Plasma Discharge Experiments Research Articles

Plasma discharge experiments of a Faraday shieldless driver for high-power and long-pulse radio frequency ion source for NBI application.

Due to the high stability and less maintenance, the radio frequency (RF) driven ion source is preferred for the neutral beam injection (NBI) system. In a popular design of the RF ion source for NBI application, a Faraday shield (FS) is installed inside the RF plasma driver to protect the discharge tube. However, the FS also brings some drawbacks, such as lowering the RF power transfer and increasing the processing difficulty. A prototype of the RF plasma driver without FS and with mature manufacturing technology has been developed by using a water-cooled discharge tube. After basic testing, this prototype was further tested under the RF plasma discharge experiments in views of high power, long pulse, and long term. The reliability and its plasma characteristics were the focus of these experiments, also for the hidden issues. The results show that the prototype could generate stable and high-density plasma without any damage or sputtering mark. An RF plasma discharge of 50 kW and 20s has been achieved. The expected frequency tuning was equally effective on the prototype. Moreover, compared to the RF plasma driver with FS, the prototype could produce higher electron density in the extraction region under the same RF power. Of course, some of the shortcomings of the prototype have also been exposed in the experiments and will be improved in subsequent experiments.

Long-time measurements of line-integrated plasma electron density using a two-color homodyne optical fiber interferometer

A two-color homodyne Mach-Zehnder (M-Z) optical fiber interferometer with wavelengths of 1.55 and 1.31 µm was developed for long-time measurement of line-integrated plasma electron density. A novel phase difference demodulation algorithm based on a single 3 × 3 optical coupler was implemented in a two-color optical fiber interferometer scheme for the first time. Our laboratory tests showed that this new optical fiber interferometer could determine the phase shift due to the low-frequency ambient vibration and could maintain high phase resolution measurement. The resolution of the new interferometer was less than 0.04rad in 1000s, corresponding to a line-averaged electron density of less than 1.0 × 1019m-2. Actual plasma discharge experiments performed on KTX-CTI, which is a new compact torus injector (CTI) constructed at the Keda Torus eXperiment (KTX), showed that this interferometer has excellent several-second stability.

Development of a real-time magnetic island reconstruction system based on PCIe platform for HL-2A tokamak

A real-time magnetic island reconstruction (MIR) system based on PCI express platform for HL-2A tokamak is introduced. The front-end analog circuit and high performance analog-to-digital converters complete high-precision synchronous sampling of 18 channel Mirnov signals, and the application of PCIe platform and direct memory access technology enables high speed data transmission between graphics processing unit and field programmable gate array (FPGA). FPGA, as a mainstream high speed parallelizable computing tool, was used to implement the MIR algorithm, while a parameter table is established in an external double data rate SDRAM to improve the computational efficiency. The software of the MIR system is developed with Compute Unified Device Architecture 8.0 in Centos 6 system, which mainly realizes driver development, data transmission, network communication, parameter calculation and system control. This system has been tested in HL-2A plasma discharge experiment, and the reconstructed magnetic island structure can achieve a spatial resolution of 1.02 cm while the time resolution can reach 2 ms.

Development of a 28-GHz/50-kW/30-s Gyrotron System for Fusion Application

A complete 28-GHz/50-kW gyrotron system was developed by the Institute of Applied Electronics, China Academy of Engineering Physics, Mianyang, China, as a plasma heating source for the EXL50 fusion device, a compact spherical tokamak constructed by the ENN Group in 2019. The gyrotron employs a triode magnetron injection gun, a TE02-mode cylindrical cavity, a built-in quasi-optical mode converter, a single-stage depressed collector, and a single-disk boron nitride window. The gyrotron system features a match optical unit, a superconducting magnet, a calorimetric dummy load, a high-voltage power supply system, and other auxiliary components, was delivered to the EXL50 site. In the acceptance test, five successive shots of each power over 50 kW were generated for 30 s at 28 GHz. An average power of 51 kW, a maximum power of 55 kW, and 46% overall efficiency were obtained. The gyrotron system was assembled in the EXL50 and applied to a plasma discharge experiment. The gyrotron is the first practical domestic gyrotron produced for fusion applications in China. The design and test results of this instrument are presented in this article.

