Advances in the subject of plasma containment in cusp-shaped magnetic fields over the past six or more years are reviewed. Particular emphasis is placed on the structure and thickness of the sheath separating a field-free plasma from a vacuum magnetic field. Recent experimental results suggest possibly that the thickness could be the geometric mean of the electron and ion Larmor radii, compared to earlier results in which the thickness measured was nearer the ion Larmor radius. The electric field in the sheath is important not only in determining its structure but also in problems associated with flow, end shorting, losses, and plasma rotation and stability. The present status of radio-frequency and electrostatic plugging of the ends is presented. Liner implosion to obtain extreme, and perhaps optimal, conditions for controlled fusion is a new aspect of this subject which deserves some attention. Hybrid schemes which employ cuspshaped magnetic fields as an essential part of their containment configuration include the linear picket fence, the spherical multipole magnetic trap, the stuffed cusp, the caulked stuffed cusp, the caulked cusp torus (CCT), the toroidal line cusp (or Tormac), and the containment of a flowing plasma in toroidal multiple cusps (the Polytron). These are reviewed, and their advantages and disadvantages for fusion are discussed.

http://iopscience.iop.org/article/10.1088/0029-5515/17/4/015/pdf

Plasma containment in cusp-shaped magnetic fields

Emissive Langmuir probes are used to map the plasma potential in the neighborhood of a picket fence cusped magnetic field. Positive potential ’’islands’’ are found in front of the picket fence conductors showing ion confinement by electrostatic self‐plugging of the cusp leaks. This mechanism results in channels whose widths are the order of the hybrid cusp leak widths recently reported.

Electrostatic self‐plugging of a picket fence cusped magnetic field

A single‐cell Lagrangian model has been developed for calculating the ionization and expansion dynamics of high‐density clouds in magnetic fields or in magnetically confined plasmas. The model was tested by means of data from magnetospheric barium cloud experiments and approximately reproduced such global characteristics as expansion rate, stopping radius, stopping time, and magnetic cavity lifetime. Detailed calculations were performed for hydrogen clouds associated with the injection of frozen hydrogen pellets into tokamak plasmas. The dynamic characteristics of the cloud expansion, such as ionization radius, stopping time, lifetime, oscillation frequencies, and amplitudes, etc., are computed as functions of the magnetic field strength, the background plasma temperature, and the cloud mass. The results are analyzed and compared with experimental observations.

Expansion of dense particle clouds in magnetically confined plasmas

Etude theorique de l'influence des electrons primaires rapides sur la largeur de perte d'un plasma dans une geometrie palissade ou cuspidee en presence de gaz neutre

The role of fast electrons for the confinement of plasma by magnetic cusps

The influence of fast primary electrons on the leak width of a plasma in a picket fence or cusp geometry in the presence of a neutral gas background is discussed theoretically. Without primary electrons a leak width of the order of an ion Larmor radius (ai) is found, whereas with fast primary electrons a leak width of the order of a hybrid width (2 square root aiae) can be obtained. This is in agreement with experimental data in the literature.

http://iopscience.iop.org/article/10.1088/0741-3335/26/2/004/pdf

Preliminary results of the experimental determination of the particle loss aperture sizes of the spindle cusp magnetic bottle for laser‐produced high‐beta deuterium plasmas are presented. The fully ionized dense plasma (with maximum density ∼1016 cm−3 and ion energy ∼200 V) is produced at the null‐field center of the magnetic bottle, from a freely falling isolated deuterium ice pellet by a focused giant laser pulse. In preparation, the free expansion of laser‐produced plasma has been studied experimentally, and the results indicate that the plasmas produced can maintain the fully ionized condition with prescribed plasma parameters until it expands to a certain prescribed volume if the combination of the physical parameters is properly chosen by considering the electron‐ion recombination as one of the important factors. The experimental results of cusp confinement for the plasma thus produced suggest that the loss aperture size of the ring cusp is much smaller than the local ion gyroradius, in sharp contra...

Cusp confinement of high‐beta plasmas produced by a laser pulse from a freely‐falling deuterium ice pellet

The equipment producing a cusp type magnetic field, which is used to trap high temperature plasma, by a pair of superconducting magnets, was constructed.The working space of a cusp field is 20mm between the two magnets and 10mm in the inner diameter of each magnet. The rated magnetic field at the center of each coil is 40kG. The maximum tested field is 55kG. Each superconducting coil is sealed in a liquid helium vessel, to which liquid helium is supplied from a liquid helium supplying vessel. The capacity of the liquid helium supplying vessel is about 5l, and the average helium loss is about 1.8l/h at 40kG of the field strength.

https://www.jstage.jst.go.jp/article/jcsj1966/3/5/3_5_214/_pdf

Akio Kitsunezaki

Papers

Cusp confinement of high‐beta plasmas produced by a laser pulse from a freely‐falling deuterium ice pellet

Superconducting Magnet Producing a Cusp Type Magnetic Field for High Temperature Plasma Trapping