ISTTOK Tokamak Research Articles

Plasma density fluctuations measurements by heavy ion beam probe with a multiple cell array detector on the tokamak ISTTOK

It is recognized that the broad-band fluctuations observed in plasmas of thermonuclear devices are the cause of anomalously large energy and particle transport. Strong progress has been achieved on different plasma devices using a heavy ion beam probe (HIBP) diagnostic measuring the plasma potential and density fluctuations. Better understanding of turbulence properties can be obtained with a multiple cell array detector (MCAD) as plasma density fluctuations are measured simultaneously at a number of sample volumes. In this paper, the capabilities of the HIBP with MCAD for plasma density fluctuations measurements on the tokamak ISTTOK are analyzed (including restrictions due to sample volume size and path effects) and compared with preliminary experimental data. The upgrade of ISTTOK HIBP for better diagnostic performance is also discussed.

Liquid gallium jet–plasma interaction studies in ISTTOK tokamak

Liquid metals have been pointed out as a suitable solution to solve problems related to the use of solid walls submitted to high power loads allowing, simultaneously, an efficient heat exhaustion process from fusion devices. The most promising candidate materials are lithium and gallium. However, lithium has a short liquid state temperature range when compared with gallium. To explore further this property, ISTTOK tokamak is being used to test the interaction of a free flying liquid gallium jet with the plasma. ISTTOK has been successfully operated with this jet without noticeable discharge degradation and no severe effect on the main plasma parameters or a significant plasma contamination by liquid metal. Additionally the response of an infrared sensor, intended to measure the jet surface temperature increase during its interaction with the plasma, has been studied. The jet power extraction capability is extrapolated from the heat flux profiles measured in ISTTOK plasmas.

Experimental evidence of coupling between local turbulent transport and large scale fluctuations in the ISTTOK edge plasma

The interplay between large scale fluctuations and the local turbulent transport has been investigated in the tokamak ISTTOK [C. A. F. Varandas et al., Fusion Technol. 29, 105 (1996)] plasma boundary region. It has been found that the floating potential fluctuations, dominated by low frequency oscillations, exhibit a significant toroidal correlation at large distances that can be attributed to the geodesic acoustic mode. The level of long-distance correlations is strongly bursty, showing a significant degree of coupling with the local electrostatic turbulent transport. Experimental findings show the key role of multiscale physics in the regulation of transport in the edge region of fusion plasmas.

Observation of plasma density fluctuations by a heavy ion beam probe diagnostic with multiple cell array detector on the tokamak ISTTOK

The heavy ion beam probe (HIBP) diagnostic permits the measurements of electron density and plasma potential as well as their fluctuations. The use of a multiple cell array detector (MCAD) spreads the HIBP capabilities to make simultaneous measurements across the plasma column. Two sets of current-to-voltage converters with respective bandwidths of 400 kHz and 4 MHz have been tested. This upgrade of the MCAD electronics has allowed the observation of quasicoherent fluctuations in bulk plasma at frequencies between 100 and 200 kHz. The localization and frequency of bulk plasma fluctuations correlate with the magnetohydranamic activity of the plasma.

Use of Cherenkov-type detectors for measurements of runaway electrons in the ISTTOK tokamak

Gas, fluid, or solid Cherenkov-type detectors have been widely used in high-energy physics for determination of parameters of charged particles, which are moving with relativistic velocities. This paper presents experimental results on the detection of runaway electrons using Cherenkov-type detectors in the ISTTOK tokamak discharges. Such detectors have been specially designed for measurements of energetic electrons in tokamak plasma. The technique based on the use of the Cherenkov-type detectors has enabled the detection of energetic electrons (energies higher than 80 keV) and determination of their spatial and temporal parameters in the ISTTOK discharges. Obtained experimental data were found in adequate agreement to the results of numerical modeling of the runaway electron generation in ISTTOK.

