Prototype high resolution multienergy soft x-ray array for NSTX

A novel diagnostic design seeks to enhance the capability of multienergy soft x-ray (SXR) detection by using an image intensifier to amplify the signals from a larger set of filtered x-ray profiles. The increased number of profiles and simplified detection system provides a compact diagnostic device for measuring T(e) in addition to contributions from density and impurities. A single-energy prototype system has been implemented on NSTX, comprised of a filtered x-ray pinhole camera, which converts the x-rays to visible light using a CsI:Tl phosphor. SXR profiles have been measured in high performance plasmas at frame rates of up to 10 kHz, and comparisons to the toroidally displaced tangential multi-energy SXR have been made.     

A dual-band adaptor for infrared imaging

A novel imaging adaptor providing the capability to extend a standard single-band infrared (IR) camera into a two-color or dual-band device has been developed for application to high-speed IR thermography on the National Spherical Tokamak Experiment (NSTX). Temperature measurement with two-band infrared imaging has the advantage of being mostly independent of surface emissivity, which may vary significantly in the liquid lithium divertor installed on NSTX as compared to that of an all-carbon first wall. In order to take advantage of the high-speed capability of the existing IR camera at NSTX (1.6-6.2 kHz frame rate), a commercial visible-range optical splitter was extensively modified to operate in the medium wavelength and long wavelength IR. This two-band IR adapter utilizes a dichroic beamsplitter, which reflects 4-6 μm wavelengths and transmits 7-10 μm wavelength radiation, each with >95% efficiency and projects each IR channel image side-by-side on the camera's detector. Cutoff filters are used in each IR channel, and ZnSe imaging optics and mirrors optimized for broadband IR use are incorporated into the design. In-situ and ex-situ temperature calibration and preliminary data of the NSTX divertor during plasma discharges are presented, with contrasting results for dual-band vs. single-band IR operation.     

A real-time velocity diagnostic for NSTX

A new system for fast measurements of the plasma toroidal velocity has been installed on the National Spherical Torus Experiment, NSTX [M. Ono et al., Nucl. Fusion 40, 557 (2000)]. The diagnostic, based on active charge-exchange recombination spectroscopy, can measure at up to six radial locations with maximum sampling rate of 5 kHz. The system is interfaced in real time with the NSTX plasma control system, in order to feed back on plasma velocity by means of actuators such as neutral beams and external coils. The paper describes the design criteria and implementation of the diagnostic. Examples from the initial tests of the system during neon glows are also discussed.     

Doppler spectroscopy and D-alpha emission diagnostics for the C-2 FRC plasma

Two Doppler spectroscopy diagnostics with complementary capabilities are developed to measure the ion temperatures and velocities of FRC plasmas in the C-2 device. First, the multichord ion doppler diagnostic can simultaneously measure 15 chords of the plasma using an image intensified camera. Second, a single-chord fast-response ion Doppler diagnostic provides much higher faster time response by using a 16-channel photo-multiplier tube array. To study the neutral density of deuterium under different wall and plasma conditions, a highly sensitive eight-channel D-alpha diagnostic has been developed and calibrated for absolute radiance measurements. These spectroscopic diagnostics capabilities, combined with other plasma diagnostics, are helping to understand and improve the field reversed configuration plasmas in the C-2 device.     

A tangentially viewing fast ion D-alpha diagnostic for NSTX

A second fast ion D-alpha (FIDA) installation is planned at NSTX to complement the present perpendicular viewing FIDA diagnostics. Following the present diagnostic scheme, the new diagnostic will consist of two instruments: a spectroscopic diagnostic that measures fast ion spectra and profiles at 16 radial points with 5-10 ms resolution and a system that uses a band pass filter and photomultiplier to measure changes in FIDA light with 50 kHz sampling rate. The new pair of FIDA instruments will view the heating beams tangentially. The viewing geometry minimizes spectral contamination by beam emission or edge sources of background emission. The improved velocity-space resolution will provide detailed information about neutral-beam current drive and about fast ion acceleration and transport by injected radio frequency waves and plasma instabilities.     

