脉冲射频容性耦合氩等离子体的发射探针诊断

利用工作在浮点模式下的发射探针,对500 Hz脉冲调制的27.12 MHz容性耦合氩气等离子体的空间电位和电子温度的时变特性进行了诊断.等离子体空间电位是通过测量强热状态下的发射探针电位获得的,而电子温度则是由发射探针在冷、热状态下的电位差来估算得到.测量结果表明:脉冲开启时,空间电位会快速上升并在300μs内趋于饱和;当脉冲关断后,空间电位经历了快速下降后趋于稳定的过程.电子温度在脉冲开启时存在过冲并趋于稳定的特征;而在脉冲关断期间,电子温度在300μs内则快速下降到0.45 e V后略有上升.无论在脉冲开启或关断期间,空间电位基本上都随功率和气压的变化存在有线性的依赖关系;而放电功率对脉冲开启期间过冲电子温度与稳态电子温度差异的影响较大.针对空间电位和电子温度在各阶段及不同放电条件下的时变特性,给出了相应的解释.     

Experimental study of the creation of a fire‐rod II: Emissive probe measurements

A planar electrode (collector) is immersed into a weakly magnetized discharge plasma column. Positive voltage steps are applied to the collector and time resolved measurements of the plasma response are performed by emissive probes using standard boxcar technique. On the emissive probe characteristics an interesting phenomenon is observed: a few μs after the application of the voltage step to the collector (time depends on the distance between the collector and the probe), the emissive probe starts to collect additional positive current in the potential region between plasma potentials of the high and low potential plasma. This phenomenon is investigated experimentally. We give a possible qualitative explanation that additional positive current collection is due to a combination of an interaction between the collector and the probe and space charge effects of positive ions around the probe.     

Direct measurements of the plasma potential by katsumata-type probes

By taking advantage of the inherent difference between the gyro-radii of ions and electrons in a magnetised plasma, we developed a method to measure the plasma potential directly. The principle is based on the concept of the Katsumata probe. The probe collector is hidden inside a tube which screens an adjustable fraction of the electron flux whereas it lets pass most of the ions. In this paper an arrangement of three Katsumata-type probes with different diameters is used in the edge plasma region of the CASTOR tokamak to investigate the influence of the dimension of the probe. The results show almost no influence of the probe diameter on the values of the plasma potential.     

真空中微波等离子体喷流电子数密度分布规律实验研究

为了准确诊断真空中微波等离子体喷流的电子数密度，利用统一的发射和单郎缪尔探针测量等离子体的空间电位，再测量等离子体的电流-电压特性曲线.根据空间电位测量结果，在等离子体的电流-电压特性曲线上能准确地获取饱和电流，从而处理出电子数密度.最后的诊断实验表明，当真空环境压强为2—6 Pa、等离子体发生器以60 W以下的微波功率击穿流量范围是42—106 mg/s的氩气时，所产生的微波等离子体喷流中电子数密度分布在1×10¹⁶—7.2×10¹⁶/m³范围内.     

Nonlinear collisional monte carlo simulations for high-temperature SOL plasma

A nonlinear Monte Carlo collisional model is applied to to investigate scrape-off layer (SOL) plasmas with high temperatures. In the proposed SOL modeling, A steady state SOL plasma, which satisfies the particle and energy balances and neutrality constraint, is determined in terms of total particle and heat fluxes across the separatrix, the edge plasma temperature, the secondary electron emission coefficient, and the SOL size. A conductive heat flux into the SOL is effectively modeled via random exchange of source particles and the SOL plasma particles. It is found that the potential drop and the electron transmission factor in the collisional SOL plasma are in good agreement with the theoretical prediction. The cooling effect of secondary electrons in the high temperature divertor operation is investigated. In such a collisionless plasma, the present nonlinear collision model is useful because the electron distribution function deviates far from a Maxwellian distribution. In the presence of strong secondary electron emission, the electron sheath energy transmission factor in the collisionless regime is found to be significantly smaller than that in the collisional regime. This fact suggests that a high-temperature divertor operation can be possible.     

