power [Peterson et al., Rev. Sci. Instrum. 79, 10E301 (2008)]. This assembly is also expected to improve foil uniformity, as the Pt layer is obtained via deposition rather than mechanical processes [Mukai et al., Rev. Sci. Instrum. 87, 2014 (2016)].Given its multi-material composition, measuring the thermal properties of the foil assembly is vital. Various methods using a calibrated laser as a heat source have been developed, analyzing the temperature profile shape [Sano et al., Plasma and Fusion Res. 7, 2405039 (2012)] and [Mukai et al., Rev. Sci. Instrum. 89, 10E114 (2018)] or fitting the calculated laser power for different intensities and frequencies [Fderici et al., Rev. Sci. Instrum. 94, 033502 (2023)]. Here, a simpler approach is presented, which relies on analyzing the separate components of the foil heat equation for a single laser exposure in a given area. This can then be iterated over the entire foil to capture local deviations. [ABSTRACT FROM AUTHOR]

A new type of large area sensor for infrared imaging bolometers has been developed. It replaces the thin and fragile free-standing metal foils, which typically have been used, with a multi-layer coated sapphire (or diamond) substrate. Sapphire is transparent to mid-infrared wavelengths, is robust against transients, and can be thick enough to even be the vacuum window. The primary radiation absorber is still a thin deposited metal layer, but now it is partially insulated from the supporting sapphire substrate by a black (carbon-based) layer, which also acts as a blackbody remitter. Test results indicate 6× more noise equivalent power density (estimated NEPD = 23 W/m2 at 5 ms camera exposure time, foil temperature decay time 60 ms) for a 2 μm gold-coated sapphire disk compared to estimated NEP = 4 W/m2 at 1.8 ms exposure time, with foil decay time 420 ms, for a nominal 2.5 μm thick platinum-free-standing foil. [ABSTRACT FROM AUTHOR]

A diagnostic for extreme ultraviolet spectroscopy was fielded on the sheared-flow-stabilized (SFS) fusion Z-pinch experiment (FuZE-Q) for the first time. The spectrometer collected time-gated plasma emission spectra in the 5–40 nm wavelength (30–250 eV) range for impurity identification, radiative power studies, and for plasma temperature and density measurements. The unique implementation of the diagnostic included fast (10 ns risetime) pulsed high voltage electronics and a multi-stage differential pumping system that allowed the vacuum-coupled spectrometer to collect three independently timed spectra per FuZE-Q shot while also protecting sensitive internal components. Analysis of line emission identifies oxygen (N-, C-, B-, Be-, Li-, and He-like O), peaking in intensity shortly after maximum current (>500 kA). This work provides a foundation for future high energy spectroscopy experiments on SFS Z-pinch devices. [ABSTRACT FROM AUTHOR]

Integrated magnetic modeling and design are important to meet the requirements for (1) formation, (2) translation, and (3) compression of a field reversed configuration (FRC) for magnetized target fusion. Off-the-shelf solutions do not exist for many generic design issues. A predictive capability for time-dependent magnetic diffusion in realistically complicated geometry is essential in designing the experiment. An eddy-current code was developed and used to compute the mutual inductances between driven magnetic coils and passive magnetic shields (flux excluder plates) to calculate the self-consistent axisymmetric magnetic fields during the first two stages. The plasma in the formation stage was modeled as an immobile solid cylinder with selectable constant resistivity and magnetic flux that was free to readjust itself. It was concluded that (1) use of experimentally obtained anomalously large plasma resistivity in magnetic diffusion simulations is sufficient to predict magnetic reconnection and FRC formation, (2) comparison of predicted and experimentally observed timescales for FRC Ohmic decay shows good agreement, and (3) for the typical range of resistivities, the magnetic null radius decay rate scales linearly with resistivity. The last result can be used to predict the rate of change in magnetic flux outside of the separatrix (equal to the back-emf loop voltage), and thus estimate a minimum θ-coil loop voltage required to form an FRC. [ABSTRACT FROM AUTHOR]

Wendelstein 7-X (W7-X) is the most advanced fusion experiment in the stellarator line and is aimed at proving that the stellarator concept is suitable for a fusion reactor. One of the most important issues for fusion reactors is the monitoring of plasma facing components when exposed to very high heat loads, through the use of visible and infrared (IR) cameras. In this paper, a new image processing system for the analysis of the strike lines on the inboard limiters from the first W7-X experimental campaign is presented. This system builds a model of the IR cameras through the use of spatial calibration techniques, helping to characterize the strike lines by using the information given by real spatial coordinates of each pixel. The characterization of the strike lines is made in terms of position, size, and shape, after projecting the camera image in a 2D grid which tries to preserve the curvilinear surface distances between points. The description of the strike-line shape is made by means of the Fourier Descriptors. [ABSTRACT FROM AUTHOR]

