Soft X-ray Camera Research Articles

Soft x-ray tomography on the high field spherical tokamak ST40.

As part of its roadmap to developing commercial fusion plants, Tokamak Energy Ltd. operates the high field spherical tokamak ST40. Studies on this device will help to expand the high field spherical tokamak physics basis by characterizing confinement and the fusion triple product. In support of this, bolometers and broadband and x-ray sensitive diodes can provide information on key energy loss mechanisms of the plasma. These mechanisms include core magnetohydrodynamic activity that deteriorates confinement, such as sawtooth crashes that can be used to characterize relaxations in the q-profile. In addition, combinations of these diagnostics can be used to infer the total radiated power losses and plasma composition. Here, we present results from a new, midplane, tangential, Be-filtered diode with 16 channels spanning the radial extent of the plasma. The system is shown to resolve magnetohydrodynamic instabilities (up to 100kHz) and be able to provide radiation profiles through tomography. The tomographic inversion routine is compared against other diagnostics on ST40 and provides emissivity measurements across a variety of operating scenarios. Finally, we look ahead to implementing multiple soft x-ray cameras on ST40 and the improvements this will have on the diagnostic capabilities.

Flux and estimated spectra from a low-intensity laser-driven X-ray source

Abstract Laser-driven X-rays as probes for high-energy-density physics spans an extremely large parameter space with laser intensities varying by 8 orders of magnitude. We have built and characterized a soft X-ray source driven by a modest intensity laser of 4 × 1013 W/cm2. Emitted X-rays were measured by diamond radiation detectors and a filtered soft X-ray camera. A material-dependence study on Al, Ti, stainless steel alloy 304, Fe, Cu and Sn targets indicated 5-μm-thick Cu foils produced the highest X-ray yield. X-ray emission in the laser direction and emission in the reverse direction depend strongly on the foil material and the thickness due to the opacity and hydrodynamic disassembly time. The time-varying X-ray signals are used to measure the material thinning rate and is found to be ∼1.5 μm/ns for the materials tested implying thermal temperature around 0.6 eV. The X-ray spectra from Cu targets peaks at ∼2 keV with no emission >4 keV and was estimated using images with eight different foil filters. One-dimensional hydrodynamic and spectral calculations using HELIOS-CR provide qualitative agreement with experimental results. Modest intensity lasers can be an excellent source for nanosecond bursts of soft X-rays.

Experiments on excitation of Alfvén eigenmodes by alpha-particles with bump-on-tail distribution in JET DTE2 plasmas

Dedicated experiments were performed in JET DTE2 plasmas for obtaining an α-particle bump-on-tail (BOT) distribution aiming at exciting Alfvén eigenmodes (AEs). Neutral beam injection-only heating with modulated power was used so that fusion-born α-particles were the only ions present in the MeV energy range in these DT plasmas. The beam power modulation on a time scale shorter than the α-particle slowing down time was chosen for modulating the α-particle source and thus sustaining a BOT in the α-particle distribution. High-frequency modes in the toroidicity-induced Alfven eigenmode (TAE) frequency range and multiple short-lived modes in a wider frequency range have been detected in these DT discharges with interferometry, soft x-ray cameras, and reflectometry. The modes observed were localised close to the magnetic axis, and were not seen in the Mirnov coils. Analysis with the TRANSP and Fokker-Planck FIDIT codes confirms that α-particle distributions with BOT in energy were achieved during some time intervals in these discharges though no clear correlation was found between the times of the high-frequency mode excitation and the BOT time intervals. The combined magneto-hydrodynamic (MHD) and kinetic modelling studies show that the high-frequency mode in the TAE frequency range is best fitted with a TAE of toroidal mode number n= 9. This mode is driven mostly by the on-axis beam ions while the smaller drive due to the pressure gradient of α-particles allows overcoming the marginal stability and exciting the mode (Oliver et al 2023 Nucl. Fusion submitted). The observed multiple short-lived modes in a wider frequency range are identified as the on-axis kinetic AEs predicted in Rosenbluth and Rutherford (1975 Phys. Rev. Lett. 34 1428).

Initial Experimental Results of Three-Dimensional Plasma Structure Reconstructed from Seven Independent X-Ray Images on RELAX

In this study, we demonstrated the principle underlying the three-dimensional (3D) reconstruction of plasma structure by using a soft X-ray camera with multiple pinholes. Time evolutions of seven independent soft X-ray images were captured by the camera. The obtained data were used to reconstruct a 3D soft X-ray emission profile in the reversed-field pinch produced in the RELAX machine, which appeared to be consistent with the forecast in our feasibility study.

