Soft X-ray Array Research Articles

Note: Measurement of the runaway electrons in the J-TEXT tokamak

The runaway electrons have been measured by hard x-ray detectors and soft x-ray array in the J-TEXT tokamak. The hard x-ray radiations in the energy ranges of 0.5-5 MeV are measured by two NaI detectors. The flux of lost runaway electrons can be obtained routinely. The soft x-ray array diagnostics are used to monitor the runaway beam generated in disruptions since the soft x-ray is dominated by the interaction between runaway electrons and metallic impurities inside the plasma. With the aid of soft x-ray array, runaway electron beam has been detected directly during the formation of runaway current plateau following the disruptions.

Soft x-ray array system with variable filters for the DIII-D tokamak

Recent upgrades to the soft x-ray (SXR) array system on the DIII-D tokamak are described. The system consists of two 32-channel arrays at one toroidal location and three toroidally distributed 12-channel arrays. The 32-channel arrays have been completely rebuilt to allow the switching of SXR filters without breaking vacuum. The 12-channel arrays have had upgrades performed to detectors, view slits, and data acquisition. Absolute extreme ultraviolet (AXUV) photodiodes are used as detectors in all arrays, allowing detection of photons ranging in energy from 2 eV to 10 keV. In the fixed-filter arrays, 127 μm Be filters are used. In the variable-filter arrays, filter wheels are used to switch between five different possible pinhole/filter combinations.

Soft x-ray measurements of resistive wall mode behavior in NSTX**This paper was originally submitted as part of the special issue on MHD stability control in toroidal devices, see Plasma Phys. Control. Fusion 2010 52 100301–104006.

A multi-energy soft x-ray (ME-SXR) array is used for the characterization of resistive wall modes (RWMs) in the National Spherical Torus Experiment (NSTX). Modulations in the time history of the ME-SXR emissivity profiles indicate the existence of edge density and core temperature fluctuations in good agreement with the slow evolution of the n = 1 magnetic perturbation measured by the poloidal and radial RWM coils. The characteristic 20–25 Hz frequency in the SXR diagnostics is approximately that of the n = 1 stable RWM, which is also near the measured peak of the resonant field amplification (RFA) and inversely proportional to the wall time. Together with the magnetics, the ME-SXR measurements suggest that in NSTX the RWM is not restricted exclusively to the reactor wall and edge, and that acting with the stabilizing coils on its global structure may result in density and temperature fluctuations that can be taken into account when designing the feedback process.

A new double-foil soft x-ray array to measure Te 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.

Features of ion and electron fishbone instabilities on HL-2A**The 11th IAEA Technical Meeting on Energetic Particles in Magnetic Confinement Systems, 21–23 September 2009, Kyiv, Ukraine, OT16.

The features of ion and electron fishbone instabilities have been investigated during neutral beam injection (NBI) and electron cyclotron resonance heating (ECRH) on HL-2A. Some new phenomena, such as frequency jumps and V-font-style sweeping, have been presented in the paper. Three kinds of i-fishbones, including hybrid sawtooth–fishbone (sawbone), run-on fishbone and classical fishbone, have been identified during NBI. During high power (PECRH > 0.7 MW) ECRH, the experimental results indicate that the e-fishbone frequencies are higher than those during low power ECRH, and are provided with up- and down-chirping behaviours, and sometimes also with V-font-style sweeping. The periodic mode frequency jumps have also been detected by a soft x-ray array. It is possible to correlate these phenomena with the redistribution of energetic electrons.

Calculation of the Poloidal Magnetic Field Configuration for the J-TEXT Tokamak

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Two approximate analytical methods are used to calculate and analyze the poloidal magnetic field configuration for the J-TEXT tokamak, an iron core tokamak. The methods are based on a spool model and an infinite length model, respectively. By comparing the calculated results with the experimental data, the two methods are modified by the least square method. The results given by the modified method based on the spool model shows good agreements with the experimental data. The modified method based on the spool model replaces the similar function in a general equilibrium code, EFIT. The displacements of the centriod of the plasma current in the J-TEXT tokamak reconstructed by the revised EFIT are coincident with the results measured by the soft-X ray array and magnetic probes. </para>

Diagnostics for first plasma and development plan on KSTAR

The first plasma with target values of the plasma current and the pulse duration was finally achieved on June 13, 2008 in the Korea Superconducting Tokamak Advanced Research (KSTAR). The diagnostic systems played an important role in achieving successful first plasma operation for the KSTAR tokamak. The employed plasma diagnostic systems for the KSTAR first plasma including the magnetic diagnostics, millimeter-wave interferometer, inspection illuminator, H(alpha), visible spectrometer, filterscope, and electron cyclotron emission (ECE) radiometer have provided the main plasma parameters, which are essential for the plasma generation, control, and physics understanding. Improvements to the first diagnostic systems and additional diagnostics including an x-ray imaging crystal spectrometer, reflectometer, ECE radiometer, resistive bolometer, and soft x-ray array are scheduled to be added for the next KSTAR experimental campaign in 2009.