X-ray spectroscopy of Cu impurities on NSTX and comparison with Z-pinch plasmas

X-ray spectroscopy of mid-Z metal impurities is important in the study of tokamak plasmas and may reveal potential problems if their contribution to the radiated power becomes substantial. The analysis of the data from a high-resolution x-ray and extreme ultraviolet grating spectrometer, XEUS, installed on NSTX, was performed focused on a detailed study of x-ray spectra in the range 7-18 A?. These spectra include not only commonly seen iron spectra but also copper spectra not yet employed as an NSTX plasma impurity diagnostic. In particular, the L-shell Cu spectra were modeled and predictions were made for identifying contributions from various Cu ions in different spectral bands. Also, similar spectra, but from much denser Cu plasmas produced on the UNR Z-pinch facility and collected using the convex-crystal spectrometer, were analyzed and compared with NSTX results.     

A new double-foil soft x-ray array to measure T(e) on the MST reversed field pinch

A soft x-ray (SXR) diagnostic to measure electron temperature on the Madison Symmetric Torus using two complementary methods is presented. Both methods are based on the double-foil technique, which calculates electron temperature via the ratio of SXR bremsstrahlung emission from the plasma in two different energy ranges. The tomographic emissivity method applies the double-foil technique to a tomographic reconstruction of SXR emissivity, creating a two-dimensional map of temperature throughout the plasma. In contrast, the direct brightness method applies the double-foil technique directly to the measured brightness and generates vertical and horizontal radial profiles. Extensive modeling demonstrates advantages and limitations in both techniques. For example, although the emissivity technique provides a two-dimensional mapping of temperature, its reliance on multiple tomographic inversions introduces some artifacts into the results. On the other hand, the more direct brightness technique avoids these artifacts but is only able to provide a radial profile of electron temperature.     

Wall reflection issues for optical diagnostics in fusion devices

The problem of light reflection has been raised as a high priority issue for optical diagnostics in next step fusion devices where metallic wall environment will generate significant perturbations in the diagnostics measurements. Tore Supra is a large size tokamak equipped with water-cooled stainless-steel panels used to sustain the plasma long shot radiations. These panels are highly reflective and affect significantly optical systems. In particular, we show that the infrared imaging diagnostic, which surveys the plasma facing component surface temperature for safety purposes, can give incorrect information due to reflected light coming from the bottom limiter. In the visible range, motional Stark effect and Zeff measurements experience important drifts during the plasma heating phases due to parasitic light coming from the limiter, but also from the plasma itself when the viewing lines are facing the reflecting walls. In the next step fusion devices such as ITER, the possibility to use optical measurements needs to be accessed by a modeling of the diagnostic light in its machine environment and the development of new techniques of online correction.     

2D soft x-ray system on DIII-D for imaging the magnetic topology in the pedestal region

A new tangential two-dimensional soft x-ray imaging system (SXRIS) is being designed to examine the edge island structure in the lower X-point region of DIII-D. Plasma shielding and/or amplification of the calculated vacuum islands may play a role in the suppression of edge-localized modes via resonant magnetic perturbations (RMPs). The SXRIS is intended to improve the understanding of three-dimensional (3D) phenomena associated with RMPs. This system utilizes a tangential view with a pinhole imaging system and spectral filtering with beryllium foils. SXR emission is chosen to avoid line radiation and allows suitable signal at the top of a H-mode pedestal where T(e)～1-2?keV. A synthetic diagnostic calculation based on 3D SXR emissivity estimates is used to help assess signal levels and resolution of the design. A signal-to-noise ratio of 10 at 1 cm resolution is expected for the perturbed signals, which are sufficient to resolve most of the predicted vacuum island sizes.     

