Soft X-ray Emission Profile Research Articles

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.

Development of a soft x-ray (SXR) array diagnostic system on versatile experiment spherical torus (VEST).

A new soft x-ray (SXR) array diagnostic system has been developed on versatile experiment spherical torus (VEST) for measurements of 2D SXR emissivity profile and identification of poloidal mode structure. Through tomographic inversion techniques, 2D SXR emissivity profile can be acquired from the line-integrated SXR data, which enables the visualization of mode structure of plasma instability, such as the magnetohydrodynamics mode. The SXR array diagnostic system consists of two 20-channel arrays positioned at the middle and the top on the same poloidal plane for horizontal and vertical lines of sight, respectively. Each array of the diagnostic system uses absolute extreme ultraviolet photodiode array as the detector. To apply appropriate filters (up to four) for different energy regimes without breaking the vacuum, a filter wheel and its rotatable vacuum feed-through are installed behind the pinhole. SXR data are acquired with a digitizer at the sampling rate of up to 125MHz. Finally, we discuss initial measurement data obtained from Ohmic plasma in VEST.

Incorporating magnetic equilibrium information in Gaussian process tomography for soft X-ray spectroscopy at WEST.

Gaussian process tomography (GPT) [J. Svensson, JET Internal Report EFDA-JET-PR(11)24, 2011 and D. Li, J. Svensson, H. Thomsen, F. Medina, A. Werner, and R. Wolf, Rev. Sci. Instrum. 84, 083506 (2013)] is a recently developed tomography method applied earlier to soft X-ray (SXR) spectroscopy on WEST-Tungsten (W) Environment in Steady-state Tokamak. The short execution time of the algorithm makes GPT an important candidate for providing real-time information on impurity transport and for fast MHD control. In earlier work, GPT has shown its flexibility by providing good reconstruction results without background information about the magnetic equilibrium. On the other hand, information about the magnetic flux surface geometry can in general be useful for additional regularization of the solution. In this paper, we develop a way to take into account the equilibrium information, by constructing a covariance matrix of the prior Gaussian process depending on the flux surface geometry. The GPT method is validated using synthetic SXR emissivity profiles relevant to WEST plasmas and compares favorably with the classical algorithm based on minimization of the Fisher information.

Gaussian process tomography for soft x-ray spectroscopy at WEST without equilibrium information.

Gaussian process tomography (GPT) is a recently developed tomography method based on the Bayesian probability theory [J. Svensson, JET Internal Report EFDA-JET-PR(11)24, 2011 and Li et al., Rev. Sci. Instrum. 84, 083506 (2013)]. By modeling the soft X-ray (SXR) emissivity field in a poloidal cross section as a Gaussian process, the Bayesian SXR tomography can be carried out in a robust and extremely fast way. Owing to the short execution time of the algorithm, GPT is an important candidate for providing real-time reconstructions with a view to impurity transport and fast magnetohydrodynamic control. In addition, the Bayesian formalism allows quantifying uncertainty on the inferred parameters. In this paper, the GPT technique is validated using a synthetic data set expected from the WEST tokamak, and the results are shown of its application to the reconstruction of SXR emissivity profiles measured on Tore Supra. The method is compared with the standard algorithm based on minimization of the Fisher information.

Progress in stellarator research at IPP-Kharkov

Abstract A new antenna of ‘crankshaft’ type has been installed in the Uragan-2M device in order to increase the plasma density and heating below the ion cyclotron frequency. Antenna operation is modelled by 1D code, which solves boundary problem for time-harmonic Maxwell’s equations in radially non-uniform plasma cylinder. In recent experiments with this antenna, the SXR, CV, OV and OII emission measurements indicate that the light impurity radiation barrier is overcame at this device. Plasma with a temperature of ~50 eV exists during a short period of a few milliseconds. Then the radiation collapse comes owing to strong influx of impurities to the plasma column. A new magnetic diagnostics has been installed at Uragan-3M. Using it the poloidal magnetic field is measured and the shift of toroidal current in major radius is registered. A miniature pinhole camera array for spatially and temporally resolved measurements of soft X-ray (SXR) plasma emission has been recently installed on the U-3M. Different shapes of the SXR emission profile has been observed in different discharge conditions.