Overview of diagnostic system in the experimental advanced superconducting tokamak

The first plasma was achieved in the experimental advanced superconducting tokamak (EAST) on September 26, 2006. Single-and double-null diverter plasma discharges were also achieved successfully in the EAST in January 2007. The plasma diagnostics for the first EAST plasma study have been developed as follows: a vertical one-channel far-infrared hydrogen cyanide (HCN) laser interferometer for measuring the line average density, a ten-channel soft X-ray array for intensity measurement, a 16-channel heterodyne electric cyclotron emission for measuring the profile of the electron temperature, an eight channel XUV bolometer array to measure plasma radiation losses, a three-channel hard X-ray array for intensity measurement, an electromagnetic measurement system, a 35-channel Hα radiation array, 20 triple probes for diverter plasma, a one-channel visible bremsstrahlung emission, an impurity optical spectrum measurement system, and two optical spectroscopic multichannel analyzers. The first experimental results of the diagnostic system and the plan of what is to come are summarized in this paper.

Diagnostics for first plasma study on EAST tokamak

Abstract The first plasma was obtained in the EAST on September 26th, 2006. Single-null (SN) and double-null (DN) diverted plasmas were achieved successfully in the EAST tokamak on January 22nd, 2007. The employed plasma diagnostics for first plasma study of EAST are as follows: a vertical one-channel far-infrared (FIR) hydrogen cyanide (HCN) laser interferometer for measuring the line average density, a 10-channel soft X-ray array for intensity measurement, a 16-channel heterodyne Electron Cyclotron Emission (ECE) for measuring the electron temperature profile, a 8-channel XUV bolometer array to measure plasma radiation losses, a 3-channel hard X-ray array for intensity measurement, an electromagnetic measurement system, a 35-channel H α radiation array, 20 probes for divertor plasma, a one-channel visible bremsstrahlung emission, an impurity optical spectrum measurement system and two optical spectroscopic multi-channel analyzers (OMA). The first experimental results of diagnostic systems are summarized in this Letter.

Fast electron temperature measurements using a “multicolor” optical soft x-ray array

A fast (⩽0.1ms) and compact “multicolor” scintillator-based optical soft x-ray (OSXR) array has been developed for time- and space-resolved measurements of the electron temperature [Te(R,t)] profiles in magnetically confined fusion plasmas. The 48-channel tangential multicolor OSXR prototype was tested on the National Spherical Torus Experiment. Each sight line views the same plasma volume at the midplane (0⩽r∕a⩽1), in three distinct energy ranges determined by beryllium foils with different thicknesses. A tangential view of the toroidally (circular) symmetric plasma allows an Abel inversion of the line-integrated SXR brightness to obtain the x-ray emissivity profiles which are then used to constrain the reconstruction of the fast Te(R,t). The first assessment of the electron temperature is obtained by modeling the slope of the continuum radiation with the ideal double-foil method using both the line-integrated intensity measurements as well as the inverted SXR emissivity profiles.

High-efficiency fast scintillators for "optical" soft x-ray arrays for laboratory plasma diagnostics

Scintillator-based "optical" soft x-ray (OSXR) arrays have been investigated as a replacement for the conventional silicon (Si)-based diode arrays used for imaging, tomographic reconstruction, magnetohydrodynamics, transport, and turbulence studies in magnetically confined fusion plasma research. An experimental survey among several scintillator candidates was performed, measuring the relative and absolute conversion efficiencies of soft x rays to visible light. Further investigations took into account glass and fiber-optic face-plates (FOPs) as substrates, and a thin aluminum foil (150 nm) to reflect the visible light emitted by the scintillator back to the optical detector. Columnar (crystal growth) thallium-doped cesium iodide (CsI:Tl) deposited on an FOP, was found to be the best candidate for the previously mentioned plasma diagnostics. Its luminescence decay time of the order of approximately 1-10 micros is thus suitable for the 10 micros time resolution required for the development of scintillator-based SXR plasma diagnostics. A prototype eight channel OSXR array using CsI:Tl was designed, built, and compared to an absolute extreme ultraviolet diode counterpart: its operation on the National Spherical Torus Experiment showed a lower level of induced noise relative to the Si-based diode arrays, especially during neutral beam injection heated plasma discharges. The OSXR concept can also be implemented in less harsh environments for basic spectroscopic laboratory plasma diagnostics.