Fringe-jump corrected far infrared tangential interferometer/polarimeter for a real-time density feedback control system of NSTX plasmas

The far infrared tangential interferometer/polarimeter (FIReTIP) of the National Spherical Torus Experiment (NSTX) has been set up to provide reliable electron density signals for a real-time density feedback control system. This work consists of two main parts: suppression of the fringe jumps that have been prohibiting the plasma density from use in the direct feedback to actuators and the conceptual design of a density feedback control system including the FIReTIP, control hardware, and software that takes advantage of the NSTX plasma control system (PCS). By investigating numerous shot data after July 2009 when the new electronics were installed, fringe jumps in the FIReTIP are well characterized, and consequently the suppressing algorithms are working properly as shown in comparisons with the Thomson scattering diagnostic. This approach is also applicable to signals taken at a 5 kHz sampling rate, which is a fundamental constraint imposed by the digitizers providing inputs to the PCS. The fringe jump correction algorithm, as well as safety and feedback modules, will be included as submodules either in the gas injection system category or a new category of density in the PCS.     

High density Langmuir probe array for NSTX scrape-off layer measurements under lithiated divertor conditions

A high density Langmuir probe array has been developed for measurements of scrape-off layer parameters in NSTX. Relevant scale lengths for heat and particle fluxes are 1-5 cm. Transient edge plasma events can occur on a time scale of several milliseconds, and the duration of a typical plasma discharge is ～1?s. The array consists of 99 individual electrodes arranged in three parallel radial rows to allow both swept and triple-probe operation and is mounted in a carbon tile located in the lower outer divertor of NSTX between two segments of the newly installed liquid lithium divertor. Initial swept probe results tracking the outer strike point through probe flux measurements are presented.     

Experimental setup for tungsten transport studies at the NSTX tokamak

Tungsten particles have been introduced into the National Spherical Torus Experiment (NSTX) in Princeton with the purpose to investigate the effects of tungsten injection on subsequent plasma discharges. An experimental setup for the study of tungsten particle transport is described where the particles are introduced into the tokamak using a modified particle dropper, otherwise used for lithium-powder injection. An initial test employing a grazing-incidence extreme ultraviolet spectrometer demonstrates that the tungsten-transport setup could serve to infer particle transport from the edge to the hot central plasmas of NSTX.     

Spectroscopic diagnostics for liquid lithium divertor studies on National Spherical Torus Experiment

The use of lithium-coated plasma facing components for plasma density control is studied in the National Spherical Torus Experiment (NSTX). A recently installed liquid lithium divertor (LLD) module has a porous molybdenum surface, separated by a stainless steel liner from a heated copper substrate. Lithium is deposited on the LLD from two evaporators. Two new spectroscopic diagnostics are installed to study the plasma surface interactions on the LLD: (1) A 20-element absolute extreme ultraviolet (AXUV) diode array with a 6 nm bandpass filter centered at 121.6 nm (the Lyman-α transition) for spatially resolved divertor recycling rate measurements in the highly reflective LLD environment, and (2) an ultraviolet-visible-near infrared R=0.67?m imaging Czerny-Turner spectrometer for spatially resolved divertor D I, Li I-II, C I-IV, Mo I, D(2), LiD, CD emission and ion temperature on and around the LLD module. The use of photometrically calibrated measurements together with atomic physics factors enables studies of recycling and impurity particle fluxes as functions of LLD temperature, ion flux, and divertor geometry.     

Determination of radial location of rotating magnetic islands by use of poloidal soft x-ray detector arrays in the STOR-M tokamak

A technique is presented for determining the radial location of the rotating magnetic islands in the STOR-M tokamak by use of soft x-ray (SXR) detector arrays. The location is determined by examining the difference in the integrated SXR emission intensities through two adjacent lines of sight. A model for calculating dependence of the line integrated SXR emission intensity on the radius, the mode numbers and the magnetic island geometry, has been developed. The SXR difference signal shows phase inversion when the impact parameter of the line of sight sweeps across the magnetic islands. Experimentally, the difference SXR signals significantly reduce noise and suppress the influence of background plasma fluctuations through common mode rejection when a dominant mode exists in the STOR-M tokamak. The radial locations of the m = 2 magnetic islands have been determined under several experimental conditions in the STOR-M discharges. With the decrease in the tokamak discharge current and thus the increase of the safety factor at the edge, the radial location of the m = 2 magnetic islands has been found to move radially inward.     