Comparison of two regularization methods for soft x-ray tomography at Tore Supra

Soft x-ray (SXR) emission in the range 0.1–20 keV is widely used to obtain valuable information on tokamak plasma physics, such as particle transport, magnetic configuration or magnetohydrodynamic activity. In particular, 2D tomography is the usual plasma diagnostic to access the local SXR emissivity. The tomographic inversion is traditionally performed from line-integrated measurements of two or more cameras viewing the plasma in a poloidal cross-section, like at Tore Supra (TS). Unfortunately, due to the limited number of measured projections and presence of noise, the tomographic reconstruction of SXR emissivity is a mathematical ill-posed problem. Thus, obtaining reliable results of the tomographic inversion is a very challenging task. In order to perform the reconstruction, inversion algorithms implemented in present tokamaks use a priori information as additional constraints imposed on the plasma SXR emissivity. Among several potential inversion methods, some of them have been identified as well suited to tokamak plasmas. The purpose of this work is to compare two promising inversion methods, i.e. the minimum fisher information method already used at TS and planned for WEST configuration, and the alternative 2nd order Phillips–Tikhonov regularization with smoothness constraints imposed on the second derivative norm. Respective accuracy of both reconstruction methods as well as overall robustness and computational time are studied, using several synthetic SXR emissivity profiles. Finally, a real case is studied through tomographic reconstruction from TS SXR database.

Non-inductive initiation of closed flux surfaces by ECH/ECCD on KSTAR using an oblique fundamental O-mode injection from the low-field side

An experiment on non-inductive plasma current start-up by electron cyclotron (EC) heating and current drive (ECH/ECCD) has been carried out on KSTAR by injecting the fundamental O-mode wave from the low-field side obliquely to the toroidal magnetic field. A plasma current up to 14.5 kA is generated by 180 kW of 84 GHz microwave power and the magnetic measurement shows the formation of a large last-closed flux surface with a diameter of 0.4 m. The soft x-ray emission profile and fast CCD images also support the existence of closed flux surfaces. The current of the cross-field-passing electrons (CFPEs) is calculated according to the paper Nucl. Fusion 52 083008 in these experimental conditions, and it is shown that a CFPE current can produce the initial closed flux surfaces. The observed large increase of EC emission supports the generation of energetic electrons, like CFPEs. After the formation of the closed flux surfaces, the pressure-driven current and CFPE current do not flow in the closed flux surfaces. EC-driven current should flow in these surfaces and ramp up the plasma current. It is estimated that an EC-driven current of about one third of the total plasma current flows in the closed flux surface at the last stage.

Observation of Helical Structure by Imaging Diagnostics in a Low-Aspect-Ratio Reversed Field Pinch

Study of helical structure has been in progress in shallow reversal reversed field pinch (RFP) plasmas in a low-aspect-ratio machine RELAX ( R / a = 0.5 m/0.25 m, A = 2) by using visible light and soft-X-ray (SXR) imaging diagnostics. Simple helix and its toroidal rotation have been observed in tangential visible light images taken by a high-speed camera. Similar simple filament structures have also been observed in tangential images obtained by a pin-hole SXR camera assembled with Image-intensified charged coupled device (ICCD). The SXR experimental images have been compared with calculated tangential images using model profiles for emissivity to identify the plausible SXR emissivity profile. It has been found that a model profile of helical core having SXR emissivity 2–4 times higher than the background provides 2D SXR images in reasonable agreement with the experimental results. Time evolution of the SXR emissivity profile measured with a photodiode array has shown that the emissivity is the highest in...

