Sino-UNIted Spherical Tokamak Research Articles

Compressive sensing-based tomography for Absolute X-ray UltraViolet (AXUV) diagnostics.

Absolute x-ray ultraviolet diagnostics ensures 2D coverage of the radiation emission region that enables tomographic reconstruction. However, retrieving the local emissivity via tomography remains a challenge due to its ill-posed nature. Tikhonov regularization with smoothness operation generally performs well but tends to over-smooth regions with steep gradients and local structure in the radiation profile and may introduce artifacts. In this paper, a tomography method based on compressive sensing theory with Tikhonov regularization terms is developed. Experimental results on multiple phantom sets show that the proposed method improves the reconstruction accuracy and quality in regions with steep gradients compared with the Tikhonov regularization method and suppresses the unphysical negative emissivity. The analysis of reconstruction uncertainty shows that the dictionary learning process provides more accurate prior information about steep gradients to improve the quality of reconstructed images, and compressive sensing has the denoising capability to reduce the impact of noise. Finally, the method is validated by data from the Sino-UNIted Spherical Tokamak, showing fewer artifacts and more reliable reconstruction images than the earlier method.

Quasi-coherent mode in core plasma of SUNIST spherical tokamak

A quasi-coherent (QC) mode was observed in the core region of low-density ohmic plasmas in Sino-UNIted Spherical Tokamak. In experiments on the QC mode, two sets of moveable Langmuir probes (LPs) were used to measure the local parameters including floating potential, electron temperature, electron density, and so on, as well as their profiles. To monitor the magnetohydrodynamic activities, a Mirnov probe was used to measure the poloidal magnetic fluctuation. The QC mode can be seen in the spectra of floating potential, but there is no similar peak in the spectra of magnetic fluctuation. Thus, the QC mode is probably electrostatic. By analyzing the electrostatic potential fluctuations from the LPs, the features of the QC mode including frequency, wavenumber, propagation direction, and dependence on collisionality are identified, which are consistent with the characteristics of dissipative trapped electron mode.

Development of a thin high-frequency and high-precision magnetic probe array in Sino-United Spherical Tokamak

Since the major/minor radius of the Sino-United Spherical Tokamak (SUNIST) is 0.3/0.23m, respectively, the space left for magnetic diagnostics in the high field side (HFS) is quite limited. At the same time, a good signal-to-noise ratio and a high-frequency response (up to 1MHz) are required for equilibrium reconstruction (ER) and Alfven eigenmode studies. Such a magnetic probe array must be extremely thin and tightly close to the central column, not exceeding the inner limiter and leaving the aspect ratio of the spherical tokamak unchanged. Therefore, the turn number and the shape of windings should be highly optimized to enable both a high-frequency response and an enough effective area. A 32-channel magnetic probe array fulfilling these requirements on the central column is designed, calibrated, and installed in the SUNIST. The array consists of 16 probes. Each of them consists of two perpendicular windings, which can measure toroidal and poloidal magnetic fields simultaneously. The effective area and frequency response of each probe are calibrated using a Helmholtz coil and an LCR bridge based on an equivalent probe-and-cable circuit model. After that, an expression of the magnetic diagnostic response to the field coil currents is used to calibrate the installation error. With the full coverage of magnetic probes in the poloidal direction, more reliable ER can be obtained, and the features of magnetohydrodynamic activities in the HFS can be studied.

Implementation and data processing of a five-channel microwave interferometer with high temporal resolution and low noise on Sino-UNIted Spherical Tokamak.

A five-channel microwave interferometer with high temporal resolution and high phase resolution has been developed for measuring electron density profiles and fluctuations on the Sino-UNIted Spherical Tokamak. A correction algorithm, based on the low signal amplitude detection, is proposed to eliminate the fringe jump errors. The correction algorithm achieves an accuracy of 92%. The density profile is reconstructed through the Park-matrix method, with the geometry of magnetic surfaces calculated by the equilibrium fitting. The reconstructed density profiles for discharges with supersonic molecular beam injection are in good agreement with the results of another 94GHz single-channel horizontal interferometer and the Langmuir probes. The temporal resolution of the system is 0.5 µs and the line-integrated electron density resolution is 1 × 1015m-2, which benefits from the single sideband modulation technique. Therefore, the multichannel interferometer system is capable of studying the details of the high-frequency (up to 200 kHz) density fluctuation such as that in the minor disruption event.

