Preliminary study of divertor particle exhaust in the EAST superconducting tokamak

The particle exhaust of the upper tungsten and lower carbon divertors in EAST has been preliminarily studied during the 2016 experimental campaign. The density decay time during terminating gas puffing has been employed as a key parameter to evaluate the divertor particle exhaust performance. Comparative plasma discharges have been carried out on the particle exhaust performance between two toroidal field directions in the upper single null and lower single null divertor configurations. This work has enhanced the understanding of the effects of the in–out asymmetry and divertor geometry on the efficiency of the divertor particle exhaust. In addition, the sensitivity of the particle exhaust capability on different strike point locations has been analyzed. The experimental results are expected to provide important information on the future upgrade of EAST bottom divertor and facilitate the realization of longer pulse operation.     

The Construction of Plasma Density Feedback Control System on J-TEXT Tokamak

The plasma density feedback control system(PDFCS) has been established on the Joint Texas Experimental Tokamak(J-TEXT) for meeting the need for an accurate plasma density in physical experiments.It consists of a density measurement subsystem,a feedback control subsystem and a gas puffing subsystem.According to the characteristic of the gas puffing system,a voltage amplitude control mode has been applied in the feedback control strategy,which is accomplished by the proportion,integral and differential(PID) controller.In this system,the quantity calibration of gas injection,adjusted responding to the change of the density signal,has been carried out.Some experimental results are shown and discussed.     

Energy Confinement of both Ohmic and LHW Plasma on EAST

Study on the characters of energy confinement in both ohmic and Lower Hybrid Wave (LHW) discharges on EAST is conducted and the linear ohmic confinement (LOC), saturated ohmic confinement (SOC) and improved ohmic confinement (IOC) regimes are investigated in this paper. It is observed that an improved confinement mode characterized by both a drop of Dα line intensity and an increase in line average density can be triggered by a gas puffing pulse.     

Improvement of Plasma Performance Using Carbon Pellet Injection in Large Helical Device

A cylindrical carbon pellet with a size of 1.2L?1.2? to 1.8L?1.8?mm and a velocity of 100 to 300 m/s was injected into Large Helical Device (LHD) for an efficient fueling based on its deeper deposition instead of hydrogen gas puffing and ice pellet injection. Electron density increment of ?ne=1014cm-3 is successfully obtained by single carbon pellet injection without plasma collapse. Typical density and temperature of the ablation plasma of the carbon pellet, e.g., 6.5x1016cm-3 and 2.5eV for CII, are examined respectively by spectroscopic method. A confinement improvement up to 50% compared to ISS-95 stellarator scaling is clearly observed in a relatively low-density regime of ne=2 to 4?1013cm-3, and high ion temperature Ti(0) of about 6keV is also observed with an internal transport barrier at ne=1.2?1013cm-3. In particular, the improvement in the ion temperature largely exceeds that observed in hydrogen gas- puffed discharges, which typically ranges below 3 keV.     

Recent ICRF coupling experiments on EAST

Recent ion cyclotron resonance frequency(ICRF) coupling experiments for optimizing ICRF heating in high power discharge were performed on EAST. The coupling experiments were focus on antenna phasing and gas puffing, which were performed separately on two ports of the ion cyclotron resonance heating(ICRH) system of EAST. The antenna phasing was performed on the I-port antenna, which consists of four toroidally spaced radiating straps operating in multiple phasing cases; the coupling performance was better under low wave number ∣k_‖∣(ranging from 4.5 to 6.5). By fuelling the plasma from gas injectors, placed as uniformly spaced array from top to bottom at each side limiter of the B-port antenna, which works in dipole phasing, the coupling resistance of the B-port antenna increased obviously.Furthermore, the coupling resistance of the I-port antenna was insensitive to a smaller rate of gas puffing but when the gas injection rate was more than a certain value(1021 s~(-1)), a sharp increase in the coupling resistance of the I-port antenna occurred, which was mainly caused by the toroidal asymmetric boundary density arising from gas puffing. A more specific analysis is given in the paper.     