Analysis of thermal and structural responses of a new diagnostic probe to repeated exposure in ASDEX upgrade tokamak

Abstract Thermal and structural responses of a new diagnostic probe inside the AUG (ASDEX Upgrade) tokamak has been analysed numerically. This new reciprocating probe has been developed as a multi-diagnostic device for measuring several plasma parameters simultaneously. It is constructed as a universal probe that can be used in different tokamaks by applying corresponding adapters. The probe head design has to go through a number of tests (numerical and experimental) before it can be used in the actual tokamak environment, where it has to withstand repeated exposure to energetic particle fluxes in the scrape-off layer plasma. In this respect, the probe resilience to the heat loads in AUG has been investigated numerically. The performed transient analyses correspond to a typical triple insertion of the probe head into the plasma during a plasma discharge experiment in AUG. Thermal and mechanical responses of the most critical parts of the probe have been analysed. To avoid the potential pollution of the vacuum vessel, thermal resistance and structural integrity of the probe`s protective shroud have been studied in particular. The protective shroud has been constructed specifically for AUG. The simulations have shown that the shroud is able to withstand the expected plasma exposure and provides sufficient protection for the probe diagnostic and structural parts.

Application of LHD Post Data Analysis Systems to the KSTAR Project

Abstract Most of the experiments of the LHD (Large Helical Device) Project are repetitive short pulse experiments. That is, a 10 seconds-long plasma discharge experiment is executed every three minutes. Therefore, it is important to grasp the results of the experiment as soon as possible to direct the ongoing experiment. The post analysis tools used for the LHD project provide important information to the scientists during the experiment. The first tool is the AutoAna. The AutoAna runs the programs automatically to maintain the dependency among the analyzed data. Another tool is MyView2. MyView2 is a python-based data visualization tool, and it has been developed mainly for visitors so that they may use MyView2 to retrieve and analyze the necessary data for their study soon after they arrive at NIFS. The graph layout and the data source can be flexibly customized using a GUI operation. In the central control room for the LHD project, approximately 10 computers are located at the center and display the latest results of the ongoing experiment using MyView2. The authors believe these tools are useful for other experiments other than the LHD project, and prove the usefulness by porting them to the KSTAR project.

A visible light high-speed imaging system and tomographic reconstruction of plasma emissivity on the J-TEXT tokamak

A visible light high-speed imaging system has been installed on the J-TEXT tokamak to measure the spatial distribution of visible emission in the vacuum chamber. The system views the plasma tangentially from an outer port at the height of the mid-plane of the vacuum vessel. An optical system is designed for the consideration that the camera must be installed away from the magnetic field. A spatial calibration based on Tsai’s method is adopted to build the relationship between the tokamak 3-D space coordinate and the 2-D image coordinate. Tomographic reconstruction techniques are used to calculate the 2-D poloidal emission profiles from the camera image data assuming toroidal symmetry of the radiation. Sample results are presented to view the discharge process in typical plasma discharge and methane injection experiments with a CIII-filter added. And the 2-D poloidal CIII emission profiles reconstructed from imaging results in typical plasma discharge experiments are also obtained.

Design of the Power Supply for Alfvén Eigenmodes Excitation on J-TEXT

The Alfven eigenmodes (AEs) excitation system has been designed and installed on the J-TEXT tokamak. The system consists of a series resonant power supply and an excitation antenna. Under the typical operating conditions of the J-TEXT, the excitation frequency is thought to be in the range of 200–700 kHz. In order to match the AEs frequency, the external excitation system needs to scan its frequency automatically in experiments. In the implementation process, the whole frequency band is divided into several segments. For each one of them, the resonant circuit matches different capacitors. Specifically, the resonant capacitor is arranged in the form of a fixed capacitor in parallel with a variable capacitor. In the last campaign, a 2.3-nF fixed capacitor and a 1-nF variable capacitor are picked up, so that the resonant capacitance could change from 2.3 to 3.3 nF. In addition, the matching units were applied to optimize the antenna currents. Finally, the formal plasma discharge experiments have been carried out on the J-TEXT. The results show that the excitation frequency can be changed from 445 to 510 kHz in 350 ms, and the output current is able to reach 50 A. This paper is focused on the design of the excitation system, particularly the strategy of frequency scanning.