The edge magnetic structure in the Helically Symmetric eXperiment (HSX) and Wendelstein 7X (W7-X) stellarators has been shown to have a significant impact on the particle fueling and exhaust of the plasma main species (hydrogen) as well as impurity helium. For HSX, the plasma sourcing to exhaust ratio, quantified by the effective and global particle confinement times τ p * and τ p , H , respectively, increases when a magnetic island chain is located in the plasma edge. The fueling efficiency is reduced by 25% when the plasma boundary is deformed by the magnetic islands. The X-point geometry also yields higher plasma temperatures in front of the main recycling region. When the island is moved radially inward, both τ p * and τp decrease by 10 % – 25 % depending on plasma density. The τ p , H results rely heavily on EMC3-EIRENE modeling which confirms reduced fueling efficiency due to more rapid ionization in the outward shifted island position. These findings suggest that for a helically optimized system like HSX, the plasma fueling from the recycling source, as well as from active gas injection, can be controlled by the magnetic island chain in the plasma edge—which is a basic requirement for a divertor system. This process is also effective for the control of effective helium exhaust times, as τ p , H e * measured by perturbative gas puff experiments is reduced by up to 40% when the islands are shifted inwards. For Wendelstein 7-X, a similar reduction of τ p , H e * was inferred when magnetic islands were moved from the far plasma edge into the confined plasma region. However, the effective confinement features of H as the main plasma species were not affected due to the non-optimal position of the magnetic islands with respect to the highly localized ionization domain during the limiter startup campaign. [ABSTRACT FROM AUTHOR]

Laser-driven neutrons are generated by the conversion of laser-accelerated ions via nuclear reactions inside a converter material. We present results from an experimental campaign at the PHELIX laser at GSI in Darmstadt where protons and deuterons were accelerated from thin deuterated plastic foils with thicknesses in the μm and sub-μm range. The neutrons were generated inside a sandwich-type beryllium converter, leading to reproducible neutron numbers around 1011 neutrons per shot. The angular distribution was measured with a high level of detail using up to 30 bubble detectors simultaneously. It shows a laser forward directed component of up to 1.42 × 1010 neutrons per steradian, corresponding to a dose of 43 mrem scaled to a distance of 1 m from the converter. [ABSTRACT FROM AUTHOR]

Wendelstein 7-X (W7-X) aims to demonstrate the reactor capability of the stellarator concept, by creating plasmas with pulse lengths of up to 30 min at a heating power of up to 10 MW. The divertor plasma facing components will see convective steady state heat flux densities of up to 10 MW/m2. These high heat flux target elements are actively cooled and are covered with carbon fibre reinforced carbon (CFC) as plasma facing material. The CFC is bonded to the CuCrZr cooling structure. Over the life time of the experiment this interface may weaken and cracks can occur, greatly reducing the heat conduction between the CFC tile and the cooling structure. Therefore, there is not only the need to monitor the divertor to prevent damage by overheating but also the need to detect these fatigue failures of the interface. A method is presented for an early detection of fatigue failures of the interface layer, solely by using the information delivered by the IR-cameras monitoring the divertor. This was developed and validated through experiments made with high heat flux target elements prior to installation in W7-X. [ABSTRACT FROM AUTHOR]

Time-dependent, two-dimensional profiles of the axial flow velocity, density, electron temperature, and magnetic field components are measured at two axial locations in a screw pinch plasma column of the reconnection scaling experiment. The results show that the ion momentum flux for a given column radius is dissipated by the ion-ion Coulomb scattering viscosity due to a significant radial shear of the axial velocity. By comparing the terms of the magnetohydrodynamic momentum balance equation, radial profile of ion viscosity is determined. Chord-integrated ion temperature measurements performed at several radial locations using Doppler broadening spectroscopy show ion temperature of about 1 eV. Measured ion viscosity agrees within a factor of 2 with the classical Braginskii expectations. [ABSTRACT FROM AUTHOR]

Alcator C-Mod is a particularly challenging environment for thermography. It presents issues that will similarly face ITER, including low-emissivity metal targets, low-Z surface films, and closed divertor geometry. In order to make measurements of the incident divertor heat flux using IR thermography, the C-Mod divertor has been modified and instrumented. A 6° toroidal sector has been given a 2° toroidal ramp in order to eliminate magnetic field-line shadowing by imperfectly aligned divertor tiles. This sector is viewed from above by a toroidally displaced IR camera and is instrumented with thermocouples and calorimeters. The camera provides time histories of surface temperatures that are used to compute incident heat-flux profiles. The camera sensitivity is calibrated in situ using the embedded thermocouples, thus correcting for changes and nonuniformities in surface emissivity due to surface coatings. [ABSTRACT FROM AUTHOR]

A high-resolution imaging system, consisting of megapixel mid-IR and visible cameras along the same line of sight, has been prepared for the new W7-X stellarator and was operated during Operational Period 1.1 to view one of the five inboard graphite limiters. The radial line of sight, through a large diameter (184 mm clear aperture) uncoated sapphire window, couples a direct viewing 1344 × 784 pixel FLIR SC8303HD camera. A germanium beam-splitter sends visible light to a 1024 × 1024 pixel Allied Vision Technologies Prosilica GX1050 color camera. Both achieve sub-millimeter resolution on the 161 mm wide, inertially cooled, segmented graphite tiles. The IR and visible cameras are controlled via optical fibers over full Camera Link and dual GigE Ethernet (2 Gbit/s data rates) interfaces, respectively. While they are mounted outside the cryostat at a distance of 3.2 m from the limiter, they are close to a large magnetic trim coil and require soft iron shielding. We have taken IR data at 125 Hz to 1.25 kHz frame rates and seen that surface temperature increases in excess of 350 °C, especially on leading edges or defect hot spots. The IR camera sees heat-load stripe patterns on the limiter and has been used to infer limiter power fluxes (∼1-4.5 MW/m2), during the ECRH heating phase. IR images have also been used calorimetrically between shots to measure equilibrated bulk tile temperature, and hence tile energy inputs (in the range of 30 kJ/tile with 0.6 MW, 6 s heating pulses). Small UFO's can be seen and tracked by the FLIR camera in some discharges. The calibrated visible color camera (100 Hz frame rate) has also been equipped with narrow band C-III and H-alpha filters, to compare with other diagnostics, and is used for absolute particle flux determination from the limiter surface. Sometimes, but not always, hot-spots in the IR are also seen to be bright in C-III light. [ABSTRACT FROM AUTHOR]