Modular bolometric/soft x-ray diagnostic in Sino-UNIted Spherical Tokamak.

A bolometer/soft x-ray diagnostic based on modular absolute extreme ultraviolet photodiodes has been built to obtain the local emission profile of a poloidal cross section in Sino-UNIted Spherical Tokamak (SUNIST). The system consists of seven photodiode arrays of 112 channels located around the low field side of a poloidal cross section. All arrays are mounted into 81 × 36 × 33mm3 modules, which could be installed flexibly on the wall of the vacuum vessel. Tungsten shields with adjustable pinholes restrict the radiation flux. Compact flexible printed circuits with multilayer copper polygon poured on both sides are designed to transmit signals with electronic noise, which is reduced to less than 1/10 before. A two-stage transimpedance amplifier array of 700 kHz bandwidth is fixed on the atmosphere side of a vacuum electrical feedthrough and is powered by battery packs. A synchronized switch shuts down the circuit to reduce the heat production and increase the battery life. A line-integrated radiation emission profile with 2cm spatial resolution and temporal resolution up to 2 µs is obtained, and the local emission profile of SUNIST is reconstructed with the help of a newly developed tomography scheme for movable detector modules.

A low noise power supply based on buck converter for current regulation in an inductive load.

High current regulations are widely required in tokamaks to generate a specific magnetic field for plasma confinement which is challenging for power electronics. For high current regulation, the stray inductance will cause high noise and surge voltage that may damage the power electronics. In addition, it is difficult to ensure both a fast response and a steady evolution. In this paper, a power supply based on a buck converter for high current regulation in an inductive load is described, in which insulated gate bipolar transistors are used as switches and two capacitor banks are used for energy storage. The first stage capacitor bank at a higher voltage ensures the fast current growth rate at the beginning of discharge and plays the role of wave filter later to reduce the noise. The second stage capacitor bank at a lower voltage provides the main energy required during discharge. A microcontroller is used to regulate the current by a feedback mechanism. This power supply can regulate the high current in an inductive load with low noise and fast response, which has been applied to the poloidal field coils and the ultrafast reciprocating probe system in the SUNIST (Sino-UNIted Spherical Tokamak) spherical tokamak.

Radiation diagnostics for plasma current ramp-up and ramp-down research.

The plasma current ramp-up and ramp-down are the basic processes in the tokamak operation. In order to research these processes in SUNIST (Sino-UNIted Spherical Tokamak), some diagnostic systems that detect the plasma radiation ranging from hard X-rays to visible light are developed. CdZnTe and silicon drift detectors measure the energy spectrum of hard X-rays and soft X-rays coming from the plasma. A pinhole camera equipped with absolute extended ultraviolet array photodiodes has been installed on the top of SUNIST to observe the radiation power loss and the magneto-hydrodynamic activities with high temporal and spatial resolution. The spectrum of vacuum ultraviolet is acquired by using a CCD camera, and the intensity of the lines can be measured by using a photomultiplier tube with a scintillator. The full spectrum of the visible light can be acquired in every 3 ms, and the intensity of some lines, such as Hα, Hγ, can be measured by filter scopes with high time response. Additionally, a Doppler broadening measurement system is developed to measure the ion temperature of edge plasma.

Development of a triple probe array for mode conversion study of electron cyclotron wave.

A triple probe array has been designed with a modified triple probe technique and utilized on the Sino-UNIted Spherical Tokamak for mode conversion research from the electron cyclotron wave to the electron Bernstein wave. The probe array is designed to measure the radial profile of the electron density in the edge region. The probe array consists of 36 single molybdenum tips, constituting 12 sets of triple probes with an interval of 6 mm in the radial direction. The three tips in each triple probe are displaced slightly in the radial direction, and thus they are not at the same magnetic surface to avoid shadow effects in the toroidal direction. A modified triple probe technique is used. The radial profile of the electron density of ohmic heated plasmas has been measured by the triple probe array. The temporal evolution of the density profile is presented, and the mode conversion processes are discussed.