Progress and scientific results in the TCV tokamak

The TCV tokamak has the dual mission of supporting ITER and exploring alternative paths to a fusion reactor. Its most unique tools are a 4.5 MW electron cyclotron resonance heating system with seven real-time controllable launchers and a plasma control system with 16 independent shaping coils. Recent upgrades in temperature, density and rotation diagnostics are being followed by new turbulence and suprathermal electron diagnostics, and a new digital real-time network has been commissioned. The shape control flexibility of TCV has enabled the generation and control of the first ‘snowflake’ divertor, characterized by a null point in which both the poloidal field and its gradient vanish. The predicted increases in flux expansion and edge magnetic shear have been verified experimentally, and stable EC-heated snowflake ELMy H-modes have been obtained and characterized. ECCD modulation techniques have been used to study the role of the current profile in energy transport, and simulations reproduce the results robustly. The relation between impurity and electron density gradients in L-mode is explained in terms of neoclassical and turbulent drives. Studies of torqueless plasma rotation have continued, highlighting the important role of MHD and sawtooth relaxations in determining the rotation profiles. A newly predicted mechanism for turbulent momentum transport associated with up–down plasma asymmetry has been verified in TCV. Sawtooth period control, neoclassical tearing mode control and soft x-ray emission profile control have been demonstrated in TCV using the new digital control hardware, as a step on the way to more complex applications.

Soft X-ray Emission Profile and Mode Structure During MHD Events in the TST-2 Spherical Tokamak

Mode structures during MHD events in the TST-2 spherical tokamak are revealed by means of singular value decomposition. Soft X-ray and magnetic coil signals show growths of modes with toroidal mode numbers n = 1 and 2. Slow evolution of the soft X-ray profile indicates beginning of localized crash. Occurrence of these events is correlated with the spatial gradient of the soft X-ray profile, suggesting that these events are driven by the pressure gradient. Increases in impurity ion temperatures (CIII and OV) are observed. These are positively correlated with the increment of the plasma current during these events.

Simultaneous soft x-ray emissivity profile measurements in poloidally separate locations of the Alcator C-Mod edge plasma

Two high resolution edge x-ray imaging diagnostics have been installed in the Alcator C-Mod tokamak. One array measures the radial soft x-ray emissivity profiles at the top of the plasma with 1.2 mm radial resolution, mapped along flux surfaces to the midplane, whereas the other measures the radial soft x-ray emissivity profiles at the outboard edge with 1.5 mm radial resolution mapped to the midplane. The two diagnostics measure the chord brightness profiles, which are then inverted to get soft x-ray emissivity simultaneously with a 12 μs sampling time. This allows us to determine if the soft x-ray emissivity, and therefore the fluorine density, is constant on a flux surface during steady state high confinement mode conditions, as well as during fast transient edge events, such as edge localized modes or transitions from the high confinement mode to the low confinement mode. Measurements are presented showing that the soft x-ray emissivity is not constant on a flux surface, but instead shows a large poloidal variation, contrary to what is assumed in the inversion routine. The effects of the poloidal variation on the inversion accuracy are estimated numerically. It is found that the emissivity is systematically overestimated at the top, and underestimated at the outboard edge, by less than 15%. The width of the x-ray emissivity pedestal is accurate to within 15%, and the location of the pedestal is accurate to within 1 mm. Measurements showing a poloidal propagation delay for the onset of the transition from high confinement mode (H mode) to low confinement mode (L mode) are also presented.

Suppression of a pressure driven m=1 mode in a lower hybrid current drive plasma by electron cyclotron heating in the WT-3 tokamak

A pressure driven m=1/n=1 mode is excited by lower hybrid current drive in the WT-3 tokamak [T. Maehara et al., Nucl. Fusion 38, 39 (1998)]. The excitation of the mode is accompanied with the decrease of the magnetic shear and with the peaking of the soft x-ray emissivity profile inside the q=1 surface. The crescent-shaped mode structure appeared on the contour map of the soft x-ray emissivity is consistent with that of the quasi-interchange mode. The m=1 mode can be suppressed by electron cyclotron heating near the q=1 surface. The range of the location of the electron cyclotron resonance layer effective for the complete suppression is much wider and the time scale for the suppression is much faster than those in the case of the suppression of the tearing mode in the ohmic heating plasma.