A combined IR and visible camera system [G. A.Wurden et al., "A high resolution IR/visible imaging system for the W7-X limiter," Rev. Sci. Instrum. (these proceedings)] and a filterscope system [R. J. Colchin et al., Rev. Sci. Instrum. 74, 2068 (2003)] were implemented together to obtain spectroscopic data of limiter and first wall recycling and impurity sources during Wendelstein 7-X startup plasmas. Both systems together provided excellent temporal and spatial spectroscopic resolution of limiter 3. Narrowband interference filters in front of the camera yielded C-III and Hα photon flux, and the filterscope system provided Hα, Hβ, He-I, He-II, C-II, and visible bremsstrahlung data. The filterscopes made additional measurements of several points on the W7-X vacuum vessel to yield wall recycling fluxes. The resulting photon flux from both the visible camera and filterscopes can then be compared to an EMC3-EIRENE synthetic diagnostic [H. Frerichs et al., "Synthetic plasma edge diagnostics for EMC3-EIRENE, highlighted for Wendelstein 7-X," Rev. Sci. Instrum. (these proceedings)] to infer both a limiter particle flux and wall particle flux, both of which will ultimately be used to infer the complete particle balance and particle confinement time τP. [ABSTRACT FROM AUTHOR]

Interpretation of spectroscopic measurements in the edge region of high-temperature plasmas can be a challenge since line of sight integration effects make direct interpretation in terms of quantitative, local emission strengths often impossible. The EMC3-EIRENE code--a 3D fluid edge plasma and kinetic neutral gas transport code--is a suitable tool for full 3D reconstruction of such signals. A versatile synthetic diagnostic module has been developed recently which allows the realistic 3D setup of various plasma edge diagnostics to be captured.We highlight these capabilities with two examples for Wendelstein 7-X (W7-X): a visible camera for the analysis of recycling, and a coherent-imaging system for velocity measurements. [ABSTRACT FROM AUTHOR]

A convenient method to accelerate simultaneously hundreds of micron-size dust particles to a few km/s over a distance of about 1 m is based on plasma drag. Plasma jets which can deliver sufficient momentum to the dust particles need to have speeds of at least several tens of km/s, densities of the order of 1022 m-3 or higher, and low temperature ∼1 eV, in order to prevent dust destruction. An experimental demonstration of dust particles acceleration to hypervelocities by plasma produced in a coaxial gun is presented here. The plasma flow speed is deduced from photodiode signals while the plasma density is measured by streaked spectroscopy. As a result of the interaction with the plasma jet, the dust grains are also heated to high temperatures and emit visible light. A hypervelocity dust shower is imaged in situ with a high speed video camera at some distance from the coaxial gun, where light emission from the plasma flow is less intense. The bright traces of the flying microparticles are used to infer their speed and acceleration by employing the time-of-flight technique. A simple model for plasma drag which accounts for ion collection on the grain surface gives predictions for dust accelerations which are in good agreement with the experimental observations. [ABSTRACT FROM AUTHOR]

A global power balance analysis has been performed for the Field Reversed Experiment with Liner high density (>5×1022 m-3) field reversed configuration (FRC) plasma. The analysis was based on a zero-dimensional power balance model [D. J. Rey and M. Tuszewski, Phys. Fluids 27, 1514 (1984)]. The key findings are as follows. First, the percentage of radiative losses relative to total loss is an order of magnitude lower than previous lower density FRC experiments. Second, Ohmic heating was found to correlate with the poloidal flux trapping at FRC formation, suggesting that poloidal flux dissipation is primarily responsible for plasma heating. Third, high density FRCs analyzed in this work reinforce the low-density adiabatic scaling, which shows that particle confinement time and flux confinement time are approximately equal. [ABSTRACT FROM AUTHOR]

A field-reversed configuration (FRC) gains angular momentum over time, eventually resulting in an n=2 rotational instability (invariant under rotation by π) terminating confinement. To study this, a laser interferometer probes the time history of line integrated plasma density along eight chords of the high-density (∼1017 cm-3) field-reversed configuration experiment with a liner. Abel and tomographic inversions provide density profiles during the FRC’s azimuthally symmetric phase, and over a period when the rotational mode has saturated and rotates with a roughly fixed profile, respectively. During the latter part of the symmetric phase, the FRC approximates a magnetohydrodynamic (MHD) equilibrium, allowing the axial magnetic-field profile to be calculated from pressure balance. Basic FRC properties such as temperature and poloidal flux are then inferred. The subsequent two-dimensional n=2 density profiles provide angular momentum information needed to set bounds on prior values of the stability relevant parameter α (rotational to ion diamagnetic drift frequency ratio), in addition to a view of plasma kinematics useful for benchmarking plasma models of higher order than MHD. [ABSTRACT FROM AUTHOR]