Experimental study of the tearing mode instability during the current ramp up stage in Sino-United Spherical Tokamak

The fierce magnetohydrodynamic (MHD) activity observed during the current ramp up stage of a tokamak discharge, which significantly limits the performance of the plasma, has been experimentally studied in Sino-United Spherical Tokamak (SUNIST). The mode structure extracted by magnetic measurements appears to be typically decreasing from to , where m and n are the poloidal and toroidal mode numbers. A phase reversal layer in the spatial distribution of the magnetic perturbation extracts the approximate position of the magnetic island chain, which gives insight on the evolution of the internal structure of the MHD activity. A matrix equation method that includes corrections for the shadowing effect of the probe array has been developed so as to derive a more accurate estimation of the current density profile. The resulting current profile evolution qualitatively explains the decrease in poloidal mode number and the spontaneous suppression of the MHD activity. Statistical results of the MHD response suggest that the deterioration of the plasma confinement is closely related to the phase inversion position of the poloidal magnetic perturbation.

Design and implementation of an interferometer with high stability and wide dynamic range for steady-state plasmas

Microwave interferometry is an effective and reliable way to measure line integrated plasma electron density. An extremely low noise heterodyne interferometer has been developed for routine operation in Sino-UNIted Spherical Tokamak (SUNIST) and acts as the prototype design of a microwave inteferometer for Experimental Advanced Superconducting Tokamak (EAST) in the near future. The system has been designed with optimum frequency deduced from detailed calculation in real geometry and discharge parameters. In contrary to traditional heterodyne interferometers, the application of a single sideband modulator (SSBM) eliminates the necessity of the second millimeter wave (MMW) oscillator, which averts the intermediate frequency (IF) stability problem aroused by the two high frequency oscillators in the traditional heterodyne configuration. A pair of specially designed spot focusing antenna is also applied to boost the signal to noise ratio (SNR), whose near field scan data displays excellent focusability. The bench test data and plasma electron density measurement results from SUNIST discharges consistently show excellent performance of the interferometer, which is expected to be suitable for steady-state plasma experiments due to the high stability.

Note: Development of a multichannel magnetic probe array for magnetohydrodynamic activity studies in Sino-United Spherical Tokamak.

A 30-channel movable magnetic probe radial array measuring the poloidal magnetic field's time derivative Ḃθ has been developed and installed on the Sino-United Spherical Tokamak to investigate the magnetohydrodynamic (MHD) activities in ohmic discharges. The probe array consists of thirty identical commercial chip inductors mounted on a slim printed circuit board and shielded by a customized quartz tube of 14 mm in outer diameter. With the application of instrumentation amplifiers, the system exhibits a good signal to noise ratio and the measured vertical field spatial distribution agrees well with the simulation result. The measured spatial and temporal distribution of Ḃθ during the MHD activities exhibits a clear phase reversal layer, which is a direct proof of tearing mode and provides a reliable indication of the magnetic island chain position.

Experimental measurements of energy transfer and nonlinear interaction in turbulence at the sino-united spherical tokamak

The characteristics of the energy transfer and nonlinear coupling among edge electromagnetic turbulence have been dedicatedly studied in various discharge stages at the sino-united spherical tokamak using multiple Langmuir and magnetic probe arrays. The wavelet bispectral analysis and the modified Kim's method are applied to investigate turbulence properties and their linear growth/damping and nonlinear energy transfer rates, along with multi-field turbulence interactions. The results show diverse features in the linear growth and nonlinear energy transfer between multi-field fluctuations during the current ramp-up, stationary, and internal connection event discharge phases. The diversity implies the importance to develop more sophisticated multi-field models to directly estimate the energy transfer rate among multiple turbulent fields.

Nonlinear Energy Cascading in Turbulence during the Internal Reconnection Event at the Sino-United Spherical Tokamak**Supported by the National Basic Research Program of China under Grant No 2012CB934303, the Natural Science Foundation of Shanghai under Grant No 15ZR1403300 and the National Natural Science Foundation of China under Grant No 11275051.

