S. Hiroe

Radio-frequency plugging of a high density plasma

Mirror end loss can be suppressed by applying an rf field at the mirror throat through low impedance coils. This method is verified to be effective for a high density plasma up to 1014 cm−3. Experimental results show that the rf field strength required for the plugging is dependent on the plasma density. The dependence differs according to the geometry of the coil. The mechanism of giving the density dependence is theoretically clarified for each coil. Particularly, it is shown that the electric field induced in the direction of the static magnetic field is intrinsic for the result of type‐III coil where its efficiency for rf plugging is insensitive to the plasma density.

Theory and experiment on radio‐frequency plugging of magnetically confined plasma in an open‐ended system

The theory of the rf plugging of a magnetically confined plasma in an open‐ended system is presented; it takes explicit account of the collective effect brought about by the dynamic screening action of the plasma. The theory predicts a maximum efficiency of the rf plugging at an optimum frequency corresponding to a zero of the longitudinal dielectric constant. The plugging efficiency is calculated as a function of the plasma density, the strength of the rf field, and the intensity of the magnetic field. These theoretical predictions are compared with experimetal observations, demonstrating satisfactory agreement in over‐all features. The results provide a useful scaling law associated with the concept of the rf confinement. The theory also offers an explanation for the discrepancy between the experimental values of the optimum frequency and those predicted by a conventional theory based on an individual‐particle model.

Dynamic Control of a Cusped Plasma near the Ion Cyclotron Frequency

An r f electric field near the ion cyclotron frequency is applied perpendicularly to the magnetic field of cusp regions in order to plug up cusp ends and to heat ions. A helium plasma is steadily fed from one of point cusps into the containment region whose density is varied from 1×10 8 to 1×10 11 cm -3 . For the line cusp loss good plugging is satisfied up to 1×10 11 cm -3 , but for the point cusp loss it is not obtained for the density above 1×10 10 cm -3 . Under the resonance condition the ion energies consist of two components both of which display Maxwellian-like distribution of T i of about 40eV and 150∼200eV respectively when E r f =100V/cm.

A slow-wave heating experiment on RFC-XX using an array of phased antennas

Experimental data for ion cyclotron resonance heating in the RFC-XX machine in IPP-Nagoya are presented. The achieved ion temperature is as high as 100 eV at n = 1013 cm−3 and 1 keV at n = 1012 cm−3. The ion energy confinement becomes worse by the application of a longer pulse, which is found to be due to the enhanced charge-exchange loss and/or electron drag. Axially and azimuthally arrayed antennas are used in the heating, and the importance of the phasing is demonstrated. A simple model of the multiple-antenna problem is also given and used to interpret the experimental data.

Experiment on nonadiabatic rf plugging in a cusped field

Under nonadiabatic conditions, rf plugging is accompanied by effective ion heating, which in turn causes increased particle losses from a cusp configuration. An empirical scaling relation for rf plugging of line cusp loss is obtained among the required rf electric field, the static magnetic field, and the plasma density, and is compared with the relation obtained by a linear theory.

Radio frequency heating of multi-ion species plasma

Experimental results of ion heating by the use of an electrostatic ion cyclotron wave are presented. When a plasma is composed of several ion species, it is possible to heat the desired one by selecting the frequency of the externally applied r.f. field. A theory of the r.f. plasma heating including the multiple ion species is also presented. A computation carried out for the appropriate experimental parameters agree with the experimental results.

Radio-frequency preferential plugging of multi-ion-species plasma in cusped magnetic devices

The r.f. preferential plugging of multi-ion-species plasma is studied theoretically and experimentally in a magnetic-cusp plasma. In a He+, N+ and plasma He+ is effectively plugged in, while N+ and are allowed to escape freely from a container by the applied r.f. field. In a H+, and plasma, all ion species are plugged in the container. These experiments can be explained by theory.

Excitation of obliquely propagating cyclotron harmonic waves

Obliquely propagating cyclotron harmonic waves are excited and their behavior is studied. The waves can be excited within the region that satisfies the condition, ω p 2 >ω 2 -ω c 2 . Contrary to the commonly accepted notion, it is confirmed that the cyclotron harmonic wave is in a propagating mode rather than in a standing mode. The wavenumbers in the direction parallel to the magnetic field lines are determined by the geometrical length of the exciter.

Radio-Frequency Heating Using the Electrostatics Cyclotron Wave

The theory of the plasma heating by the use of an electrostatic ion cyclotron wave is presented. It provides the optimum heating condition as the interrelation between the plasma density and the r f frequency. An experiment is carried out based on the theoretical prediction and the result agrees with the theory. The feedback control of the heating is also carried out with the afterglow plasma.

Excitation of Bernstein Mode in an Inhomogeneous Collisionless Plasma

The cyclotron harmonic waves propagating at right angles to the static magnetic lines of forces, are excited in an inhomogeneous plasma. The measured dispersion relations are in good agreement with a collisionless theory, not only qualitatively but also quantitatively. It is made clear that the excited waves propagate within the core of the plasma bounded by the upper hybrid (ω P 2 ( r )>ω 2 -ω c 2 ), and these waves are propagating rather than standing.