Pulse-signal digitizer for high-temperature plasma diagnostic systems

A pulse-signal digitizer for laser, neutron, and gamma diagnostics of high-temperature plasma at the GOL-3 and GDL (Budker Institute of Nuclear Physics Siberian Branch, Russian Academy of Sciences) is described. The architecture of a two-channel 12-bit ADC12500PXIe digitizer with a sampling rate of 500MHz, a PXI Express interface, and a built-in reconfigurable digital signal processing unit is described. The results of ADC12500PXIe metrological tests are presented: its signal-to-noise ratio (SNR), jitter, effective number of bits, and differential and integral nonlinearities are shown. The high-temperature plasma diagnostic systems based on ADC12500PXIe are considered. The structure of the digital signal processing unit that was modified for a prototype of the ITER vertical neutron camera (Cadarache, France) is described.     

Concept of a charged fusion product diagnostic for NSTX

The concept of a new diagnostic for NSTX to determine the time dependent charged fusion product emission profile using an array of semiconductor detectors is presented. The expected time resolution of 1-2 ms should make it possible to study the effect of magnetohydrodynamics and other plasma activities (toroidal Alfve?n eigenmodes (TAE), neoclassical tearing modes (NTM), edge localized modes (ELM), etc.) on the radial transport of neutral beam ions. First simulation results of deuterium-deuterium (DD) fusion proton yields for different detector arrangements and methods for inverting the simulated data to obtain the emission profile are discussed.     

Investigation of the optical characteristics of laser plasma by photochronography methods

The results of photochronographic studies of spatiotemporal and spectral characteristics of laser plasma in the optical region, which were obtained at the Iskra-5 facility in different experiments, are presented. The plasma diagnostics was performed using a slit photochronographic streak camera, which is based on a PIM-112 streak tube with microsecond sweep durations.     

A Complex of Imaging Diagnostic Devices of Vacuum UV Radiation for the GOL-3 Multimirror Trap

A complex of imaging diagnostic devices of vacuum UV radiation based on the GOL-3 multimirror trap is described. The characteristics of the devices included in the complex and examples of their operation are presented. The diagnostic techniques are intended for obtaining information on the basic plasma parameters and monitoring the operating modes of the facility. In combination with spectroscopic systems, these diagnostics allow one to determine the impurity diffusion dynamics in a plasma.     

Diagnostics for ion beam driven high energy density physics experiments

Intense beams of heavy ions are capable of heating volumetric samples of matter to high energy density. Experiments are performed on the resulting warm dense matter (WDM) at the NDCX-I ion beam accelerator. The 0.3 MeV, 30 mA K(+) beam from NDCX-I heats foil targets by combined longitudinal and transverse neutralized drift compression of the ion beam. Both the compressed and uncompressed parts of the NDCX-I beam heat targets. The exotic state of matter (WDM) in these experiments requires specialized diagnostic techniques. We have developed a target chamber and fielded target diagnostics including a fast multichannel optical pyrometer, optical streak camera, laser Doppler-shift interferometer (Velocity Interferometer System for Any Reflector), beam transmission diagnostics, and high-speed gated cameras. We also present plans and opportunities for diagnostic development and a new target chamber for NDCX-II.     

Transmission grating based extreme ultraviolet imaging spectrometer for time and space resolved impurity measurements

A free standing transmission grating based imaging spectrometer in the extreme ultraviolet range has been developed for the National Spherical Torus Experiment (NSTX). The spectrometer operates in a survey mode covering the approximate spectral range from 30 to 700 A? and has a resolving capability of δλ/λ on the order of 3%. Initial results from space resolved impurity measurements from NSTX are described in this paper.