The characteristics of the energy transfer and nonlinear coupling among edge electromagnetic turbulence in thermal quench sub-period of the internal reconnection event (IRE) are studied at the sino-united spherical tokamak device using multiple Langmuir and magnetic probe arrays. The wavelet bispectral analysis and the modified Kim method are applied to investigate linear growth/damping and nonlinear energy transfer rates, along with multi-field turbulence interactions. The results show a multi-field nonlinear energy transfer from electrostatic to magnetic turbulence that results in two-mode coupling in magnetic turbulence, which may play a crucial role to trigger the IRE.

An ultrafast reciprocating probe.

For tokamak plasma diagnostics, an ultrafast reciprocating probe system driven by magnetic field coils, achieving a maximum velocity of 21 m/s, is introduced. The probes are attached with a driving hoop made of carbon steel and accelerated by three acceleration coils in series, then decelerated by two deceleration coils and buffer springs and return slowly. The coils with a current of about 1 kA generate a magnetic field of about 1 T. This probe system has been tested on the SUNIST (Sino-UNIted Spherical Tokamak) spherical tokamak. Radial profiles of the floating potential and other plasma parameters measured by this probe system are given.

A high speed compact microwave interferometer for density fluctuation measurements in Sino-UNIted Spherical Tokamak.

A single-channel 3 mm interferometer has been developed for plasma density diagnostics in the Sino-UNIted Spherical Tokamak (SUNIST). The extremely compact microwave interferometer utilizes one corrugated feed horn antenna for both emitting and receiving the microwave. The beam path lies on the equatorial plane so the system would not suffer from beam path deflection problems due to the symmetry of the cross section. A focusing lens group and an oblique vacuum window are carefully designed to boost the signal to noise ratio, which allows this system to show good performance even with the small-diameter central column itself as a reflector, without a concave mirror. The whole system discards the reference leg for maximum compactness, which is particularly suitable for the small-sized tokamak. An auto-correcting algorithm is developed to calculate the phase evolution, and the result displays good phase stability of the whole system. The intermediate frequency is adjustable and can reach its full potential of 2 MHz for best temporal resolution. Multiple measurements during ohmic discharges proved the interferometer's capability to track typical density fluctuations in SUNIST, which enables this system to be utilized in the study of MHD activities.

Assessment of the beam path deflection for a vertically installed microwave interferometer in SUNIST.

Microwave interferometry has been widely employed to provide reliable line averaged electron density measurement on plasma devices. For a vertically installed interferometer on a tokamak, the refraction problem, which distorts the beam path and aggravates power loss at the receiving antenna, may become significant if taking the cross section shape into account. Increasing the frequency of the probing microwave can alleviate the distortion, but at the expense of losing the density resolution. To seek for an optimized frequency, previous calculations are mainly based on the cylindrical column geometry which grossly underestimates the deflection of the beam path induced by the plasma shape, and empirical suggestions indicating ne0/nc = 1/2 ∼ 1/3 may not always be the appropriate option. Here a single ray tracing method is applied to estimate the final horizontal deviation at the receiving antenna, which is supposed to represent the level of power loss. The calculation is carried out under the real tokamak geometry in Sino-UNIted Spherical Tokamak (SUNIST) with the cross section parameters obtained from the equilibrium reconstruction, and the result indicates that for a target density of 1.2 × 10(19) m(-3), a frequency of at least 100 GHz is desirable to reduce the power loss to an acceptable level. This would be helpful for the design of a vertically installed interferometer on SUNIST.

Measurement of eddy-current distribution in the vacuum vessel of the Sino-UNIted Spherical Tokamak.