Real-time digital heterodyne interferometer for high resolution plasma density measurements at ISTTOK

With the implementation of alternating discharges (ac) at the ISTTOK tokamak, the typical duration of the discharges increased from 35 to 250 ms. This time increase created the need for a real-time electron density measurement in order to control the plasma fueling. The diagnostic chosen for the real-time calculation was the microwave interferometer. The ISTTOK microwave interferometer is a heterodyne system with quadrature detection and a probing frequency of 100 GHz (lambda(0)=3 mm). In this paper, a low-cost approach for real-time diagnostic using a digital signal programmable intelligent computer embedded system is presented, which allows the measurement of the phase with a 1% fringe accuracy in less than 6 micros. The system increases its accuracy by digitally correcting the offsets of the input signals and making use of a judicious lookup table optimized to improve the nonlinear behavior of the transfer curve. The electron density is determined at a rate of 82 kHz (limited by the analog to digital converter), and the data are transmitted for each millisecond although this last parameter could be much lower (around 12 micros--each value calculated is transmitted). In the future, this same system is expected to control plasma actuators, such as the piezoelectric valve of the hydrogen injection system responsible for the plasma fueling.

Real-time plasma control based on the ISTTOK tomography diagnostic

The presently available processing power in generic processing units (GPUs) combined with state-of-the-art programmable logic devices benefits the implementation of complex, real-time driven, data processing algorithms for plasma diagnostics. A tomographic reconstruction diagnostic has been developed for the ISTTOK tokamak, based on three linear pinhole cameras each with ten lines of sight. The plasma emissivity in a poloidal cross section is computed locally on a submillisecond time scale, using a Fourier-Bessel algorithm, allowing the use of the output signals for active plasma position control. The data acquisition and reconstruction (DAR) system is based on ATCA technology and consists of one acquisition board with integrated field programmable gate array (FPGA) capabilities and a dual-core Pentium module running real-time application interface (RTAI) Linux. In this paper, the DAR real-time firmware/software implementation is presented, based on (i) front-end digital processing in the FPGA; (ii) a device driver specially developed for the board which enables streaming data acquisition to the host GPU; and (iii) a fast reconstruction algorithm running in Linux RTAI. This system behaves as a module of the central ISTTOK control and data acquisition system (FIRESIGNAL). Preliminary results of the above experimental setup are presented and a performance benchmarking against the magnetic coil diagnostic is shown.

Tomographic Visualization for Plasma Position Control in ISTTOK

Plasma positioning on the ISTTOK tokamak has usually been done, resorting to magnetic pickup coils. However, during alternating current (ac)-type discharges with plasma current reversal, this method has been found to be inadequate. The recently installed tomography diagnostic can be an alternative for determining the plasma position. The presented image shows tomographic reconstructions of the ISTTOK plasma during ac discharges, showing the position of maximum and average emissivity positions.

Fast Camera Images of Surviving Gallium Droplets During Plasma–Liquid Gallium Jet Interaction Experiments in ISTTOK

Liquid metals have received a renewed interest in fusion research due to the advantages that these materials could offer to overcome the erosion problem of plasma facing components presently used in large-size fusion devices. In ISTTOK tokamak, the plasma-liquid gallium jet interaction is currently being studied. One of the main technical problems faced during the implementation of the experimental setup was to minimize the amount of gallium that remains in the vacuum chamber. Despite the large effort put in fulfilling this task, liquid-metal droplets could not be completely eradicated from the plasma. Here, pictures of one of those remaining droplets traveling through the plasma are presented and discussed.

Multi-Rate DSP/FPGA-Based Real-Time Acquisition and Control on the ISTTOK Tokamak

Data acquisition on the ISTTOK tokamak is being upgraded from VME to PCI using a general purpose especially developed transient recorder PCI acquisition board with 8 galvanic isolated 14 bit, 2 MSamples/sec channels, 512 Mbytes of SDRAM, and integrated FPGA/DSP capabilities. Due to the wide range of ISTTOK signal spectrum, data-reduction is crucial to efficiently use data retrieval and analysis systems. To avoid long latency offline processing and also to allow real-time control implemented in the DSP, multi-rate real time decimation in the FPGA has been implemented. This paper describes the data processing techniques developed using either FIR or cascaded-integrator-comb CIC filters. A digital DC baseline restorer was also implemented in the FPGA.