Effect of cold electron emission on diffusion plasma parameters and the sheath structure in a double plasma device

It is observed experimentally that by injecting cold electrons in the discharge region of a double plasma device, the plasma parameters and sheath structure can be controlled in the other region, which is devoid of any electrical discharge. The main discharge region is separated from the region under investigation by a grounded mesh grid. Both cold and hot ionizing electrons are emitted from separate sets of filaments in the discharge region. With an increase in the cold electron emission current, the plasma parameters in the discharge region get changed, which in turn alter the plasma parameters in the other region. Two important effects caused by cold electrons in the diffusion region are the increase in the plasma density and decrease in the plasma potential. The increase in the plasma density and decrease in the sheath potential drop therefore cause the contraction of the sheath.     

Investigation of a Drifting Plasma Containing Negative Ions by Cylindrical Langmuir Probe Ion Mass Spectrometer

The study of drifting plasma in the laboratory or the ionsphere is not a straightforward problem because of the many variables such plasma will encounter. The presence of negative ions will add to the complication due to the detachment mechanism and other physical processes. It is not easy to separate the negative ion current component from the electron current component. To study negative ion population and mass spectra in the lower ion-sphere the value of the mean free path ? and the Debye shielding radius ?o of the medium should be taken into consideration together with the collision frequency of the different charged particles. Cylindrical Langmuir probes have been used successfully as dianostic techniques for the drifting plasma. A cylindrical probe of suitable radius in relation to ? and ?o was chosen. The probe was swept by a composite wave-form which contained a sawtooth sweep and two audio -frequency signals. The experiment was flown on board a stabilised rocket "Centaure" at high latitudes at Norway. The outputs yielded information about : 1. The first derivative of the current i with respect to the applied probe voltage -V 2. Percent modulation from which d2 i/dv2 was obtained. The second derivative is a function of the energy distribution function of the charged particles. The negative ion Gaussian peaks of energy are easily detached on the energy distribution function graph from those of thermal electrons.     

Effect of Discharge Plasma Potential on Diffusion Plasma Parameters Controlled by a Mesh Grid in a Double Plasma Device

The plasma region under investigation is separated from the discharge region by a mesh grid. Plasma potential and electron number densities and electron temperatures under bi‐Maxwellian approximation for electron distribution function of the multi‐dipole argon plasma are measured. The cold electrons in the diffusion region are produced by local ionization. The hot electrons are the ionizing electrons behaving as Maxwellian. The electron trapping process in the discharge region is produced by potential well due to positive plasma potential with respect to the anode and by a repulsive grid. The dependence of ratios of the density of the hot to the cold electrons N_E (=N_eh/N_ec) and hot to cold electron temperature T(=T_eh/T_ec) in the diffusion region on the depth of the potential well has been investigated. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The observation of field-aligned current during type-III ELM in EAST

Measurements of current-carrying filaments associated with Type-III edge localized mode (ELM) have recently been made in the Experimental Advanced Superconducting Tokamak by direct probing of the edge plasma using an advanced, fast-moving electrostatic and magnetic probe system. Contrary to expectations, no current filaments were detected near the separatrix. However, a clear signature of current filament has been observed in the far scrape-off layer (SOL) where the difference of the voltage between the divertor plates connecting a filament is sufficiently large, thus strongly suggesting that the current-carrying filaments form in the SOL, rather than being ejected from the plasma inside the separatrix. These findings provide, for the first time, information on the formation and sustainment of current filaments during type-III ELMy H-modes.     

One-dimensional solution to the stable, space-charge-limited emission of secondary electrons from plasma-wall interactions

Numerical solutions to the stable, space-charge-limited emission of secondary electrons from plasma-wall interaction are found based on one-dimensional plasma moment equations that assume cold ions, Maxwellian electrons and cold secondary electrons. The numerical method finds a range of plasma parameters that permit stable emission of secondary electrons in the absence of normal electric fields to the wall. These solutions were not obtained with previous method that solves only for the marginally stable plasma sheath. Range of the ion Mach number at the sheath edge, the floating wall potential relative to the plasmas, and secondary electron emission coefficients corresponding to the vanishing normal electric fields are found for hydrogen, argon and xenon plasmas. The results show that a relatively small range of secondary electron emission coefficient exists to allow stable sheaths structures along with larger ranges of ion injection speed at the sheath edge and floating potential of the emitting wall.     