A 0.633 μm laser interferometer provides detailed time resolved information about the spatial distribution of the plasma density of field reversed configurations (FRC’s) produced by the FRX-L experiment at Los Alamos National Laboratory. This experiment is an effort to produce a magnetized plasma with closed field lines suitable for compression by a solid metal liner imploded by the Shiva Star capacitor bank at the Air Force Research Laboratory. The interferometer probes a fanned array of eight chords through the FRC midplane, measuring the line integrated free electron density via its effect on optical phase shift relative to eight reference beams as a function of time. The reference beams are given nominally identical optical paths, except that they are folded for compactness and given an 80 MHz higher optical frequency by use of a Bragg cell beam splitter. After the beams are recombined, interference results in 80 MHz electromagnetic beat waves with dynamic phase shifts equal to those of the corresponding optical probes. Quadrature mixing of the electronically monitored light is then performed with rf components. Noteworthy features of the interferometer’s design are the unique compact folding scheme of the reference paths, inclusion of a fused quartz tube in the reference path similar to that of the FRC’s vacuum vessel to compensate for cylindrical lensing, and transmission of the interfering light via optical fibers to a rf shielded room for processing. Extraneous contributions to the phase shift due to vibration resulting from the system’s pulsed magnetic field, and dynamic refractive changes in or near the fused quartz tube wall (possibly due to radiation heating) are corrected for. [ABSTRACT FROM AUTHOR]

The focus of the field-reversed configuration (FRC) experiment with a liner (FRX-L) is the formation of a target FRC plasma for magnetized target fusion experiments. An FRC plasma with density of 1023 m-3, total temperature in the range of 150–300 eV, and a lifetime of ≈20 μs is desired. Field-reversed θ-pinch technology is used with programed cusp fields at θ-coil ends to achieve non-tearing field line reconnections during FRC formation. Well-formed FRCs with density between (2–4)×1022 m-3, lifetime in the range of 15–20 μs, and total temperature between 300–500 eV are reproducibly created. Key FRC parameters have standard deviation in the mean of 10% during consecutive shots. The FRCs are formed at 50 mTorr deuterium static fill using 2 kG net reversed bias field inside the θ-coil confinement region, with external main field unexpectedly ranging between 15–30 kG. The high-density FRCs confinement properties are approximately in agreement with empirical scaling laws obtained from previous experiments with fill pressure mostly less than 20 mTorr. Analyses in this paper reveal that reducing the external main field modulation and/or extending the θ-coil length in the FRX-L device are critical in achieving higher FRC parameters for application in magnetized target fusion. [ABSTRACT FROM AUTHOR]

Magnetic pickup coils and single turn flux loops are installed on the FRX-L device. The combination of the two measurements provides the excluded flux radius that approximates the separatrix radius of the field-reversed configuration (FRC) plasma. Arrays of similar probes are used to map out local magnetic field dynamics beyond both ends of the theta-coil confinement region to help understand the effects of cusp locations on flux trapping during the FRC formation process. Details on the probe design and system calibrations are presented. The overall system calibration of excluded flux radius measurement is examined by replacing FRC plasma with a known radius aluminum conductor cylinder. [ABSTRACT FROM AUTHOR]

Magnetized target fusion (MTF) is a potentially low cost path to fusion, intermediate in plasma regime between magnetic and inertial fusion energy. It requires compression of a magnetized target plasma and consequent heating to fusion relevant conditions inside a converging flux conserver. To demonstrate the physics basis for MTF, a field reversed configuration (FRC) target plasma has been chosen that will ultimately be compressed within an imploding metal liner. The required FRC will need large density, and this regime is being explored by the FRX-L (FRC-Liner) experiment. All theta pinch formed FRCs have some shock heating during formation, but FRX-L depends further on large ohmic heating from magnetic flux annihilation to heat the high density (2-5 ×1022m³), plasma to a temperature of Te+Ti≈500 eV. At the field null, anomalous resistivity is typically invoked to characterize the resistive like flux dissipation process. The first resistivity estimate for a high density collisional FRC is shown here. The flux dissipation process is both a key issue for MTF and an important underlying physics question. [ABSTRACT FROM AUTHOR]

The reconnection scaling experiment (RSX), a linear device for studying three-dimensional magnetic reconnection in both collisional and collisionless laboratory plasmas, has been constructed at Los Alamos National Laboratory. Advanced experimental features of the RSX that lead to scientific advantages include the use of simple technology (commercial plasma guns) to create plasma and current channels. Physics motivations, design and construction features of the RSX, are presented. Basic plasma parameters that characterize the RSX are shown together with preliminary measurements of visible light emission during the merging of two parallel current channels. [ABSTRACT FROM AUTHOR]