Compact cantilever force probe for plasma pressure measurements

A simple, compact cantilever force probe (CFP) has been developed for plasma pressure measurements. It is based on the pull-in phenomenon well known in microelectromechanical-system electrostatic actuators. The probe consists of a thin (25 mum) titanium foil cantilever (38 mm of length and 14 mm of width) and a fixed electrode separated by a 0.75 mm gap. The probe is shielded by brass box and enclosed into boron nitride housing with a 9 mm diameter window for exposing part of cantilever surface to the plasma. When the voltage is applied between the cantilever and the electrode, an attractive electrostatic force is counterbalanced by cantilever restoring spring force. At some threshold (pull-in) voltage the system becomes unstable and the cantilever abruptly pulls toward the fixed electrode until breakdown occurs between them. The threshold voltage is sensitive to an additional externally applied force, while a simple detection of breakdown occurrence can be used to measure that threshold voltage value. The sensitivity to externally applied forces obtained during calibration is 0.28 V/microN (17.8 VPa for pressure). However, the resolution of the measurements is +/-0.014 mN (+/-0.22 Pa) due to the statistical scattering in measured pull-in voltages. The diagnostic temporal resolution is approximately 10 ms, being determined by the dynamics of pull-in process. The probe has been tested in the tokamak ISTTOK edge plasma, and a plasma force of approximately 0.07 mN (plasma pressure approximately 1.1 Pa) has been obtained near the leading edge of the limiter. This value is in a reasonable agreement with the estimations using local plasma parameters measured by electrical probes. The use of the described CFP is limited by a heat flux of Q approximately 10(6) W/m(2) due to uncontrollable rise of the cantilever temperature (DeltaT approximately 20 degrees C) during CFP response time.

Interaction of a liquid gallium jet with the tokamak ISTTOK edge plasma

The interaction of a liquid gallium jet with plasma has been investigated in the tokamak ISTTOK. This paper presents a description of the conceived experimental setup, a detailed characterization of the produced jets and the first experimental results related to the gallium–plasma interaction. A stable jet has been obtained, which was not noticeably affected by magnetic field transients. ISTTOK has been successfully operated with the gallium jet without degradation of the discharge or a significant plasma contamination by liquid metal. This observation is supported by spectroscopic measurements showing that gallium radiation is limited to the region around the jet. Furthermore, the power deposited on the jet has been evaluated at different radial locations and its temperature increase estimated.

Compact retarding field energy analyzer for the tokamak ISTTOK boundary plasma

The retarding field energy analyzer (RFEA) remains the more reliable diagnostic to measure the ion temperature in the boundary plasmas of magnetic fusion devices. A compact, simple design and inexpensive RFEA has been developed for investigations on the tokamak ISTTOK. It consists of a stainless steel pinhole (with a diameter of 0.6mm), three fine nickel grids with a separation of 1mm, and a collector, all insulated by mica. All the components are placed inside a boron nitride housing with dimensions of 14×14×23mm3. The RFEA has been tested in both ion and electron modes. The conditions of the RFEA operation are discussed, and preliminary measurements of the ion and electron temperature profiles presented.

Magnetohydrodynamic and turbulence activity analysis in the ISTTOK tokamak using empirical mode decomposition

The understanding of the nature, driving mechanisms, and dynamics of turbulence activity in tokamak plasmas is a major challenge in fusion research. In the ISTTOK tokamak, edge turbulence activity has been extensively studied using both limiter and electrode biasing to induce local sheared electric fields, thus affecting turbulence and consequently particle confinement. Due to the fast time scales involved plasmas (pulse length of 30ms and particle confinement of the order of 0.3ms) and the turbulent nature of ISTTOK plasmas, not only turbulence but also magnetohydrodynamic (MHD) activity occurs at rapid bursts. Therefore, resolving the modes both spatially and during time (in particular, regarding the energy/frequency spectrum) requires advanced time-frequency analysis tools, capable of tracking short lasting (0.1ms) MHD modes. In this article, we explore the capabilities of the empirical mode decomposition method as a tool for turbulence and MHD instability analysis. The time evolution of the energy content associated with fluctuating potential, edge plasma density (obtained from Langmuir probes), and perturbed magnetic fields (Mirnov coil signals) is obtained, and a comparison is made with other advanced time-frequency signal estimators.