Inter-application communication during LHD consecutive short pulse discharge experiment

LHD short pulse experiments are executed every three minutes. After the end of the discharge, the scientists must collect, analyze, visualize the last acquired data of the discharge, and prepare for the next discharge. From the beginning, the computer environment of the LHD (Large Helical Device) experiment has been built as a network distributed system, and various computers have been used for data acquisition or physical analysis. When one program is finished on one computer, that computer must send the results in order to the other computers to run programs. Smooth communication is required in order to finish all the tasks before the next discharge. To exchange the information among the applications running on the different computers, the authors have tried various methods, such as a commercial software to share the memory over the network, simple network file sharing method, IP multicast, web interfaces, and others. The purpose of this paper is to share our experiences of trial and error to build the network distributed systems for the consecutive plasma discharge experiments.

A fiber-optic analog isolator of up to 10 MHz bandwidth

The electromagnetic interference (EMI) around the Experimental Advanced Superconducting Tokamak (EAST) device is very harsh during plasma discharge experiment. In order to reduce the effect of EMI and enhance signal to noise ratio (SNR), a fiber-optic high speed isolator has been designed. Each channel of the isolator has a transmitter unit connected via fiber-optic cable to the receiver unit. A transmitter unit which consists of a 6th order Bessel low-pass anti-aliasing filter, an analog-to-digital converter (ADC), a Field Programmable Gate Array (FPGA), a serializer and an optical transceiver, while a receiver unit comprises an optical transceiver, a deserializer, a FPGA, a digital–to-analog converter (DAC) and a 6th order Bessel low-pass filter. Data from the ADC is serialized by the serializer using 8b/10b encoding before transmission, and 10b/8b decoding was used by the deserializer in the receiver unit. A prototype of this isolator has been realized and tested. The test results show that the transmission bandwidth of analog signal is up to 10MHz, the transmission delay is typically 500ns, the transmission distance is up to 250m and the output noise is less than 5mV, which can meet the requirements of EAST experimental campaigns.

A precision control method for plasma electron density and Faraday rotation angle measurement on HL-2A

The precision of plasma electron density and Faraday rotation angle measurement is a key indicator for far-infrared laser interferometer/polarimeter plasma diagnosis. To improve the precision, a new multi-channel high signal-to-noise ratio HCOOH interferometer/polarimeter has been developed on the HL-2A tokamak. It has a higher level requirement for phase demodulation precision. This paper introduces an improved real-time fast Fourier transform algorithm based on the field programmable gate array, which significantly improves the precision. We also apply a real-time error monitoring module (REMM) and a stable error inhibiting module (SEIM) for precision control to deal with the weak signal. We test the interferometer/polarimeter system with this improved precision control method in plasma discharge experiments and simulation experiments. The experimental results confirm that the plasma electron density precision is better than 1/3600 fringe and the Faraday rotation angle measurement precision is better than 1/900 fringe, while the temporal resolution is 80 ns. This performance can fully meet the requirements of HL-2A.

Study on divertor heat flux of L-H transition with LHCD and NBI in EAST

An infrared (IR)/visible endoscope system was built on the Experimental Advanced Superconducting Tokamak (EAST) in 2014. The temperature distributions of the lower divertor have been measured. Based on the IR data of EAST, the heat fluxes on the lower outer divertor were calculated with a code named DFLUX developed by ASIPP, aimed to research the heat fluxes on divertor under different auxiliary heating schemes. Peak heat fluxes of the lower outer divertor show a reduction of several hundred kilowatts during L-H transition and an increase of nearly 1MW/m2 during H-L transition, corresponding with the Dα signal, electron density and plasma stored energy under LHCD heating scheme(PLHCD∼1.9MW). The heat flux profile of lower outer divertor was broad before the first L-H transition and narrowed rapidly after L-H transition but without obvious change during the next L-H transitions. Narrower heat flux profile and higher heat flux than LHCD heating scheme were induced by NBI heating scheme (0.9MW at first and 1.7MW before L-H transition), with the heat flux profile narrowed after L-H transition. The study results can afford reference for the optimization of H-mode plasma discharge experiments on EAST.

