Low-density Background Plasma Research Articles

Effect of plasma properties on magnetic bubbles

In this work, a three-dimensional hybrid simulation program is used to investigate the process of the explosion and expansion of a large number of thermal debris ions in a low density background plasma. By quantitatively calculating the variation of the magnetic bubble and the bubble constraint in the motion of the debris cloud, studied are the influences of the background plasma charge density, the background ion atomic weight, and the charge-mass ratio of the fragment ion on the magnetic bubble. The results show that the background charge density has an important effect on the motion of the bubble and debris cloud. In the early stage of the debris cloud expansion, the background ion atomic weight has little effect on the bubble expansion, but it affects the movement of debris cloud at a later time. When the charge-mass ratio of the debris ion is small, the radius of the ion gyration is larger than the bubble radius, and the bubble radius is small so that it cannot restrain the debris cloud. When the charge-mass ratio of the debris ion is large, the radius of the ion gyration is smaller than the bubble radius. If the background charge density is low in this condition, the early expansion of the bubble and the debris cloud are slightly affected by the charge-mass ratio of the fragment ions, and only the subsequent evolution of the system is influenced. If the background charge density is large in this condition, the charge-mass ratio of the fragment ion has a prominent influence on the motion of the bubble and the debris cloud.

Modelling of tungsten contamination and screening in WEST plasma discharges

The WEST experiment is currently operating with tungsten plasma-facing components and testing ITER-like divertor monoblocks. In order to support WEST experiments interpretation, numerical analyses were carried out. Starting from WEST experimental data, realistic background plasma conditions were reproduced through SolEdge-EIRENE and used as input for ERO2.0 simulations to investigate tungsten migration. Tungsten contamination due to the different plasma-facing components was modelled under different plasma conditions, highlighting a non-negligible contribution of tungsten coming from the tokamak main chamber. Tungsten penetration factor was computed and used as an indication for tungsten screening by the background plasma at the different tokamak plasma-facing components. Simulations showed the main chamber components to be very weakly screened. Light impurities charge was showed to influence not only tungsten sputtering, but also its probability to enter the confined plasma. Simulations results indicated that even when the tungsten source is not heavily influenced by self-sputtering, contamination of the confined plasma can be strongly impacted by it in low density background plasma conditions. Finally, a one-to-one comparison between tungsten visible spectroscopy at the lower divertor from experimental data and from synthetic diagnostics was performed, showing that it is possible to reproduce a realistic lower divertor signal following experimental evidence on light impurities asymmetry between the targets.

Formation, Focusing and Transport of Highintensity, Low-Energy Metal Ion Beams

High-intensity implantation of low-energy ions into various materials demonstrates the formation of long, ionalloyed layers having a thickness of tens and hundreds of micrometers. A deep incorporation of alloying elements in materials is achieved, first of all, due to a stronger radiation-induced ion diffusion at ultrahigh current densities and radiation fluence ranging from 1020 to 1022 cm2, when the diffusion coefficient exceeds its classical value derived from Arrhenius theory by several orders of magnitude. Nevertheless, the generation of low-energy ions with energies of several or a few kiloelectronvolts at high current densities of several hundreds of milliamperes per square metre and their effective transport, is a sophisticated problem. The paper studies the ballistic focusing of high-intensity pulsed ion beams at 2 kV accelerating voltage, 800 μs pulse time, 0.8 duty cycle, and their propagation through the preliminary injected low-density background plasma.

Формирование, фокусировка и транспортировка высокоинтенсивных пучков ионов металлов низкой энергии

Формирование, фокусировка и транспортировка высокоинтенсивных пучков ионов металлов низкой энергии

Generation and dissipation of runaway electrons in ASDEX Upgrade experiments

The paper describes under which plasma and machine conditions runaway electrons (REs) are generated during ASDEX Upgrade plasma disruptions. The REs are created by argon injection to investigate methods of current and energy dissipation. The RE beams are stable and last up to a few hundred milliseconds. The experimental findings are described and made available for the validation of theoretical models. In the following, a simple 0D fluid model is used to simulate the observed RE current magnitude and time behavior. In spite of its simplicity, the model is consistent with the measured RE current. The injection of heavy gases into the RE beam is then discussed in some detail: the injected gas penetrates into the low density background plasma and through the beam without ionizing significantly. Therefore its effect on the REs can be tested directly and it is found to be consistent with the known Coulomb collisional theory.

Runaway electron beam dynamics at low plasma density in DIII-D: energy distribution, current profile, and internal instability

Parameters of the post-disruption runaway electron (RE) beam in the low density background plasma achieved after secondary deuterium injection are investigated in DIII-D. The spatially resolved RE energy distribution function is measured for the first time during the RE plateau stage by inverting hard x-ray bremsstrahlung spectra. It has maximum energy up to 20 MeV and a non-monotonic feature at 5–6 MeV observed only in the core of the beam supporting the possibility of kinetic instabilities. Results of Fokker-Plank modelling qualitatively support the formation of the non-monotonic distribution function. The RE current profile is reconstructed for the first time using the spatially resolved RE energy distribution. It is found to be more peaked than the pre-disruption plasma current, with higher internal inductance, suggesting preferential formation of REs in the core plasma or potentially a radially inward motion of REs. The accessed relatively low current (180 kA) RE beam is found to be MHD stable, likely due to its elevated safety factor profile. From this base stable equilibrium, an internal beam instability is accessed by ramping up the current. The instability leads to a sawtooth-like relaxation of the RE current profile, but drives no RE loss. An internal kink mode proposed as a candidate instability is supported by results of MARS-F modelling. Electron cyclotron emission (ECE) spectrum measured during the low density RE plateau is found to be bifurcated, with a break point at ≈ 100 GHz, suggesting resonant absorption of the ECE at low frequencies.