Measurements of the high confinement mode pedestal region on Alcator C-Mod

Measurements of the steep transport barrier at the edge of the Alcator C-Mod tokamak [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] are presented. The parameters at the top of this barrier are in the range Te=150–750 eV and ne=0.5−3.3×1020 m−3, depending on the confinement regime. Type III edge localized modes (ELMs) have an upper temperature limit. Te pedestal profiles show a barrier width ΔT≃8 mm. Soft x-ray emissivity profiles are narrower, with Δ=2–4 mm. Edge currents are calculated to alter the ideal stability boundary favorably, leading to ideally stable pedestal profiles. High frequency, broadband, edge density fluctuations are sometimes observed in H-mode (high-confinement mode) and are associated with enhanced particle transport. Coherent magnetic fluctuations localized near the pedestal are also seen.

Maximum entropy based reconstruction of soft X-ray emissivity profiles in W7-AS

The reconstruction of 2-D emissivity profiles from soft X-ray tomography measurements constitutes a highly underdetermined and ill-posed inversion problem, because of the restricted viewing access, the number of chords and the increased noise level in most plasma devices. An unbiased and consistent probabilistic approach within the framework of Bayesian inference is provided by the maximum entropy method, which is independent of model assumptions, but allows any prior knowledge available to be incorporated. The formalism is applied to the reconstruction of emissivity profiles in an NBI heated plasma discharge to determine the dependence of the Shafranov shift on beta , the reduction of which was a particular objective in designing the advanced W7-AS stellarator

Investigation of plasma transport processes using the dynamical response of soft X-ray emission

The dynamical response of a soft X-ray emission profile to different external perturbations - gas feed, impurity injection, RF power, surface loop voltage - has been studied on the TCA tokamak and analysed using a single technique. The frequency dependence of the response has been exploited to distinguish between the dominant transport processes. Remarkably similar phase response profiles were obtained with different stimuli; they show a link with sawtooth activity. The model that most plausibly explains these experimental observations requires diffusive transport with a diffusive coefficient locally modulated by the perturbation

Comparison of experimental results and calculated detector responses for PBFA II thermal source experiments

We performed experiments using proton beams to heat foam-filled cylinders on the Particle Beam Fusion Acceleration II. Preliminary analysis of these diagnostic results provides reasonable agreement between prediction and experiment. The diagnostic package allowed us to benchmark target response as well as diagnose driver performance. Soft x-ray images, both time and space resolved, and the results of broadband spectral measurements on bolometers and x-ray diodes provide qualitative agreement with LASNEX predictions of target features such as Au motion and soft x-ray emission profile. The analysis is consistent with a total beam deposition of 40–50 kJ and power depositions of approximately 180 TW/g. Estimates of target motion confirm the ability of the foam to retard wall motion.

Image reconstruction methods for the PBX-M pinhole camera

We describe two methods that have been used to reconstruct the soft x-ray emission profile of the PBX-M tokamak from the projected images recorded by the PBX-M pinhole camera [Proc. Soc. Photo-Opt. Instrum. Eng. 691, 111 (1986)]. Both methods must accurately represent the shape of the reconstructed profile while also providing a degree of immunity to noise in the data. The first method is a simple least-squares fit to the data. This has the advantage of being fast and small and thus easily implemented on the PDP-11 computer used to control the video digitizer for the pinhole camera. The second method involves the application of a maximum entropy algorithm to an overdetermined system. This has the advantage of allowing the use of a default profile. This profile contains additional knowledge about the plasma shape that can be obtained from equilibrium fits to the external magnetic measurements. Additionally the reconstruction is guaranteed positive, and the fit to the data can be relaxed by specifying both the amount and distribution of noise in the image. The algorithm described has the advantage of being considerably faster for an overdetermined system than the usual Lagrange multiplier approach to finding the maximum entropy solution [J. Opt. Soc. Am. 62, 511 (1972); Rev. Sci. Instrum. 57, 1557 (1986)].