The transient electric field measurement system for EAST device

The electromagnetic environment around the Experimental Advanced Superconducting Tokamak (EAST) device is very complex during plasma discharge experiment. In order to fully monitor the changes of electric field around the EAST device during plasma discharge, a transient electric field measurement system based on PCI eXtensions for Instrumentation (PXI) platform has been designed. A digitizer is used for high-speed data acquisition of raw signals from electric field sensors, and a Field Programmable Gate Array (FPGA) module is used for realizing an on-the-fly fast Fourier transform (FFT) and inverse fast Fourier transform (IFFT) algorithm including a beforehand identified antenna factor (AF) to achieve finally a calibrated and filtered electric field measurement, then these signals can be displayed and easily analyzed. The raw signals from electric field sensors are transferred through optical fiber by optical isolation to reduce electromagnetic interference (EMI). The high speed data streaming technology is used for data storage. A prototype of this system has been realized to measure the transient electric field strength, with the real-time processing and continuous acquisition ability of one channel of 14-bit resolution and up to 50MHz sampling rate, and 6 KHz FFT frequency resolution.

A multi-channel real-time digital integrator for magnetic diagnostics in HL-2A tokamak

A novel full-digital integrator has been developed for the magnetic diagnostics in HL-2A. Based on the pipeline processing of the field-programmable gate array and high-speed PCI extensions for instrumentation platform, the digital integrator has realized octal-channel 10-kHz real-time integration and data transmission. In order to reduce the integration drift, a 24-bit analog-to-digital converter and simple analog processing circuits are applied for high-precision sampling, while certain correction algorithms are used to minimize the drift. With simple and highly integrated circuits and high-performance digital processor, the digital integrator is of high stability and functional expansibility which greatly simplifies the operation procedure. The digital integrator has been tested in the plasma discharge experiments, and the experimental results have confirmed that the drift performance and accuracy of the digital integrator could fully meet the requirements of HL-2A.

Conditioning of SST-1 Tokamak Vacuum Vessel by Baking and Glow Discharge Cleaning

Steady-state Superconducting Tokamak (SST-1) vacuum vessel (VV) adopts moderate baking at 110±10°C and the limiters baking at 250±10°C for ∼ 200h followed by glow discharge cleaning in hydrogen (GDC-H) with 0.15 A/m2 current density towards its conditioning prior to plasma discharge experiment. The baking in SST-1 reduces the water (H2O) vapor by 95% and oxygen (O2) by 60% whereas the GDC reduces the water vapor by an additional 57% and oxygen by another 50% as measured with residual gas analyzer. The minimum breakdown voltage for H-GDC in SST-1 tokamak was experimentally observed to 300V at 8 mbar cm. As a result of these adherences, SST-1 VV achieves an ultimate of 4.5×10−8 mbar with two turbo-molecular pumps with effective pumping speed of 3250 l/s. In the last campaign, SST-1 has achieved successful plasma breakdown, impurity burn through and a plasma current of ∼ 40 kA for 75ms.

Analysis and optimization on in-vessel inspection robotic system for EAST

Since China has successfully built her first Experimental Advanced Superconducting TOKAMAK (EAST) several years ago, great interest and demand have been increasing in robotic in-vessel inspection/operation systems, by which an observation of in-vessel physical phenomenon, collection of visual information, 3D mapping and localization, even maintenance are to be possible. However, it has been raising many challenges to implement a practical and robust robotic system, due to a lot of complex constraints and expectations, e.g., high remanent working temperature (100°C) and vacuum (10−3pa) environment even in the rest interval between plasma discharge experiments, close-up and precise inspection, operation efficiency, besides a general kinematic requirement of D shape irregular vessel. In this paper we propose an upgraded robotic system with redundant degrees of freedom (DOF) manipulator combined with a binocular vision system at the tip and a virtual reality system. A comprehensive comparison and discussion are given on the necessity and main function of the binocular vision system, path planning for inspection, fast localization, inspection efficiency and success rate in time, optimization of kinematic configuration, and the possibility of underactuated mechanism. A detailed design, implementation, and experiments of the binocular vision system together with the recent development progress of the whole robotic system are reported in the later part of the paper, while, future work and expectation are described in the end.

Search for neutron flux generation in a plasma discharge electrolytic cell

Following some recent unexpected hints of neutron production in setups like high-voltage atmospheric discharges and plasma discharges in electrolytic cells, we present a measurement of the neutron flux in a configuration similar to the latter. We use two different types of neutron detectors, poly-allyl-diglicol-carbonate (PADC, aka CR-39) tracers and Indium disks. At 95% C.L. we provide an upper limit of 1.5 neutrons cm^-2 s^-1 for the thermal neutron flux at ~5 cm from the center of the cell. Allowing for a higher energy neutron component the largest allowed flux is 64 neutrons cm^-2 s^-1. This upper limit is two orders of magnitude smaller than what previously claimed in an electrolytic cell plasma discharge experiment. Furthermore the behavior of the CR-39 is discussed to point our possible sources of spurious signals.