Injecting neutral atoms into high-temperature plasmas forms the basis for several important diagnostics, such as motional Stark effect and charge exchange recombination spectroscopy. We describe an alternative approach to seeding the plasma with neutrals, via "hypervelocity dust beam injection" (HDBI), using micron-sized dusts. Among its many potential applications, HDBI mapping of two-dimensional internal magnetic fields inside medium-sized (50-500 eV) plasmas is discussed in detail. Electrostatic acceleration at ∼100-200 kV will launch a stream of (0.2-10 µm-sized) dust grains of lithium or carbon to hypervelocities (1-10 km/s). Each dust grain, acting as a "microcomet" in the plasma, forming a plume (tail), which if photographed, will reveal the direction of the local magnetic field, with anywhere from 10-100 microcomets in the plasma at any time, a full profile of the B-field direction could be obtained per high resolution image. Due to the small dust grain size, the perturbation to the plasma will be minimal. HDBI could be a simple low cost approach to obtain internal magnetic field information in plasmas with magnetic field structures that are significantly different than vacuum fields, such as in spherical tokamaks, FRC's, RFP's, and spheromaks. [ABSTRACT FROM AUTHOR]

The gas puff imaging (GPI) diagnostic can be used to study the turbulence present at the edge of magnetically confined plasmas. In this diagnostic the instantaneous two-dimensional (2D) radial vs poloidal structure of the turbulence is measured using fast-gated cameras and discrete fast chords. By imaging a controlled neutral gas puff, of typically helium or deuterium, the brightness and contrast of the turbulent emission fluctuations are increased and the structure can be measured independently of natural gas recycling. In addition, recent advances in ultrafast framing cameras allow the turbulence to be followed in time. The gas puff itself does not perturb the edge turbulence and the neutral gas does not introduce fluctuations in the emission that could possibly arise from a nonsmooth (turbulent) neutral gas puff. Results from neutral transport and atomic physics simulations using the DEGAS 2 code are discussed showing that the observed line emission is sensitive to modulations in both the electron density and the electron temperature. The GPI diagnostic implementation in the National Spherical Torus Experiment (NSTX) and Alcator C-Mod tokamak is presented together with example results from these two experiments. [ABSTRACT FROM AUTHOR]

A newly installed density diagnostic using CO[sub 2] laser interferometry and an H[sub α] diagnostic using interference filters and photomultiplier tubes for the recently constructed sustained spheromak physics experiment (SSPX) are described. First diagnostic results of the H[sub α] diagnostic were useful to understand the breakdown physics in the new SSPX experiments. Low-noise density data validates techniques to reduce vibration and electronic pickup. The data-processing electronics of the new interferometer can yield unambiguous density data that is equivalent to 16 fringe shifts. Density data is also critical to understand the particle source, and the J/n[sub e] parameter for SSPX. [ABSTRACT FROM AUTHOR]

We are planning experiments using a field reversed configuration plasma injected into a metal cylinder, which is subsequently electrically imploded to achieve a fusing plasma. Diagnosing this plasma is quite challenging due to the short timescales, high energy densities, high magnetic fields, and difficult access. We outline our diagnostic sets in both a phase I study (where the plasma will be formed and translated), and phase II study (where the plasma will be imploded). The precompression plasma (diameter of only 8–10 cm, length of 30–40 cm) is expected to have n∼10[sup 17] cm[sup -3], T∼100–300 eV, B∼5 T, and a lifetime of 10–20 μs. We will use visible laser interferometry across the plasma, along with a series of fiber-optically coupled visible lig recently completed its first phase of operation. The primary objective for the baseline diagnostic1 set during this phase has been to aid in plasma control and machine operation, which successfully led to attainment of 1 MA of plasma current and some encouraging initial results with coaxial helicity injection (CHI). This first set of diagnostics relied on techniques previously established on tokamaks and related plasma devices. In the next phase of operation, strong auxiliary heating will become available in the form of rf heating through high harmonic fast waves (HHFWs) and neutral beam injection (NBI) for a combined total power of 11 MW. With intense auxiliary heating, accurate and reliable measurements of the plasma parameters for both machine operation and characterization of the plasma performance over a wide range of conditions will require an extended set of diagnostics. Profile diagnostics will be particularly important in this phase, and in some cases this capability can be achieved by upgrading existing diagnostics. However, many new diagnostic approaches must be implemented which take into account the constraints of a spherical torus device. An... [ABSTRACT FROM AUTHOR]

An infrared imaging system, based on an Amber Radiance 1 infrared camera, is used at Alcator C-Mod to measure the surface temperatures in the lower divertor region. Due to the supra-linear dependence of the thermal radiation with temperature it is important to make use of the 12-bit digitization of the focal plane array of the Amber camera and not be limited by the 8 bits inherent to the video signal. It is also necessary for the image capture device (i.e., fast computer) to be removed from the high magnetic field environment surrounding the experiment. Finally, the coupling between the digital camera output and the capture device should be nonconductive for isolation purposes (i.e., optical coupling). A digital video remote camera interface (RCI) coupled to a PCI bus fiber optic interface board is used to accomplish this task. Using this PCI-RCI system, the 60 Hz images from the Amber Radiance 1 camera, each composed of 256×256 pixels and 12 bits/pixel, are captured by a Windows NT computer. An electrical trigger signal is given directly to the RCI module to synchronize the image stream with the experiment. The RCI can be programmed from the host computer to work with a variety of digital cameras, including the Amber Radiance 1 camera. [ABSTRACT FROM AUTHOR]