Frequency dependence of plasma characteristics at different pressures in cylindrical inductively coupled plasma source

The effects of driving frequency on plasma parameters and electron heating efficiency are studied in cylindrical inductively coupled plasma (ICP) source. Measurements are made in an Ar discharge for driving frequency at 13.56/2 MHz, and pressures of 0.4–1.2 Pa. In 13.56 MHz discharge, higher electron density (n_e) and higher electron temperature (T_e) are observed in comparison with 2 MHz discharge at 0.6–1.2 Pa. However, slightly highern_e andT_e are observed in 2 MHz discharge at 0.4 Pa. This observation is explained by enhanced electron heating efficiency due to the resonance between the oscillation of 2 MHz electromagnetic field and electron-neutral collision process at 0.4 Pa. It is also found that the variation ofT_e distribution is different in 13.56 and 2 MHz discharge. For ICP at 13.56 MHz, T_e shows an edge-high profile at 0.4–1.2 Pa. For 2 MHz discharge,T_e remains an edge-high distribution at 0.4–0.8 Pa. However, the distribution pattern involves into a center-high profile at 0.9–1.2 Pa. The spatial profiles ofn_e remain a center-high shape in both 13.56 and 2 MHz discharges, which indicates the nonlocal kinetics at low pressures. Better uniformity could be achieved by using 2 MHz discharge. The effects of gas pressure on plasma parameters are also examined. An increase in gas pressure necessitates the rise ofn_e in both 13.56 and 2 MHz discharges. Meanwhile, T_e drops when gas pressure increases and shows a flatter distribution at higher pressure.     

Experimental Study of the Atomic and Molecular Processes Related to Plasma Detachment in Steady-State Divertor Simulator MAP-II

Atomic and molecular processes relevant to the volumetric recombination phenomena were investigated in a linear divertor plasma simulator MAP-II. Volumetric recombination is induced in He plasma by puffing of He or H2. In the He puffing case, the reduction of the ion flux is dominated by the electron-ion recombination. In the H2 puffing case, however, it is dominated by the molecule-assisted recombination (MAR), which is characterized by the disappearance of the Helium Rydberg spectra and by the existence of the hydrogen negative ions. Current achievement and the future prospect are described.     

Simulation Study on the Self-Sustained Oscillations in DC Driven Glow Discharges at Atmospheric Pressure Under Different Gas Gaps

In this paper,a one-dimensional plasma fluid model is employed to study the selfsustained oscillations in DC-driven helium glow discharges at atmospheric pressure under different gas gaps.Our simulation results indicate that a harmonic current oscillation with tiny amplitude always occur at the onset of instability and transits into a relaxation one as the conductivity of the semiconductor is decreased.It is found that the dynamics of the oscillations are dependent on the gas gaps.The discharge can only exhibit a simple oscillation with unique amplitude and frequency at smaller gas gaps(2 mm) while it can exhibit a more complex oscillation with several different amplitudes and frequencies at larger gas gaps(2 mm).The discharge modes in these current oscillations have also been analyzed.     

Mechanisms of Extending Operation Regionin the HL-1M Tokamak

1. IntroductionThe HL-1 tokamak had been modified to HL-IMby replacing the vacuum chamber from 1993 to 1994[1]. Some main modifications are'1. Removing the thick copper shell.2. Increasing the minor radius from 0.20 m to 0.26m.3. Replacing two full poloidal molybdenum limiters by graphite limiters.4. Covering about 6.5 % metallic wall surface withsome graphite components, such as two sets oftoroidal belt limiter, four sets of poloidal halfring protecting bellows, one set of plates protecti…     

Optical Spectroscopic Investigation of Ar/CH_3OH and Ar/N_2/CH_3OH Atmospheric Pressure Plasma Jets