Measurement of high-energy electrons by means of a Cherenkov detector in ISTTOK tokamak

The paper concerns detectors of the Cherenkov radiation which can be used to measure high-energy electrons escaping from short-living plasma. Such detectors have high temporal (about 1 ns) and spatial (about 1 mm) resolution. The paper describes a Cherenkov-type detector which was designed, manufactured and installed in the ISTTOK tokamak in order to measure fast runaway electrons. The radiator of that detector was made of an aluminium nitride (AlN) tablet with a light-tight filter on its front surface. Cherenkov signals from the radiator were transmitted through an optical cable to a fast photomultiplier. It made possible to perform direct measurements of the runaway electrons of energy above 80 keV. The measured energy values and spatial characteristics of the recorded electrons appeared to be consistent with results of numerical modelling of the runaway electron generation process in the ISTTOK tokamak.     

电子注在等离子体中的运动及传输特性

 从填充等离子体微波管漂移空间电子运动轨迹方程出发, 详细阐述了A区和B区电子注的聚束性能, 电子围绕平衡半径在A区和B区交替波浪式前进, 波纹的周期和幅度与等离子体密度和电子运动速度有关, 此外, 对弱加速场电子传输性能的研究表明, 调节等离子体参数、电压和场强, 可实现电子注无磁场聚束传输。     

Tonks-Langmuir problem for a bi-Maxwellian plasma

An analytical solution of the Tonks-Langmuir (TL) problem with a bi-Maxwellian electron energy distribution function (EEDF) is obtained for a plasma slab. The solution shows that the ambipolar potential, the plasma density distribution, and the ion flux to the wall are mainly governed by the cold electrons, while the ionization rate and voltage drop across the wall sheath are governed by the hot electrons. The ionization rate by direct electron impact is found to be spatially rather uniform, contrary to the T-L solution where it is proportional to the plasma density distribution. The temperature of hot electrons defined by the ionization balance is found to be close to that of the T-L solution for a mono-Maxwellian EEDF, and is in reasonable agreement with experiments carried out in a low pressure capacitance RF discharge. The energy balance for cold electrons in this discharge shows that their heating by hot electrons via Coulomb interaction is equalized by the cold electrons' escape to the RF electrodes during collapse of the RF sheath     

Plasmadiagnostik mit Emittierenden Sonden Durch Ausnutzung der Örtlichen Störung des Ionisationsgleichgewichts

The emissive probe is used generally to determine exclusively the plasma potential. The emissive probes have distinct advantages over collecting probes, because several conditions which restrict the spectrum of application fall away. In this paper the methods for the determination of plasma potentials are discussed and completed by new techniques using the local perturbation of the ionization balance. On the other hand it is shown that further plasma data can be obtained exploiting the plasma response on the emitted electrons accelerated by the probe bias voltage. In this way the total ionization rate, the portion of direct ionization, the concentration of metastable atoms and the life time of metastables are determined for selected conditions.

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Presented at the Colloquium on Plasma Physics Topics Common to the Laboratory and the Space Plasma Research, September 21–25, 1987, Skalský Dvr, Czechoslovakia.     

Design and fabrication of emissive biased limiter and its effect on tokamak plasma

In this paper, an emissive-biased limiter (EBL) was designed and fabricated then the magneto hydrodynamic activity was investigated based on Mirnov oscillations and hard X-ray spectroscopy through the tokamak plasma biasing. The EBL is positioned at r/a=0.92, and the biased voltage, which is varied from?250 to 250 V, applied between the head of the emissive limiter and vacuum chamber. Furthermore, the effects of the biased limiter for both negative and positive applied voltages are measured, and the results are compared with cold-biased limiter. As the results of IR-T1 tokamak suggest, in emissive negative polarity, the duration of plasma current is increased, compared with no bias, cold positive and emissive positive polarities. The amplitude of Mirnov fluctuations in emissive negative polarity is larger and more regular, compared with emissive positive polarity. The amplitude of the hard X-ray fluctuation in emissive negative polarity is very low compared with cold negative, cold positive and emissive positive polarities which leads to minimum energy loss.     