Control of the edge turbulent transport by emissive electrode biasing on the tokamak ISTTOK

In this paper results are presented on the changes induced by emissive electrode biasing in the ISTTOK edge transport. The boundary plasma is characterized with focus on the relation between E × B sheared flows and particle transport. We suggest that the distinct behaviour of the particle confinement for positive and negative bias observed in ISTTOK is related to the low E × B shear induced by positive bias in the core periphery region associated with the appearance of large amplitude fluctuations. In addition, the effect of the electrode bias on the edge turbulent transport has been investigated identifying the changes induced in the fluctuations frequency spectrum and probability density function. We have shown that negative electrode bias reduces the propagation of large-scale events, making the fluctuations distribution more Gaussian and resulting in low amplitude fluctuations across most of the edge plasma region. For positive bias, large amplitude broad spectrum fluctuations appear in the core periphery, which increase the cross-field diffusion and contribute to the observed asymmetry in particle transport with the bias polarity.

Multichannel time-of-flight technique for plasma potential profile measurements by heavy ion beam diagnostic on the tokamak ISTTOK

This article summarizes the current implementation of the time-of-flight (TOF) energy analysis for plasma potential measurements by the heavy ion beam diagnostic of the tokamak ISTTOK. The technique [beam modulation, TOF energy analyzer (TOFEA), and data acquisition system], the calibration procedures, the method used to obtain the plasma potential, and the resolution of the measurements are described in detail. The main TOFEA advantage is the possibility to realize simultaneous multichannel detection with a multiple cell array detector. The first measurements of the plasma potential profile obtained with a four-channel TOFEA are presented.

Efficiency of plasma biasing by movable localized limiter in tokamak ISTTOK

The correlation between the efficiency of plasma biasing and the position of a localized limiter has been investigated. It has been found that two distinct zones of limiter position can be distinguished in ISTTOK plasma: Far-zone at r lim > 7.2 cm, and Close-zone at r lim < 7.2 cm. The strongest modifications of edge plasma potential take place for both positive and negative limiter biasing when it is positioned inside Close-zone. The transition between zones is correlated with effective screening of the wall by the limiter, being more efficient as it is immersed deeper into the plasma. The indicative difference between zones can be a difference of non-ambipolar fluxes flowed onto short-circuited limiter: mostly positive (ion) inside Far-zone, and mostly negative (electron) inside Close-zone.

Comparison of limiter and emissive electrode bias on the tokamak ISTTOK

Biasing experiments have been performed on the tokamak ISTTOK with both a moving limiter and an emissive electrode. We have observed that large currents (>15A) can be drawn at negative applied voltage by both localized limiter and emissive electrode bias, leading to significant modifications in the edge plasma potential profile and to an improvement in particle confinement. However, compared with the localized limiter, the emissive electrode has the advantage of perturbing significantly less the plasma column. Furthermore, its use leads to the formation of stronger radial electric fields and consequently to a much larger improvement in particle confinement.

Time-of-flight energy analyzer for the plasma potential measurements by a heavy ion beam diagnostic

A time-of-flight (TOF) technique for the plasma potential measurements by a heavy ion beam diagnostic (HIBD) with a multiple cell array detector has been elaborated on tokamak ISTTOK as an alternative to the traditional electrostatic energy analyzer. This article describes the design and operation of a four-channel TOF energy analyzer (TOFEA). First results of plasma potential measurements by TOFEA are presented proving the feasibility of this technique in experiments with HIBD.

Real-time digital signal processor-based system for MHD mode identification in ISTTOK

A diagnostic for the magnetohydrodynamic (MHD) mode identification is being implemented on the Tokamak ISTTOK using the signals from the set of 12 equally spaced Mirnov coils. These poloidal magnetic field sensors have been recently installed inside the vessel and were designed for plasma shape and position equilibrium determination. The data acquisition and processing system is based on a locally developed galvanic isolated, eight channel fast PCI transient recorder module with a 2 Msamples/s sampling rate, 14 bit resolution, and a total memory of 256 Mwords. This module includes a digital signal processor and field programmable gate array allowing real-time determination of MHD modes using advanced identification algorithms, in a submillisecond cycle time. Poloidal m number and amplitude measurements are available in real time to feed event-driven active control systems. This solution enables high performance distributed computing, effective data reduction, and event detection at a relatively low cost.