Ar/CH3OH and Ar/N2/CH3OH plasma jets were generated at atmospheric pressure by dual-frequency excitations. Two different cases were studied with focus laid on the generation of CN radicals. In one case Ar gas passed through a bubbler with saturated methanol steam but without addition of N2 （Ar/CH3OH plasma）. In the other case N2 passed through the bubbler with saturated methanol steam （Ar/N2/CH3OH plasma）. The optical emission lines of CN radicals have been observed in these two cases of plasma discharges. The addition of N2 can significantly increase the optical emission intensity of CN bands.     

托卡马克超声分子束注入加料的进展

超声分子束注入作为一种新的托卡马克加料方法由作者在1992年首次提出并于当年在中国环流器一号(HL-1)装置演示成功,随后相继应用于中国环流器新一号(HL-1M)和中国科学院超导托卡马克HT-7装置。超声分子束注入等离子体呈现出电子密度峰化和温度中空分布的特征;等离子体流极向旋转速度提高,边缘扰动被抑制,等离子体能量约束得到改善。加料效率较常规脉冲送气提高一倍,而滞留器壁的粒子大为减少。近期开展的高气压氢超分子束注入实验,在束流中发现团簇流,可注入等离子体中心区域。多脉冲分子束注入形成电子密度的阶跃上升,如同冰弹丸注入效果。近年来该项技术已陆续应用于国外大型托卡马克和仿星器,是核聚变装置稳态运行的一种有效的加料方法。     

The investigation of OH radicals produced in a DC glow discharge by laser-induced fluorescence spectrometry

In this paper the OH radicals produced by a needle-plate negative DC discharge in water vapor,N2 + H2O mixture gas and He + H2O mixture gas are investigated by a laser-induced fluorescence (LIF) system.With a ballast resistor in the circuit,the discharge current is limited and the discharges remain in glow.The OH rotation temperature is obtained from fluorescence rotational branch fitting,and is about 350 K in pure water vapor.The effects of the discharge current and gas pressure on the production and quenching processes of OH radicals are investigated.The results show that in water vapor and He + H2O mixture gas the fluorescence intensity of OH stays nearly constant with increasing discharge current,and in N2 + H2O mixture gas the fluorescence intensity of OH increases with increasing discharge current.In water vapor and N2 + H2O mixture gas the fluorescence intensity of OH decreases with increasing gas pressure in the studied pressure range,and in He + H2O mixture gas the fluorescence intensity of OH shows a maximum value within the studied gas pressure range.The physicochemical reactions between electrons,radicals,ground and metastable molecules are discussed.The results in this work contribute to the optimization of plasma reactivity and the establishment of a molecule reaction dynamics model.     

Discharge Characteristics of SF6 in a Non-Uniform Electric Field Under Repetitive Nanosecond Pulses

The characteristics of high pressure sulphur hexafluoride (SF ₆ ) discharges in a highly non-uniform electric field under repetitive nanosecond pulses are investigated in this paper. The influencing factors on discharge process, such as gas pressure, pulse repetition frequency (PRF), and number of applied pulses, are analyzed. Experimental results show that the corona intensity weakens with the increase of gas pressure and strengthens with the increase of PRF or number of applied pulses. Spark discharge images suggest that a shorter and thicker discharge plasma channel will lead to a larger discharge current. The number of applied pulses to breakdown descends with the increase of PRF and ascends with the rise of gas pressure. The reduced electric field (E/p) decreases with the increase of PRF in all circumstances. The experimental results provide significant supplements to the dielectric characteristics of strongly electronegative gases under repetitive nanosecond pulses.     