Application of deep learning to soft x-ray tomography at EAST

Soft x-ray (SXR) cameras in a tokamak are limited spatially by ports of the vacuum vessel, and SXR tomography (SXT) technology is developed for reconstructing a two-dimensional SXR profile. However, traditional SXT is time-consuming and has difficulty achieving abundant and quick reconstructions for a tokamak. Based on experimental SXR data and Fourier–Bessel SXT codes at the EAST tokamak, three typical neural networks are built and trained. All the trained neural networks complete reconstruction within several milliseconds on a personal computer and succeed in constraining the SXR profile to match most of the data. In particular, the best-performing fully convolutional neural network provides SXR reconstruction images on the 2D evolution of a sawtooth, and shows its generalization. In the future, it is possible to provide an outstanding deep learning substitute to give abundant and quick SXT images instead of traditional SXT, after training for a few days.

Abel inversion of soft x-ray fluctuations associated with fast particle-driven fishbone instabilities in MAST plasmas

A set of soft x-ray cameras provided measurements of high frequency instabilities as well as steady-state emission in the Mega Amp Spherical Tokamak (MAST). It is shown that Abel inversion can be readily applied to fluctuating soft x-ray emission from the MAST midplane associated with fast particle-driven ‘fishbone’ instabilities, characterised by toroidal mode number n = 1. Each fishbone burst had an early phase in which high amplitude fluctuating soft x-ray signals from the plasma core were close to being in phase with each other, and there was a region close to the outboard plasma edge in which the fluctuations were relatively weak and in antiphase with those in the core. The major radius of the ‘phase axis’ at which the mode amplitude changed sign R p was initially outboard of the tokamak magnetic axis at R 0, but moved inboard during the burst, eventually becoming close to R 0, at which time the oscillations were of similar amplitude inboard and outboard of R p . The fishbone radial structure early in the burst can be understood in part by recognising that the mode is supported by energetic ions with a high average toroidal rotation rate: in a co-rotating frame, the effective magnetic axis is shifted outboard by a distance that is comparable to the difference between the major radii of the phase axis early in the burst and the laboratory frame magnetic axis. It is conjectured that the transition to a mode with R p ≃ R 0 occurred because most of the energetic ions were expelled from the plasma core region where the mode amplitude peaked, so that the instability could no longer be characterised as an energetic particle mode. Abel inversion of fishbone soft x-ray emission thus provides useful insights into the nature of energetic particle modes in tokamak plasmas and their relationship with MHD modes.

Activation and radwaste analysis for ITER soft X-ray camera

The Soft X-Ray camera (SXR) is designed to measure the polar profile of plasma X-ray emissivity with high spatial and temporal resolution. The camera is located on the Equatorial 12 Port Plug of ITER Tokamak. During the nuclear operation phase, a large number of high-energy neutrons are generated. These neutrons will activate the camera components and make them radioactive. In order to evaluate the activation of SXR's structural and shielding materials, an activation and radwaste analysis has been performed, and the results are summarized in this paper. A simplified model of all necessary parts of SXR is integrated into the ITER C-lite model for neutron transport. The activation of the components is calculated using the FISPACT-2007 code. The safety scenario SA2 for ITER activation calculations is recommended as irradiation history. The contact dose rate provides recommendations for the maintenance of SXR. The security risk of decay heat to SXR is analyzed. Specific activity and clearance index provide the basis for the classification of radwaste. The linear relationship between the neutron flux and activation parameters is obtained by expanding the neutron flux, so that the activation of SXR can be rapidly predicted according to the flux measured in the actual operation of ITER.

Determination of poloidal mode numbers of MHD modes and their radial location using a soft x-ray camera array in the Wendelstein 7-X stellarator

A forward modeling technique is developed for determining the characteristic features of observed MHD modes from the line-of-sight data of the soft x-ray (SXR) tomography diagnostics in the Wendelstein 7-X (W7-X) stellarator. In particular, forward modeling is used to evaluate the poloidal mode numbers m, radial location, poloidal rotation direction and ballooning character of the MHD modes. The poloidal mode structures have been modeled by the radially localized Gaussian-shaped emission regions rotating along the magnetic surfaces. In the present study the cases of rigid-shape emission regions and flexible emission regions are modeled. Various mode phase velocity dependences on the magnetic surface position are simulated. The modeled phase dynamics of line-integrated oscillations and the distribution of oscillation amplitudes are compared with the experimental signals of the SXR cameras which observe the plasma at various viewing angles in the poloidal cross-section. Application of this technique enables describing of the 1–50 kHz modes. In particular, in the discharge W7X-PID 20180918.045 three identified branches with the poloidal mode numbers m= 8, m= 10 and m= 11 localized at ρ ≈ 0.3 are rotating in the clockwise poloidal direction. The present paper reports the first application of the forward modeling technique to the data from the SXR diagnostics in W7-X. The high m-modes are identified by forward modeling in W7-X.