Soft X-ray measurements on the REPUTE-1 reversed field pinch

Soft X-ray measurements have been made with surface barrier diode (SBD) arrays in the REPUTE-1 RFP device (R/a = 82/22 cm). Arrays of seven and eleven SBDs are arranged so that they view a minor cross-section of the plasma from the vertical and horizontal diagnostic ports, respectively. Applying the tomography technique to the soft X-ray data, a two-dimensional emissivity profile can be reconstructed. In the sustainment phase of the RFP discharge, the soft X-ray emissivity profile exhibits a periodic peaking and a subsequent large crash across the minor cross-section. The fluctuation period is in the range of 60–150 μs, whereas the duration of the large crash is always about 30 μs (corresponding to ∼ 100 × τA, where τA is the Alfvén transit time). The soft X-ray emission is due to two modes: one is a dominantly m = 0 mode, which corresponds to the axisymmetric increase or decrease of the soft X-ray emissivity profile, and the other is an m = 1 mode. The toroidal correlation length of the m = 0 mode ranges from 20° to 80°, while the m = 1 mode has a high toroidal mode number. In the crash process, an m = 1 distortion of the emissivity profile, which indicates reconnection of the flux surfaces, can be observed. The magnetic fluctuations observed simultaneously are well correlated with the soft X-ray fluctuations. It is found that the soft X-ray fluctuations reflect the relaxation mechanism of the RFP plasma. This phenomenon is considered to be driven by global MHD activity.

Dynamical plasma response to additional heating

The response of a Tokamak plasma to an additional heating pulse is enigmatic. The paper presents results of the dynamical response on the electron temperature, deduced from the soft X-ray emissivity profile, to additional Alfven wave heating power. Most of the study used quasi-steady state sinusoidal modulation of the delivered r.f. power, but data are also included on the step-function response at the end of the r.f. pulse, and the impulse response to short periodic r.f. pulses. These different approaches all show similar behaviour of the radial profile of the response phase, which is dominated by a central flat region within the q=1 surface, with, at the higher frequencies used, a minimum phase of close to 180 degrees . Moving further outside the q=1 radius the phase increases extremely sharply. This basic character of the phase profile is not altered by varying the calculated power deposition profile but is still defined by the q=1 radius, demonstrated by a plasma current scan at constant toroidal field. In addition to the soft X-ray flux baseline modulation, the sawtooth period is modulated, but out of phase with r.f. power. These effects have been studied as a function of modulation frequency, plasma current, r.f. power and Alfven wave mode.

Measurement of central nickel density in Doublet III plasmas with a soft x-ray diode array

An array of soft x-ray diodes has been used to obtain central nickel densities for discharges in the Doublet III tokamak, during operation with an inconel primary limiter, in which nickel L-line radiation dominated the diode signals. The nature of the diode signals is determined primarily by comparison with soft x-ray spectra. The contribution of the continuum portion of the spectra to the central diode signal can be calculated and compared to the observed signal. When the diode signal is dominated by nickel L-line emission, the observed signal is considerably larger than the calculated continuum contribution. Chordal data from the array of diodes are inverted to provide the spatial profile of soft x-ray emission. Because the diodes are absolute detectors of radiation, the soft x-ray emission profile is used to obtain the absolute nickel concentration and density profile in the center of the plasma. A computer code, including over 100 nickel L-line transitions, has been developed to obtain the nickel density. The nickel L-line cooling rate, calculated with the code, is presented. The nickel density obtained by this technique agrees well with that obtained from the Kα line intensity measured with a soft x-ray spectrometer and that obtained from a bolometric measurement of central radiated power coupled with a coronal equilibrium model of the radiation.