Turbulent filaments in visible light emission corresponding mainly to density fluctuations at the edge have been observed in large aspect ratio tokamaks: TFTR, ASDEX, Alcator C-Mod, and DIII-D. This article reports on similar turbulent structures observed in the National Spherical Torus Experiment (NSTX) using a fast-framing, intensified, digital visible camera. These filaments were previously detected mainly in high recycling regions, such as at limiters or antennas, where the line emission from neutral atoms was modulated by the fluctuations in local plasma density. However, by introducing controlled edge gas puffs, i.e., gas puff imaging, we have increased the brightness and contrast in the fluctuation images and allowed the turbulent structure to be measured independently of the recycling. A set discrete fiber-optically coupled sight-lines also measured the frequency spectra of these light fluctuations with a 200 kHz bandwidth. Initial results in NSTX show that the turbulent filaments are well aligned with the magnetic field which can be up to 45° from the horizontal at the outer midplane of NSTX. The dominant wavelength perpendicular to the magnetic field is ∼7–11 cm, corresponding to a k[sub ⊥] ρ[sub s] of ∼0.3 at an assumed T[sub e]=25 eV, and the frequency spectra has a typical broad shape characteristic of edge turbulence extending to about 100 kHz. By imaging a He gas puff along a magnetic field line the characteristic radial scalelength appears to be in the 3–5 cm range. [ABSTRACT FROM AUTHOR]

In this article we present an overview of the plasma diagnostics operating or planned for the sustained spheromak physics experiment device now operating at Lawrence Livermore National Laboratory. A set of 46 wall-mounted magnetic probes provide the essential data necessary for magnetic reconstruction of the Taylor relaxed state. Rogowski coils measure currents induced in the flux conserver. A CO[sub 2] laser interferometer is used to measure electron line density. Spectroscopic measurements include an absolutely-calibrated spectrometer recording extended domain spectrometer for obtaining time-integrated visible ultraviolet spectra and two time-resolved vacuum monochrometers for studying the time evolution of two separate emission lines. Another time-integrated spectrometer records spectra in the visible range. Filtered silicon photodiode bolometers provide total power measurements, and a 16 channel photodiode spatial array gives radial emission profiles. Two-dimensional imaging of the plasma and helicity injector is provided by gated television cameras and associated image-processing software. An array of fiber-coupled photodetectors with H alpha filters view across the midplane and in the injector region to measure neutral hydrogen concentrations. Several novel diagnostics are being fielded including a transient internal probe (TIP) and an ultrashort-pulse reflectometer (USPR) microwave reflectometer. The TIP probe fires a very high velocity optical bullet through the plasma and will provide fairly nonpertabative internal magnetic field and current measurements to compare with an equilibrium code model fitted to wall-mounted probes. The USPR is being designed to study edge density and turbulent fluctuations. A multipoint Thomson scattering system is currently being installed to give radial temperature and density profiles. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Lower-hybrid waves are studied in the Princeton ACT-I steady-state toroidal plasma device using a radially scanning CO[sub 2] laser scattering system with both amplitude and phase sensitive detection techniques. Clearly defined resonance cones launched from external electrostatic antennas are seen to disappear as the plasma density is raised. Scaling of a lower-hybrid wave (LHW) laser signal with radio-frequency (rf) power in the presence of resonance cones shows nonlinearities associated with rf-induced changes in the effective laser scattering volume. Absolute fluctuation level estimates suggest that this occurs when eφ/T[sub e] ≥ 1. Wave-front curvature effects can cause a complete loss of resonance cone laser signals, even though probes indicate that cones are still present. Measurements of the wave k[sub 1] spectrum in the plasma show direct evidence for electron Landau filtering of the original wave k[sub ] spectrum launched from the antenna at the plasma edge, and strong dependence on antenna phasing. Finally, frequency shifts and loss of the resonance cone signal are associated with high levels of plasma density edge turbulence. [ABSTRACT FROM AUTHOR]

Describes a design for a bolometer system employing infrared imaging of a segmented-matrix absorber in a cooled-pinhole camera geometry, which will be prototyped and demonstrated on the large helical device. General concept and design constraints; Modeling and resulting design parameters.

As part of a USDOE/ASDEX collaboration, a detailed examination of pellet ablation in ASDEX with a variety of diagnostics has allowed a better understanding of a number of features of hydrogen ice pellet ablation in a plasma. In particular, fast-gated photos with an intensified Xybion CCD video camera allow in situ velocity measurements of the pellet as it penetrates the plasma. With time resolution of typically 100 ns and exposures every 50 μs, the evolution of each pellet in a multipellet ASDEX tokamak plasma discharge can be followed. When the pellet cloud track has striations, the light intensity profile through the cloud is hollow (dark near the pellet), whereas at the beginning or near the end of the pellet trajectory the track is typically smooth (without striations) and has a gaussian-peaked light emission profile. New, single pellet Stark broadened Dα, Dβ, and Dγ spectra, obtained with a tangentially viewing scanning mirror/spectrometer with Reticon array readout, are consistent with cloud densities of 2×1017 cm-3 or higher in the regions of strongest light emission. A spatially resolved array of Dα detectors shows that the light variations during the pellet ablation are not caused solely by a modulation of the incoming energy flux as the pellet crosses rational q surfaces, but instead are a result of dynamic, nonstationary, ablation process. [ABSTRACT FROM AUTHOR]