Flow Dynamics and Plasma Heating of Spheromaks in SSX

We report several new experimental results related to flow dynamics and heating from single dipole-trapped spheromaks and spheromak merging studies at SSX. Single spheromaks (stabilized with a pair of external coils, see Brown, Phys. Plasmas 13 102503 (2006)) and merged FRC-like configurations (see Brown, Phys. Plasmas 13, 056503 (2006)) are trapped in our prolate (R = 0.2 m, L = 0.6 m) copper flux conserver. Local spheromak flow is studied with two Mach probes (r 1 ≤ ρ i , r 2 ≥ ρ i ) calibrated by time-of-flight with a fast set of magnetic probes at the edge of the device. Both Mach probes feature six ion collectors housed in a boron nitride sheath. The larger Mach probe will ultimately be used in the MST reversed field pinch. Line averaged flow is measured by ion Doppler spectroscopy (IDS) at the midplane. The SSX IDS instrument measures with 1 μs or better time resolution the width and Doppler shift of the C III impurity (H plasma) 229.7 nm line to determine the temperature and line-averaged flow velocity (see Cothran, RSI 77, 063504 (2006)). We find axial flows up to 100 km/s during formation of the dipole trapped spheromak. Flow returns at the wall to form a large vortex. Recent high-resolution IDS velocity measurements during spheromak merging show bi-directional outflow jets at ±40 km/s (nearly the Alfven speed). We also measure T i ≥ 80 eV and T e ≥ 20 eV during spheromak merging events after all plasma facing surfaces are cleaned with helium glow discharge conditioning. Transient electron heating is inferred from bursts on a four-channel soft x-ray array. The spheromaks are also characterized by a suite of magnetic probe arrays for magnetic structure B(r,t), and interferometry for n e . Finally, we are designing a new oblate, trapezoidal flux conserver for FRC studies. Equilibrium and dynamical simulations suggest that a tilt-stable, oblate FRC can be formed by spheromak merging in the new flux conserver.

A ‘multi-colour’ SXR diagnostic for time and space-resolved measurements of electron temperature, MHD activity and particle transport in MCF plasmas

A fast (⩽0.1 ms) and compact ‘multi-colour’ soft x-ray array has been developed for time and space-resolved electron temperature (Te) measurements in magnetically confined fusion (MCF) plasmas. The electron temperature is obtained by modelling the slope of the continuum radiation from ratios of the available 1D-Abel inverted radial emissivity profiles over different energy ranges, with no a priori assumptions of plasma profiles, magnetic field reconstruction constraints or need of shot-to-shot reproducibility. This technique has been used to perform fast Te measurements in the National Spherical Torus Experiment (NSTX), avoiding the limitations imposed by the well-known multi-point Thompson scattering, electron cyclotron emission and electron Bernstein wave mode conversion diagnostics. The applicability of this ‘multi-colour’ technique for magnetohydrodynamic (MHD) mode recognition and a variety of perturbative electron and impurity transport studies in MCF plasmas is also discussed. Reconstructed ‘multi-colour’ emissivity profiles for a variety of NSTX scenarios are presented here for the first time.

Pellet Enhanced Performance on the HL-2A Tokamak

Enhanced confinement has been achieved by the centre fuellingof pellet injection on the HL-2A tokamak. The energy confinementtime increases from 50 ms to 140 ms after the pellet injection.Experimental results show that the improvement of the confinement isrelated to the decrease of the electron heat transport. Severalphenomena which may lead to the improved confinement have beenobserved in the experiments. After the pellet injection the hollowelectron temperature profile and the peaked electron density profilecan be sustained for about 200 ms, but the improved confinement remains at about500 ms. Sawtooth features and MHD modes have been observed by softx-ray array and the Mirnov probes. The weak (or reversed) magneticshear is thought to be an important cause of the low electron heattransport.

“Optical” soft x-ray arrays for fluctuation diagnostics in magnetic fusion energy experiments

We are developing large pixel count, fast (⩾100 kHz) and continuously sampling soft x-ray (SXR) array for the diagnosis of magnetohydrodynamics (MHD) and turbulent fluctuations in magnetic fusion energy plasmas. The arrays are based on efficient scintillators, high thoughput multiclad fiber optics, and multichannel light amplification and integration. Compared to conventional x-ray diode arrays, such systems can provide vastly increased spatial coverage, and access to difficult locations with small neutron noise and damage. An eight-channel array has been built using columnar CsI:Tl as an SXR converter and a multianode photomultiplier tube as photoamplifier. The overall system efficiency is measured using laboratory SXR sources, while the time response and signal-to-noise performance have been evaluated by recording MHD activity from the spherical tori (ST) Current Drive Experiment-Upgrade and National Spherical Torus Experiment, both at Princeton Plasma Physics Laboratory.