PXIe-based multi-channel data acquisition system of soft X-ray camera

PXIe-based multi-channel data acquisition system of soft X-ray camera

Application of the VUV and the soft x-ray systems on JET for the study of intrinsic impurity behavior in neon seeded hybrid discharges.

This paper reports on impurity behavior in a set of hybrid discharges with Ne seeding-one of the techniques considered to reduce the power load on reactor walls. A series of experiments carried out with light gas injection on JET with the ITER-Like-Wall (ILW) suggests increased tungsten release and impurity accumulation [C. Challis et al., Europhysics Conference Abstracts 41F, 2.153 (2017)]. The presented method relies mainly on the measurements collected by vacuum-ultra-violet and soft X-ray (SXR) diagnostics including the "SOXMOS" spectrometer and the SXR camera system. Both diagnostics have some limitations. Consequently, only a combination of measurements from these systems is able to provide comprehensive information about high-Z [e.g., tungsten (W)] and mid-Z [nickel (Ni), iron (Fe), copper (Cu), and molybdenum (Mo)] impurities for their further quantitative diagnosis. Moreover, thanks to the large number of the SXR lines of sight, determination of a 2D radiation profile was also possible. Additionally, the experimental results were compared with numerical modeling based on integrated simulations with COREDIV. Detailed analysis confirmed that during seeding experiments, higher tungsten release is observed, which was also found in the past. Additionally, it was noticed that besides W, the contribution of molybdenum to SXR radiation was greater, which can be explained by the place of its origin.

Focus on software and data acquisition

Focus on software and data acquisition

Features of the Electron-Temperature Distribution in a Low-Aspect-Ratio Reversed Field Pinch Plasmas

The features of the two-dimensional (2D) line-averaged electron-temperature (Te) distribution in low-aspect-ratio (low-A) reversed field pinch (RFP) plasmas are reported. We successfully obtain the time evolution of line-averaged Te images in a low-A RFP plasma with a soft X-ray (SXR) camera with two pin-holes for two-kinds of absorber foils in cojunction with a high-speed camera. Our obtained line-averaged Te is positively correlated with plasma current (Ip), and data are consistent with core and edge Te obtained by other methods. In multi helicity (MH) discharge, up-down symmetry is not changed as time goes by. On the other hand, tilting of the high temperature region is observed quasi-periodically in quasi-single helicity (QSH) discharge, with a steeper Te gradient appearing during the QSH state.

Multi-energy SXR cameras for magnetically confined fusion plasmas (invited).

A compact multi-energy soft x-ray camera has been developed for time, energy and space-resolved measurements of the soft-x-ray emissivity in magnetically confined fusion plasmas. Multi-energy soft x-ray imaging provides a unique opportunity for measuring, simultaneously, a variety of important plasma properties (Te, nZ, ΔZeff, and ne,fast). The electron temperature can be obtained by modeling the slope of the continuum radiation from ratios of the available brightness and inverted radial emissivity profiles over multiple energy ranges. Impurity density measurements are also possible using the line-emission from medium- to high-Z impurities to separate the background as well as transient levels of metal contributions. This technique should be explored also as a burning plasma diagnostic in-view of its simplicity and robustness.

Diagnostic systems for the nuclear fusion and plasma research in the PF-24 plasma focus laboratory at the IFJ PAN

Abstract This paper presents a set of diagnostics dedicated to PF-24 - new medium size - plasma focus (PF) device built and operated at the Institute of Nuclear Physics Polish Academy of Sciences (IFJ PAN). The PF-24 can operate at energy level up to 93 kJ and charging voltage up to 40 kV. Each condenser is connected with a specially designed spark gap with a very small jitter, which ensures a high effi ciency and a low current rise time. The working parameters of PF-24 generator make it a suitable tool for testing new detection systems to be used in fusion research. Four types of such detection systems are presented in this article: three diagnostic systems used to measure electric quantities (Rogowski coil, magnetic probe, capacitance probe), neutron counter based on beryllium activation, fast neutron pinhole camera based on small-area BCF-12 plastic scintillation detectors and high-speed four-frame soft X-ray camera with microchannel plate.