The four barrel pneumatic pellet injector originally used on the Alcator C tokamak at MIT has been reconfigured for use on the ZT-40M reversed field pinch (RFP) at Los Alamos National Laboratory. The lower temperature and density in this device require operation at velocities significantly lower than on Alcator. Allowing a 2-s warmup prior to firing the injector has enabled operation with pressures as low as 40 psig and velocities ranging as low as 200 m/s with H2 pellets, without major cryostat modification. Initial injection experiments in the ZT-40M plasma have verified that pellet refueling is effective for density control, but the net particle inventory increase is less than might be expected based on simple models. Indications of nonoptimal penetration and curvature of the pellet trajectory, in addition to comparable time scales for particle confinement and pellet transit, may account for this. [ABSTRACT FROM AUTHOR]

High-Θ (Θ≡B[variant_greek_theta](a)/) toroidal reversed-field pinch discharges in the ZT-40M device are characterized by soft x-ray sawtooth oscillations. It is demonstrated that a one-dimensional transport description is sufficient to model the rise time of sawtooth oscillations, provided the resistivity profile has significant gradients. This result suggests that the toroidal magnetic flux generation (dynamo) observed in the reversed-field pinch (RFP) may be restricted to the m=1 tearing events associated with the sawtooth crash (decrease in the soft x-ray flux). [ABSTRACT FROM AUTHOR]

Describes a personal computer-based system installed on the Alcator C-Mod tokamak for routine digitization of video images. Basic components of the video capture, storage, and retrieval system; Storage of video on removable media; Compression and file formats; Determination of viewing imagery.

Studies the wall stabilization and active control of low-n magnetohydrodynamic instabilities in a tokamak using a High Beta Tokamak-Extended Pulse experiment. Description of the tokamak and the stabilization systems; Results from the active control of residual instabilities in wall-stabilized discharges; Modulation of the rotation velocity of internal instability.

This paper documents the effects on confinement of introducing discrete limiters into the edge plasma of the ZT-40M reversed-field pinch (RFP) [Fusion Technol. 8, 1571 (1985)]. RFP confinement is not significantly degraded by appropriately designed single limiters inserted to a sufficient depth for effective local vacuum vessel wall protection. Inserting limiters too deeply into the plasma results in excess limiter heating, and a consequent increase in the impurity content of the plasma. Under these conditions the plasma loop voltage increases. The heating of the limiters is observed to be asymmetric, with the majority of the heat flux in ZT-40M being attributable to suprathermal electrons [Tsupra∼2–3×Te(0)] reaching the edge moving almost unidirectionally along magnetic-field lines. [ABSTRACT FROM AUTHOR]

The edge plasma of the ZT-40M [Fusion Technol. 8, 1571 (1985)] reversed-field pinch has been studied using a combination of three different plasma probes: a double-swept Langmuir probe, an electrostatic energy analyzer, and a calorimeter–Langmuir probe. The edge plasma has been measured both with and without a movable graphite tile limiter present nearby in the plasma. Without a limiter a fast nonthermal tail of electrons (T[bar_over_tilde:_approx._equal_to]350 eV) is detected in the edge plasma with nearly unidirectional flow along B and having a density between 2% and 10% of the cold edge plasma (T[bar_over_tilde:_approx._equal_to]20 eV). The toroidal sense of this fast electron flow is against the force of the applied electric field. A large power flux along B is measured flowing in the same direction as the fast electrons and is apparently carried by the fast electrons. With the limiter present the fast electrons are still detected in the plasma, but are strongly attenuated in the shadow of the limiter. The measured scrape-off lengths for both the fast electrons and the cold plasma indicate cross-field transport at the rate of, or less than, Bohm diffusion. Estimates indicate that the fast electrons could carry the reversed-field pinch current density at the edge and, from the measured transverse diffusion rates, could also account for the electron energy loss channel in ZT-40 M. The long mean-free-path kinetic nature of these fast electrons suggests that a kinetic process, rather than a magnetohydrodynamic process that is based upon a local Ohm’s law formulation, is responsible for their generation. [ABSTRACT FROM AUTHOR]

Steady-state current sustainment by oscillating field current drive (OFCD) utilizes a technique in which the toroidal and poloidal magnetic fields at the plasma surface are modulated at audio frequencies in quadrature. Experiments on the ZT-40M reversed field pinch [Fusion Technol. 8, 1571 (1985)] have examined OFCD over a range of modulation amplitude, frequency, and phase. For all cases examined, the magnitude of the plasma current is dependent on the phase of the modulations as predicted by theory. However, evidence of current drive has only been observed at relatively low levels of injected power. For larger modulation amplitudes, the data suggest that substantial current drive is offset by increased plasma resistance as a result of modulation enhanced plasma-wall interactions. The initial experimental results and supporting theoretical interpretations of OFCD are discussed. [ABSTRACT FROM AUTHOR]