Correlation of Electron Beams and Hard X‐ray Emissions in ISTTOK Tokamak

The paper reports on experimental studies of electron beams in the ISTTOK tokamak, those were performed by means of an improved four‐channel detector. The Cherenkov‐type detector measuring head was equipped with four radiators made of two types of alumina‐nitrate (AlN) poly‐crystals: machinable and translucent ones, both of 10 mm in diameter and 2.5 mm in thickness. The movable support that enabled the whole detectors to be placed inside the tokamak vacuum chamber, at chosen positions along the ISTTOK minor radius. Since the electron energy distribution is one of the most important characteristics of tokamak plasmas, the main aim of the study was to perform estimations of an energy spectrum of the recorded electrons. For this purpose the radiators were coated with molybdenum (Mo) layers of different thickness. The technique based on the use of Cherenkov‐type detectors enabled the detection of fast electrons (of energy above 66 keV) and determination of their spatial and temporal characteristics in the ISTTOK experiment. Measurements of hard X‐rays (HXR), which were emitted during ISTTOK discharges, have also been performed. Particular attention was paid to the correlation measurements of HXR pulses with run‐away electron beams. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Simulation of radio-frequency atmospheric pressure glow discharge in γ mode

The existence of two diffe1：ent discharge modes has been verified in an rf （radio-frequency） atmospheric pressure glow discharge （APGD） by Shi [J. Appl. Phys. 97, 023306 （2005）]. In the first mode, referred to as a mode, the discharge current density is relatively low and the bulk plasma electrons acquire the energy due to the sheath expansion. In the second mode, termed γ mode, the discharge current density is relatively high, the secondary electrons emitted by cathodc under ion bombardment in the cathode sheath region play an important role in sustaining the discharge. In this paper, a one-dimensional self-consistent fluid model for rf APGDs is used to simulate the discharge mechanisms in the mode in helium discharge between two parallel metallic planar electrodes. The results show that as the applied voltage increases, the discharge current becomes greater and the plasma density correspondingly increases, consequentially the discharge transits from the a mode into the γ mode. The high collisionality of the APGD plasma results in significant drop of discharge potential across the sheath region, and the electron Joule heating and the electron collisional energy loss reach their maxima in the region. The validity of the simulation is checked with the available experimental and numerical data.     

Study of multiple impurity seeding effect using SONIC integrated divertor code for JT-60SA plasma prediction

In order to study the potential impurity seeding operation regime of the future fusion devices, the first application of the integrated divertor code SONIC to the Ar + Ne mixed-impurity seeding operation of JT-60SA steady-state high-β plasma has been carried out. In the case, Ne is added to Ar-only seeding, the separatrix electron density has fell into the desired low separatrix electron density of the scenario. This is mainly because the D⁺ flow velocity towards the inner divertor has been increased by the Ne seeding. The resultant friction force transports Ar impurities towards the inner divertor region, while impurities are stagnated in the top of scrape-off layer (SOL) in the Ar-only seeding case. The higher impurity radiation power in the divertor regions and lower one in the SOL region above the X point have been obtained in mixed-impurity seeding cases, which show similar tendency as the Ar + Ne mixed-impurity seeding experiment in JT-60 U. At the core edge, Z_eff has been slightly increased and the radiation power has been decreased as the Ne seeding rate increases. The core plasma/impurity transport has been also evaluated by the TOPICS code using the impurity density at the core edge computed by the SONIC as a boundary parameter. The results show lower Z_eff and radiation power, and higher electron temperature in the core in the mixed-impurity seeding cases. Above possible contributors to the better energy confinement indicate that the mixed-impurity seeding operation might be more effective than Ar-only seeding operation.     

Observations of Edge Plasma on HL-2A Tokamak

The edge plasma characteristics are studied by a movable array of Mach/Reynolds stress/Langmuir 10-probes and fast reciprocating 4-probes in the boundary region. These probes can measure edge plasma temperature, density, poloidal electric field, radial electric field, Reynolds stress, poloidal rotation velocities and their profiles obtained by changing the radial positions of the probe array in different discharges. Measurement resuits are used to analyze plasma confinement, turbulent fluctuations and their correlation. The fixed flush 3-probe arrays are mounted on the 4-divertor neutralization plates at the same toroidal cross-section in divertor chamber. These probes are used to measure the profiles of the electron temperature, density and float potential in divertor chamber.