Eddy currents have an important effect on tokamak plasma equilibrium and control of magneto hydrodynamic activity. The vacuum vessel of the Sino-UNIted Spherical Tokamak is separated into two hemispherical sections by a toroidal insulating barrier. Consequently, the characteristics of eddy currents are more complex than those found in a standard tokamak. Thus, it is necessary to measure and analyze the eddy-current distribution. In this study, we propose an experimental method for measuring the eddy-current distribution in a vacuum vessel. By placing a flexible printed circuit board with magnetic probes onto the external surface of the vacuum vessel to measure the magnetic field parallel to the surface and then subtracting the magnetic field generated by the vertical-field coils, the magnetic field due to the eddy current can be obtained, and its distribution can be determined. We successfully applied this method to the Sino-UNIted Spherical Tokamak, and thus, we obtained the eddy-current distribution despite the presence of the magnetic field generated by the external coils.

Compact, battery powered, wireless digitizers for in situ data acquisitions in the sino-united spherical tokamak.

Potential isolation and long cable drive are very important in acquiring certain signals from tokamak diagnostics. Compact, battery powered, wireless digitizers for in situ data acquisition have been developed and routinely used in the sino-united spherical tokamak to solve the problems of isolation and long cables. The wireless digitizers utilize the integrated analog to digital converters and the static random access memory of microcontrollers but transfer data wirelessly. They consist of simple and concise circuits but have considerable performances of 12-16 bit in resolution and 500-1000 kS/s in sample rate. Wireless triggering and energy saving are two major challenges of the wireless digitizers. Wireless transceivers in the data link layer are used as trigger and can reduce the trigger jitters to be smaller than 1 μs. In order to reduce the energy consumption, the wireless digitizers are waken only when the tokamak is about to discharge. After discharges, they turn to a periodic checking mode with current consumption smaller than 200 μA. Because of low duty cycle, the wireless digitizers have a battery life of up to four weeks. In general, the wireless digitizers have better performance than normal isolation amplifiers and can greatly simplify the cable connections. They are very suitable for the data acquisition of dangerous and/or susceptible analog signals in tokamaks.

An ohmic field power supply based on a modified IGBT H-bridge for Sino-UNIted Spherical Tokamak

Air core solenoids are widely used in spherical tokamaks (STs), since their central stacks are quite compact and difficult to insert iron cores. However, much higher current are required to generate the same amount of flux in the solenoids with air core than in the ones with iron cores. Therefore, the power supplies of ohmic fields in STs are more challenging than those in conventional tokamaks with similar size. In this paper, a power supply based on insulated gate bipolar transistors (IGBTs) for the ohmic field coils in the Sino-UNIted Spherical Tokamak (SUNIST) is described. This power supply utilizes a modified H-bridge circuit, in which the high current switches are made by 10 IGBTs (rating 3.3kV/1.2kA) connected in parallel. A high voltage capacitor bank (1.2kV/60mF) and a low voltage capacitor bank (600V/500mF) store the energy required in different discharging stages. An adjustable low inductance high power resistor (0–0.3Ω) made by stainless steel foils is used to help the circuit to generate a high current derivative in the ohmic field coils, which are required for gas breakdown, plasma current initiation and ramp up. The small loop voltage required for maintaining the plasma current is realized by the low voltage capacitor bank. For each tokamak discharge, all the switches of this power supply only need to be switched on and off one time with an interval of several milliseconds. Such slow operations of the IGBTs lower the risk of failure and minimize the technical difficulties of dynamical current sharing. The power supply is compact and reliable, and has discharged for more than 5000 times successfully.

Effect of asymmetrical eddy currents on magnetic diagnosis signals for equilibrium reconstruction in the Sino-UNIted Spherical Tokamak.

The vacuum vessel of Sino-UNIted Spherical Tokamak was split into two insulated hemispheres, both of which were insulated from the central cylinder. The eddy currents flowing in the vacuum vessel would become asymmetrical due to discontinuity. A 3D finite elements model was applied in order to study the eddy currents. The modeling results indicated that when the Poloidal Field (PF) was applied, the induced eddy currents would flow in the toroidal direction in the center of the hemispheres and would be forced to turn to the poloidal and radial directions due to the insulated slit. Since the eddy currents converged on the top and bottom of the vessel, the current densities there tended to be much higher than those in the equatorial plane were. Moreover, the eddy currents on the top and bottom of vacuum vessel had the same direction when the current flowed in the PF coils. These features resulted in the leading phases of signals on the top and bottom flux loops when compared with the PF waveforms.