The ZT-40M [Fusion Tech. 8, 1571 (1985)] reversed-field pinch has been used to study magnetic flux perturbations during high-θ [θ=Bθ(a)/>1.6] discharges. Asymmetric toroidal magnetic flux perturbations are found to be associated with magnetic flux emerging through the toroidal shell gap and with m=0 magnetic disturbances moving toroidally. Ramping current discharges, which are a special case of high-θ operation, show the most robust self-generation of toroidal flux. The electron density fluctuations on the inside major radius and associated m=1 and m=0 fluctuations seen on edge magnetic field probes provide a clearer picture of activity during a soft x-ray sawtooth crash. During the sawtooth crash, significant magnetic energy is apparently converted into kinetic energy of the particles. [ABSTRACT FROM AUTHOR]

We are designing a new diagnostic based on laser inverse Compton scattering to study the dynamics of runaway electron formation during killer-pellet triggered disruptions in DIII-D, and their subsequent loss. We can improve the expected S/N ratio by using a high-intensity short-pulse laser combined with gated x-ray imagers. With 80 ps sampling, time-of-flight spatial resolution within the laser chord can be obtained. We will measure the time-resolved spatial profile and energy distribution of the runaway electrons while they are in the core of the tokamak plasma. [ABSTRACT FROM AUTHOR]

A bolometer array diagnostic has been developed for the University of Washington Translation, Confinement, and Sustainment (TCS) field-reversed configuration (FRC) experiment in order to measure radially resolved total radiated power per unit length of the FRC. This will provide radiation energy loss information, useful in power balance and impurity studies. The 20-element photodiode bolometer will be mounted at the midplane of the TCS cylindrical vacuum chamber to view the rotating magnetic field (RMF) generated FRC plasma. Key features of this new bolometer array are (1) extensive electrical shielding against the RMF, (2) robust electrical isolation, (3) trans-impedance amplifiers using a microcoax interface at the array and a fiber optic interface to the screen room, and (4) a custom glass-on-metal socket for the 20-element photodiode chip to ensure high vacuum compatibility. The bolometer array can be retracted behind a gate valve using a stepper motor to protect it during vacuum chamber bakeout. The slit assembly housing is interchangeable to provide flexibility for the viewing sightlines. [ABSTRACT FROM AUTHOR]

The novel hypervelocity dust injection diagnostic will facilitate our understanding of basic aspects of dust-plasma interaction and magnetic field topology in fusion plasma devices, by observing “comet tails” associated with the injected micron-size dust particles. A single projection of the tail onto an image plane will not provide sufficient information; therefore, we plan to use two views, with intensified DiCam-Pro cameras on two NSTX ports. Each camera can furnish up to five overlaying sequential images with gate times greater than 3 ns and 1280×1024 pixel resolution. A coherent fiber bundle with 1500×1200 fibers will relay the image from an imaging lens installed directly on the port to the camera optics. The lens receives light from the outer portion of the NSTX cross section and focuses a 1 cm tail onto at least 60 fibers for adequate resolution. The estimated number of photons received by the camera indicates signal-to-noise ratios of 102–104, with the use of a 10 nm bandwidth filter. The imaging system with one camera was successfully tested on NSTX in 2005. Photographing lithium pellets yielded bright and distinctive pictures of the tails nearly aligned with B lines. We also observed that the bright “filaments”—plasma cords with high density and temperature—are present in both top and bottom portions of the machine. [ABSTRACT FROM AUTHOR]

An internal magnetic field measurement technique using hypervelocity dust beam injection is described for eventual implementation on the National Spherical Torus experiment. The principle of the diagnostic has been described previously [Wang and Wurden, Rev. Sci. Instrum. 74, 1887 (2003)]. Approximately 200 kV of electrostatic potential will be used to accelerate multiple dust particles to velocities in the range of 1.0–10 km/s. A fast framing camera will be used to visualize and map two-dimensional internal magnetic field structure from the orientation of the visible light emissions generated by the plumes associated with the hypervelocity dust. [ABSTRACT FROM AUTHOR]

A new system for near infrared spectroscopic measurements (1.2–2.6 μm) is designed for the Alcator C-Mod tokamak. These measurements will provide a new window into understanding the physics of detached and/or high radiating plasmas. The system, located on top of the Alcator C-Mod concrete igloo, will view a 30×30 cm2 region of the lower divertor by means of a re-entrant 5 m long ZnSe based periscope. A McPherson 218 spectrometer will disperse the light that will then be detected by a thermoelectrically cooled InGaAs, 256-element linear image sensor. The signals from each pixel will be digitized by a 16-bit, 333 kHz analog/digital board and then transmitted over optical link to a personal computer (PC) located in Alcator C-Mod control room. The PC will remotely control the system and store the data in the MDSplus database. [ABSTRACT FROM AUTHOR]

Silicon photodiode detectors, which have nearly flat energy response from 1 eV to 6 keV [R. Korde and L. Randall Canfield, Proc. SPIE 1140, 126 (1989)], were used as bolometers in the field reversed theta pinch experiment LSX. Plasma escaping from the field reversed configuration is naturally diverted to the ends of the vacuum enclosure. There it affects the bolometer measurements either by direct energy deposition or by emission of low energy photons. These two particle effects can be avoided by optimizing the location of the bolometers and restricting their field of view. Good agreement is observed between the silicon bolometers and a gold foil calorimeter. [ABSTRACT FROM AUTHOR]