Effects of the Hot Electron Interchange Instability on Plasma Confined in a Dipolar Magnetic Field

The Levitated Dipole Experiment (LDX) explores confinement and stability of plasma created within the dipole field of a strong superconducting magnet. During initial experiments, long-pulse, quasi-steady state discharges that last more than 10 s and have peak beta of more than 20% are studied. The plasma is created by multi-frequency electron cyclotron resonance heating (ECRH) at 2.45 and 6.4 GHz. A population of energetic electrons, with mean energies above 50 keV, dominates the plasma pressure. Creation of high pressure, high beta plasma is possible only when intense hot electron interchange (HEI) instabilities are stabilized by sufficient neutral gas fueling. The instabilities resonate with the magnetic drift motion of the energetic electrons and can cause rapid radial transport. Measurements of the electrostatic and magnetic fluctuations of the HEI instability are described along with observations of the instability’s spectral characteristics. Fluctuations of the outer poloidal field induced by the HEI show a rapid evolution of the perturbed pressure profile.      

Engineering Improvements and Physics Studies of the Accelerated Compact Toroid in the CTIX Injector

The main goals of the experimental studies on the CTIX compact toroid accelerator are to improve CT parameters through engineering modification, and to quantify CT dynamics by the comparison of experimental measurements to MHD theories. Over this research period, the final CT kinetic energy has been increased by about 30%, with increased efficiency of first-cycle capacitor energy utilization. The efficiency increase is obtained through a combination of engineering modification to passive magnetic switching (saturable Metglas cores), and a fast gas system in the acceleration section of CTIX. Using the new switching system and gas puffing, the final CT density has been increased up to a factor of 5, compared to operation without puffing. The second task of the CTIX project is to compare the experimental data with the existing theoretical models. The most relevant models are that of force-free equilibrium, and a Hall-MHD model. From analysis of magnetic field data, it is determined that the Hall-MHD model gives better agreement, if effects of fluid vorticity are also included. This analysis has been reported in a recent paper (S. J. Howard et al., Submitted). Finally, a new plasma density measurement technique, deflectometry, has been compared with conventional interferometry method. A significant data base has been produced to show that the deflection measurements correlate well with the HeNe laser interferometry.     

Analysis of wall conditioning by using plasma impact desorption in the tandem mirror GAMMA 10

A method for evaluating wall condition by using plasma impact desorption (PID) technique has been developed and successfully applied to the tandem mirror GAMMA 10 as a monitor for wall conditioning. A magnetically shielded quadrupole mass spectrometer was installed in the vacuum chamber of the GAMMA 10 central cell. The behaviour of the partial pressure of various gas molecules desorbed by ICRF-heated plasma discharges were analyzed. The predominant increase of the partial pressure due to PID (ΔP_PID) was hydrogen (M = 2) and a small amount of impurity as CO (M = 28), CH₃ (M = 15), H₂O (M = 18) and CO₂ (M = 44) was observed in the wall-conditioning discharges. The reduction of hydrogen pressure as well as ΔP_PID of the above impurities was observed with the progress of wall conditioning. This behavior has a good correlation with the increase of partial pressures due to electron-impact desorption measured at the same period. The relation between ΔP_PID and the charge-exchange flux was investigated.     

Impurity Transport in a Simulated Gas Target Divertor

Future generation fusion reactors and tokamaks will require dissipative divertors to handle the high particle and heat loads leaving the core plasma (100–400 MW/m² in ITER). A radiative divertor is proposed as a possible scenario, utilizing a hydrogen target gas to disperse the plasma momentum and trace impurity radiation to dissipate the plasma heat flux. Introducing an impurity into the target hydrogen gas enhances the radiative power loss but may lead to a significant impurity backflow to the main plasma. Thus, impurity flow control represents a crucial design concern. Such impurity flows are studied experimentally in this thesis. The PISCES-A linear plasma device (n 3 × 10¹⁹ m^–3, kT _e 20 eV) has been used to simulate a gas target divertor. To study the transport of impurities, a trace amount of impurity gas (i.e., neon and argon) is puffed near the target plate along with the hydrogen gas. Varying the hydrogen gas puffing rate permits us to study the effects of various background plasma conditions on the transport of impurities. A 1-1/2-D fluid code has been developed to solve the continuity and momentum equations for a neutral and singly ionized impurity in a hydrogen background plasma. The results indicate an axial reduction in the impurity concentration upstream from the impurity puffing source. Impurity entrainment is more effective for higher hydrogen target pressures (and for higher hydrogen plasma densities). However, if there is a reversal of the background plasma flow, impurity particles can propagate past the plasma flow reversal point and are then no longer entrained.     