Preliminary design of the soft x-ray array tomographic diagnostic system for Korea Superconducting Tokamak Advanced Research (KSTAR) plasmas

Analyses of x-ray (SXR) emission from tokamak plasmas provide valuable information about magnetohydrodynamics activities and transport phenomena. In this study, design and optimization of the SXR detector arrays were performed for the KSTAR (Korea superconducting tokamak advanced research) device. Geometrical arrangement of the array system was made by coverage mapping and tomographic reconstruction tests. The space-time tomography algorithm was developed for KSTAR-like plasmas using the fast maximum entropy method combined with singular value decomposition. Spatial setup of the components in each detector array was determined and optimized by geometrical calculation. A curved beryllium window and a developed cooling system will be mounted on the detector arrays. Due to the existence of the complex structures between outer vessel and passive stabilizers, each array is designed to be miniaturized. Hardware-optimization of the array system was cross-checked with the tomographic test results.

Current profile reconstruction using electron temperature imaging diagnostics

Flux surface shape information can be used to constrain the current profile for reconstruction of the plasma equilibrium. One method of inferring flux surface shape relies on plasma x-ray emission; however, deviations from the flux surfaces due to impurity and density asymmetries complicate the interpretation. Electron isotherm surfaces should correspond well to the plasma flux surfaces, and equilibrium constraint modeling using this isotherm information constrains the current profile. The KFIT code is used to assess the profile uncertainty and to optimize the number, location and SNR required for the Te detectors. As Te imaging detectors we consider tangentially viewing, vertically spaced, linear gas electron multiplier arrays operated in pulse height analysis (PHA) mode and multifoil soft x-ray arrays. Isoflux coordinate sets provided by Te measurements offer a strong constraint on the equilibrium reconstruction in both a stacked horizontal array configuration and a crossed horizontal and vertical beam system, with q0 determined to within ±4%. The required SNR can be provided with either PHA or multicolor diagnostic techniques, though the multicolor system requires ∼×4 better statistics for comparable final errors.

Resistive wall mode identification by contrast enhancing technique of soft x-ray measurements on DIII-D

A contrast enhancing technique (CET) for soft x-ray (SXR) measurements has been developed and tested for the early identification of the low amplitude resistive wall mode (RWM) on the DIII-D tokamak. The technique is simple and fast. It utilizes the chord-by-chord difference of low-pass digitally filtered time derivatives of the signals from the twelve-chord fan-shape soft x-ray arrays located at toroidal angles of 195° and 45°. The two arrays allow a demonstration of the CET method principle, although they cannot completely resolve the RWM structure. The time derivative of the x-ray signal amplifies the effect of the temperature perturbation convected by the RWM, while naturally incorporating the equilibrium evolution effect. The correlation with the parameters measured by other diagnostics, such as the radial magnetic field δBr from the magnetic probes, the radial profiles of plasma current density j, pressure p, and safety factor q from the motional Stark effect, the radial profile of the temperature perturbation ΔTe from electron cyclotron emission, the normalized beta parameter βN=βT(I/aB)−1, and the Dα light emission, confirm the early detection of the low-amplitude RWMs by the soft x-ray CET. Upon validation the SXR CET could be implemented into a multisensor scheme using other diagnostics for the real-time identification of RWM.

Optimization of the number of soft x-ray arrays and detectors for the SST-1 tokamak by the tomographic method

An x-ray imaging system for tomographic reconstruction of two-dimensional emissivity is being developed for the SST-1 tokamak. In this article it is shown that for circular plasma, which will be generated in the first phase of operation of the machine, 7 arrays and 115 detectors are adequate to reconstruct the simulated data. Sawtooth phenomena depicted by both the Wesson model and the Kadomtsev model and the disruption phenomenon associated with m=2 mode are reconstructed well with the help of this imaging system.

Electron temperature estimation in the Saha Institute of Nuclear Physics tokamak from the soft x-ray imaging system

A soft x-ray imaging diagnostics has been developed for the study of magnetohydrodynamics activities in the Saha Institute of Nuclear Physics tokamak plasma and also to estimate the plasma electron temperature. The main feature of this diagnostics is the use of p–i–n photodiodes, and in this article we present the results of the estimated plasma electron temperature Te(eV) by the absorber foil method. These values agree very closely with the estimated values from Spitzer resistivity. We feel that this technique of using a soft x-ray array for temperature measurement has a good potential for providing very economical Te(r,t) measurements with good time and spatial resolution especially for low budget small tokamaks.