International collaboration on development of a stellarator confinement database has progressed. More than 3000 data points from nine major stellarator experiments have been compiled. Robust dependences of the energy confinement time on the density and the heating power have been confirmed. Dependences on other operational parameters, i.e. the major and minor radii, magnetic field and the rotational transform , have been evaluated using inter-machine analyses. In order to express the energy confinement in a unified scaling law, systematic differences in each subgroup are quantified. An a posteriori approach using a confinement enhancement factor on ISS95 as a renormalizing configuration-dependent parameter yields a new scaling expression ISS04; . Gyro–Bohm characteristic similar to ISS95 has been confirmed for the extended database with a wider range of plasma parameters and magnetic configurations than in the study of ISS95. It has also been discovered that there is a systematic offset of energy confinement between magnetic configurations, and its measure correlates with the effective helical ripple of the external stellarator field. Full documentation of the International Stellarator Confinement Database is available at http://iscdb.nifs.ac.jp/ and http://www.ipp.mpg.de/ISS.

https://iopscience.iop.org/article/10.1088/0029-5515/45/12/024/pdf

Characterization of energy confinement in net-current free plasmas using the extended International Stellarator Database

The paper examines the prospects of using laser plasma as a source of high-energy ions for the purpose of hadron beam therapy — an approach which is based on both theory and experimental results (ions are routinely observed to be accelerated in the interaction of high-power laser radiation with matter). Compared to therapy accelerators like synchrotrons and cyclotrons, laser technology is advantageous in that it is more compact and is simpler in delivering ions from the accelerator to the treatment room. Special target designs allow radiation therapy requirements for ion beam quality to be satisfied. (reviews of topical problems)

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Laser ion acceleration for hadron therapy

The propagation of ion-acoustic solitons in a plasma with negative ions has been observed. For sufficiently large concentration of negative ions, applied rarefactive (negative) voltage pulses break up into solitons, whereas compressive pulses evolve into wave trains, with exactly the opposite behavior as that for a plasma composed only of positive ions. There is a critical value of the negative-ion concentration for which a finite-amplitude pulse propagates without steepening.

Observation of ion-acoustic rarefaction solitons in a multicomponent plasma with negative ions

During the last decade, significant progress has been made in the field of pellet injection with (1) the identification of the drift of the deposited material in the inhomogeneous magnetic field that opened the possibility of fuelling efficiently the plasmas from the high-field side of the torus, (2) the technique to mitigate ELMS in H-mode discharges with shallow pellet injection at high frequency and (3) with the development of high density, high performance scenarios close to the ITER requirements. Both the experimental and theoretical aspects of this domain are reviewed in this paper.

https://iopscience.iop.org/article/10.1088/0741-3335/49/8/R01/pdf

Review: Pellet injection experiments and modelling

Low‐frequency electrostatic turbulence generated by the ion–ion beam instability was investigated experimentally in a double‐plasma device. Real time signals were recorded and examined by a conditional statistical analysis. Conditionally averaged potential distributions reveal the formation and propagation of structures with a relatively long lifetime. Various methods for making a conditional analysis are discussed and compared. The results are discussed with reference to ion phase space vortices and clump formation in collisionless plasmas.

Conditional Eddies in Plasma Turbulence

The expansion of a collisionless plasma with negative ions into a vacuum is considered. The flow pattern involves a rarefaction shock or a wave with an expanding front, two plateau-like regions with a discontinuity between them and a rarefaction wave. The ranges of the mass and density ratios of negative to positive ions for the existence of the rarefaction shock are found. A simple model is in a good agreement with simulation data.

https://iopscience.iop.org/article/10.1088/0741-3335/41/2/015/pdf

Expansion of a plasma with negative ions into a vacuum

The plasma motion near the ion front in an expanding collisionless plasma is studied. The boundary of quasineutrality, the range of application of the analytical quasineutral solution and the features of the space charge region are considered. Analytical solutions in combination with computer simulations give the quantity distributions in space and time in the analytical forms. The ion temperature effect and the electron cloud evolution are discussed.

https://iopscience.iop.org/article/10.1088/0741-3335/53/12/125007/pdf

Ion front in an expanding collisionless plasma

The expansion of a finite plasma into a vacuum is studied both analytically and numerically. The universal solution independent of the initial plasma size is discussed. In the case of cold ions the problem is solved completely. The region where two rarefaction waves interact and its boundaries with the rarefaction waves are described. It is found that the boundaries represent weak discontinuities. The effect of the initial ion temperature on the process is illustrated.

Expansion of a finite plasma into a vacuum

The evolution of a hump-like density disturbance in a collisionless plasma is considered. The effect caused by the initial shape of the disturbance is demonstrated. In the case of cold ions the development of a collisionless shock wave is observed over a long period of time. The formation of separate ion-acoustic solitons, their reflection from a wall and passage through each other are described. Also the ion temperature effect on the flow is studied. The ion reflection and the formation of a two-stream structure are illustrated.

Evolution of a density disturbance in a collisionless plasma

The possibility of hot dense plasma creation for the filling of open traps by thermalization of the direct energy of supersonic plasma clusters accelerated by electrodynamic accelerators has been studied theoretically and experimentally. The mean free path of particles in these traps, as a rule, exceeds typical system size, therefore, it is of great interest to study the turbulent mechanisms of counter-streaming plasmas relaxation. A calculation of relaxation length is very important because such turbulent relaxation of counter-streaming plasmas is observed experimentally. Investigation of counter-streaming plasma interaction is of general physical concern. Such situations occur, for example, in astrophysics: the shock wave structure problem, the relaxation of ionosphere plasma flow behind satellites, the spreading of laser-produced plasma in surrounding gas, etc. A detailed discussion is given of the relaxation of counter-streaming plasma due to turbulent mechanisms: the two-stream ion-acoustic instability.

Yu V Medvedev

Papers

Expansion of a plasma with negative ions into a vacuum

Ion front in an expanding collisionless plasma

Expansion of a finite plasma into a vacuum

Evolution of a density disturbance in a collisionless plasma

Interaction of counter-streaming plasmas