Flow Characteristics and Charge Exchange Effects in a Two-Dimensional Model of Electrothermal Plasma Discharges

Electrothermal (ET) plasma discharges are capillary discharges that ablate liner materials and form partially ionized plasma. ET plasma discharges are generated by driving current pulses through a capillary source with peak currents on the order of tens of kA and pulse lengths on the order of \(100\,\upmu \hbox {s}\). These plasma discharges can be used to propel pellets into magnetic confinement fusion devices for deep fueling of the fusion reaction, ELM mitigation, and thermal quench of the fusion plasma. ET plasma discharges have been studied using 0D, 1D, and semi-2D fluid models. In this work, a fully 2D model of ET plasma discharges is presented. The newly developed model and code resolve inter-species interaction forces due to elastic collisions. These forces affect the plasma flow field in the source and impede the development of plasma pressure at the exit of the source. In this work, these affects are observed for discharge current pulses peaking at 10 and 20 kA. The sensitivity of the model to the inclusion of charge exchange effects is observed. The inclusion of charge exchange has little effect on the integrated, global results of the simulation. The difference in total ablated mass for the simulations caused by the inclusion of charge exchange reactions is <1 %. Differences in local plasma parameters are observed during discharge initialization, but after initialization, these differences diminish. The physical reasoning for this is discussed and recommendations are made for future modeling efforts.     

Development of the pellet injection system on the J-TEXT tokamak

Pellet injection is an attractive technology for core-fueling and magnetohydrodynamic study in magnetic-confinement fusion devices like tokamaks and stellarators. It can inject solid hydrogen/deuterium pellets into the plasma with deeper density deposition compared with other fueling methods, such as gas puffing. A three-barrel H₂ pellet injection system was installed on the J-TEXT tokamak and experiments were carried out. The pellets are formed in three barrels cooled by a cryocooler and compressor system at around 9 K, and are 0.8 mm/1 mm diameter and 0.8 mm length. The pellet is launched by helium propellant gas and injected from the low-field side of the plasma. The normal range of pellet speed is 210–310 m s⁻¹ for different propellant gas pressures. Due to the three-barrel structure, the number of injected pellets can be adjusted between one and three. Pellets can be launched sequentially with arbitrary time intervals, which enables flexible applications. The results of the experiments show that pellet fueling efficiency can reach 50%. The energy confinement time increased by about 7.5‒10 ms after pellet injection.     

Investigation of 50 Hz Pulsed DC Nitrogen Plasma with Active Screen Cage by Trace Rare Gas Optical Emission Spectroscopy

Optical emission spectroscopy is used to investigate the nitrogen-hydrogen with trace rare gas (4% Ar) plasma generated by 50 Hz pulsed DC discharges. The filling pressure varies from 1 mbar to 5 mbar and the current density ranges from 1 mA·cm ⁻² to 4 mA·cm ⁻² . The hydrogen concentration in the mixture plasma varies from 0% to 80%, with the objective of identifying the optimum pressure, current density and hydrogen concentration for active species ([N] and [N ₂ ]) generation. It is observed that in an N ₂ -H ₂ gas mixture, the concentration of N atom density decreases with filling pressure and increases with current density, with other parameters of the discharge kept unchanged. The maximum concentrations of active species were found for 40% H ₂ in the mixture at 3 mbar pressure and current density of 4 mA·cm ⁻²