Nitrogen isotopic fractionation during abiotic synthesis of organic solid particles

The formation of organic compounds is generally assumed to result from abiotic processes in the Solar System, with the exception of biogenic organics on Earth. Nitrogen-bearing organics are of particular interest, notably for prebiotic perspectives but also for overall comprehension of organic formation in the young Solar System and in planetary atmospheres. We have investigated abiotic synthesis of organics upon plasma discharge, with special attention to N isotope fractionation. Organic aerosols were synthesized from N2–CH4 and N2–CO gaseous mixtures using low-pressure plasma discharge experiments, aimed at simulating chemistry occurring in Titan's atmosphere and in the protosolar nebula, respectively. The nitrogen content, the N speciation and the N isotopic composition were analyzed in the resulting organic aerosols. Nitrogen is efficiently incorporated into the synthesized solids, independently of the oxidation degree, of the N2 content of the starting gas mixture, and of the nitrogen speciation in the aerosols. The aerosols are depleted in 15N by 15–25‰ relative to the initial N2 gas, whatever the experimental setup is. Such an isotopic fractionation is attributed to mass-dependent kinetic effect(s).Nitrogen isotope fractionation upon electric discharge cannot account for the large N isotope variations observed among Solar System objects and reservoirs. Extreme N isotope signatures in the Solar System are more likely the result of self-shielding during N2 photodissociation, exotic effect during photodissociation of N2 and/or low temperature ion-molecule isotope exchange. Kinetic N isotope fractionation may play a significant role in the Titan's atmosphere. On the Titan's night side, 15N-depletion resulting from electron driven reactions may counterbalance photo-induced 15N enrichments occurring on the day's side. We also suggest that the low δ15N values of Archaean organic matter (Beaumont and Robert, 1999) are partly the result of abiotic synthesis of organics that occurred at that time, and that the subsequent development of the biosphere resulted in shifts of δ15N towards higher values.

Study on oxidation of hydrogen over commercial catalyst for tritium recovery system

For the establishment of the D-T fusion reactor technology, recovery of tritium released into the working area of fusion power plants is quite important. When tritium leaks to working areas, the last barrier is the wall of the building. Due to higher diffusion coefficient of tritium, it diffuses through the wall and would be readily liberated to the environment. Thus, the tritium recovery system is indispensable for the D-T fusion reactor. The objective of the present study is to develop the advanced technology of the tritium recovery system.In the near future, deuterium plasma discharge experiments scheduled be conducted with Large Helical Device (LHD) in National Institute for Fusion Science. A small amount of tritium is produced by D-D reaction in LHD. Tritium in plasma exhaust gases and process gas during discharge needs to be recovered, and thus the design and construction of the tritium recovery system used for that purpose is a matter of considerable urgency.The tritium recovery system usually consists of catalysts and adsorbents, which is the most conventional and reliable method for removing tritium that is accidentally released into the working area of these facilities. However, more recent and advanced type of catalysts on the market cannot be directly applied to the design of tritium recovery system, because of paucity of design data for tritium recovery system. In this study, the authors performed oxidation experiments of hydrogen over a catalyst. The experiments were performed by changing various experimental parameters.

PK-4 Science Activities in Micro-gravity

PK-4 is a multi-functional plasma discharge source. Two setups exist for operating plasma discharge experiments on parabolic flight campaigns as well as in laboratory. Another setup for use on the International Space Station ISS is planned. The plasma can be created by a dc-discharge as well as by rf-discharges, coupled inductively. There exist various possibilities to manipulate the introduced microparticles: mechanical (glass-nozzle), optical (high power laser), electrical (EM-pulse-electrode, rf-electrodes, polarity-switching) and thermal (heating coil) manipulation. Several different types of experiments have already been done with PK-4 in parabolic flights: Nozzle-Experiments (particle jets, transition between fluid and single particle behaviour), Lane-Formation (cloud collisions), Laser driven shear flow, Shock waves and solitons (excitation of shock waves applying electric pulses using the EM-electrode), String-Fluids (electro-rheological fluids), and many more.