Observation of rapid frequency chirping instabilities driven by runaway electrons in a tokamak

We report the first observation of chirping instabilities driven by runaway electrons (REs) in a tokamak. The instabilities are accessed during the post-disruption RE beam stage in a low density background plasma ( m−3) on DIII-D. The chirping instabilities are observed when a decelerating loop voltage is applied to the RE beam. The frequency chirping is detected in two distinct frequency bands: 0.1–10 MHz and 30–80 MHz. The mode frequency increases linearly when the toroidal magnetic field sensed by the RE beam increases. The frequency chirps by 0.3–2.4 MHz on a timescale of 1 ms. Modification of the RE distribution function is directly measured during the chirping in the low-frequency band consistent with the hole–clump model for frequency chirping. The low-frequency instabilities also correlate with an increase of intermittent RE loss from the plasma. These observations provide a novel experimental platform for fundamental studies of nonlinear chirping. They also support continued investigation of opportunities to utilize kinetic instabilities for RE mitigation in a tokamak reactor.

Magnetic reconnection between colliding magnetized laser-produced plasma plumes.

Observations of magnetic reconnection between colliding plumes of magnetized laser-produced plasma are presented. Two counterpropagating plasma flows are created by irradiating oppositely placed plastic (CH) targets with 1.8-kJ, 2-ns laser beams on the Omega EP Laser System. The interaction region between the plumes is prefilled with a low-density background plasma and magnetized by an externally applied magnetic field, imposed perpendicular to the plasma flow, and initialized with an X-type null point geometry with B=0 at the midplane and B=8 T at the targets. The counterflowing plumes sweep up and compress the background plasma and the magnetic field into a pair of magnetized ribbons, which collide, stagnate, and reconnect at the midplane, allowing the first detailed observations of a stretched current sheet in laser-driven reconnection experiments. The dynamics of current sheet formation are in good agreement with first-principles particle-in-cell simulations that model the experiments.

Influence of background plasma on operation efficiency of high-power continuous wave (cw) gyrotrons

A slow wave non-linear single mode stationary theory describing influence of background plasma which occurs in the gyrotron cavity on gyrotron operation has been developed. It is shown that the main effects of a low density background plasma on efficiency are concerned with decreasing the mismatch between operational and cyclotron frequencies and decreasing the beam-wave coupling coefficient. Numerical calculations are performed for the 140 GHz 1 MW CW gyrotron developed in Karlsruhe for fusion applications. Interpretation of some experimental effects which are not yet clearly understood is addressed.

Relativistic plasma microwave electronics: Studies of high-power plasma-filled backward-wave oscillators

The area of relativistic plasma microwave electronics has only recently generated renewed interest. New experimental data are presented demonstrating that the presence of a low-density background plasma in a relativistic backward-wave oscillator leads to several beneficial effects, including (a) enhanced interaction efficiency (40%), (b) operation at very low and possibly zero guiding magnetic field, (c) tunability by controlling the plasma density, (d) high degree of spectral coherency, and (e) operation well above the vacuum limiting current.

Observation of plasma wake-field effects during high-current relativistic electron-beam transport.

Modulation of the beam current has been observed during ion-focused-regime (IFR) transport of a high-power relativistic electron beam propagating through a low-density background plasma. In this experiment, a 1.7-MeV, 1-kA, rise-time-sharpened electron beam is transported in a KrF-excimer-laser--produced IFR channel in trimethylamine gas. The IFR channel is immersed in a low-density plasma-filled transport tube. We present experimental measurements and computer simulations demonstrating modulation of this high-current relativistic electron beam near the low-density background plasma frequency.

Electric fields and double layers in plasmas

Various mechanisms for driving double layers in plasmas are briefly described, including applied potential drops, currents, contact potentials, and plasma expansions. Some dynamic features of the double layers are discussed. These features, as seen in simulations, laboratory experiments and theory, indicate that double layers and the currents through them undergo slow oscillations, which are determined by the ion transit time across an effective length of the system in which the double layers form. It is shown that a localized potential dip forms at the low potential end of a double layer, which interrupts the electron current through it according to the Langmuir criterion, whenever the ion flux into the double is disrupted. The generation of electric fields perpendicular to the ambient magnetic field by contact potentials is also discussed. Two different situations have been considered; in one, a low-density hot plasma is sandwiched between high-density cold plasmas, while in the other a high-density current sheet permeates a low-density background plasma. Perpendicular electric fields develop near the contact surfaces. In the case of the current sheet, the creation of parallel electric fields and the formation of double layers are also discussed when the current sheet thickness is varied. Finally, the generation of electric fields (parallel to an ambient magnetic field) and double layers in an expanding plasma are discussed.