A comparison between soft x-ray and magnetic phase data on the Madison symmetric torus.

The Soft X-Ray (SXR) tomography system on the Madison Symmetric Torus uses four cameras to determine the emissivity structure of the plasma. This structure should directly correspond to the structure of the magnetic field; however, there is an apparent phase difference between the emissivity reconstructions and magnetic field reconstructions when using a cylindrical approximation. The difference between the phase of the dominant rotating helical mode of the magnetic field and the motion of the brightest line of sight for each SXR camera is dependent on both the camera viewing angle and the plasma conditions. Holding these parameters fixed, this phase difference is shown to be consistent over multiple measurements when only toroidal or poloidal magnetic field components are considered. These differences emerge from physical effects of the toroidal geometry which are not captured in the cylindrical approximation.

Cherenkov emission provides detailed picture of non-thermal electron dynamics in the presence of magnetic islands

Results from a Cherenkov probe recently installed in FTU are presented on non-thermal electron losses. A range of scenarios are investigated to prove the versatility of the diagnostics by correlation with several other diagnostics, including electron cyclotron emission (ECE), neutron and gamma ray detectors, Mirnov coils and soft x-ray cameras. The data analysed provide useful insights into the dynamics of runaway electron (RE) losses in the presence of magnetic islands, demonstrating the distinct and broad potential of this relatively new diagnostic system. The analysis focuses on the sensitivity of the Cherenkov probe to RE losses in connection with magnetohydrodynamic activity and, generally, with magnetic perturbations and reconnection events. In those cases, the Cherenkov probe signals show that the RE expulsion mechanisms are due to the magnetic perturbation of a magnetic island and its amplitude fluctuations. Importantly, the microsecond resolution of the Cherenkov diagnostics reveals an internal structure of the signal peaks, permitting, for the first time with non-magnetic diagnostics, the detection of high frequency signals linked to perturbations of the magnetic island width, known as beta-induced Alfvèn eigenmodes.

Observation of Runaway Electrons with Soft X-Ray Camera on HT-7 Tokamak

Runaway electrons produced during minor disruptions, which are confirmed by the hard x-ray system and the runaway energy spectrum system, are observed by a soft x-ray camera on the HT-7 Tokamak. In this observation, the soft x-ray system can also provide the size information and the position information of the runaway electron current directly from the signal information on the chord. This observation implies that the soft x-ray system can provide the control system with the physical information of the runaway electron current on future devices to avoid electrons hitting the first wall.

Spatial Mode Structure of Magnetohydrodynamic Instabilities Observed by a Tangentially Viewing Soft X-Ray Camera in LHD

Structure of magnetohydrodynamic instabilities are observed by a tangentially viewing soft X-ray camera system in the large helical device. Displacement at the rational surface is estimated to be an even function at the mode rational surface by comparison with synthetic image assuming the displacement of the mode.

Design and initial operation of a two-color soft x-ray camera system on the Compact Toroidal Hybrid experiment.

A multi-camera soft x-ray diagnostic has been developed to measure the equilibrium electron temperature profile and temperature fluctuations due to magnetohydrodynamic activity on the Compact Toroidal Hybrid experiment. The diagnostic consists of three separate cameras each employing two 20-channel diode arrays that view the same plasma region through different beryllium filter thicknesses of 1.8 μm and 3.0 μm allowing electron temperature measurements between 50 eV and 200 eV. The Compact Toroidal Hybrid is a five-field period current-carrying stellarator, in which the presence of plasma current strongly modifies the rotational transform and degree of asymmetry of the equilibrium. Details of the soft x-ray emission, effects of plasma asymmetry, and impurity line radiation on the design and measurement of the two-color diagnostic are discussed. Preliminary estimates of the temperature perturbation due to sawtooth oscillations observed in these hybrid discharges are given.

Development of 2D soft X-ray measurement system in the large helical device

A fast two-dimensional soft X-ray camera using silicon photo diode array is being developed in order to investigate high frequency MHD instability with high mode number. The advantage of the adopted diode is a large sensor area of 10 mm × 10 mm and small diode capacitance which enable us to measure signals with the short response time. The characteristic of the prototype is summarized as follows: Channel number is 6 × 8 = 48, detection range 1∼10 keV, the spatial resolution 128 mm at the plasma location, and frequency range DC∼100 kHz. Synthetic image of the prototype in the Large Helical Device is estimated by using perturbation model of MHD mode.