Takayoshi Okuda

Control of the tokamak edge plasma by static and rotating helical magnetic limiters

Rotation of a helical magnetic limiter in the poloidal and toroidal directions is shown to be a good method of avoiding the non-uniform wall loading that is obtained when a static local helical field is applied. Mechanisms of the static and dynamic modification of the plasma potential at the tokamak edge are discussed in terms of magnetic field structure, ambipolarity, electron transport along the stochastic magnetic fields and cross-field drift motion of plasma particles. The magnetic structure is studied by Poincare mapping of field lines traced numerically, using a code that accounts for the exact configuration of local helical coils.

ASYMMETRICAL TRIPLE PROBE METHOD FOR DETERMINING ENERGY DISTRIBUTION OF ELECTRON IN PLASMA

A new probe method is proposed. This method is an improvement of the floating triple probe method, which was proposed by us in a previous paper. In this method, the dimension of one operating probe is different from the other, so that the smaller probe behaves as the single probe. Thus, the measurable range of electron energy can be extended as much as that in the single probe method. The asymmetrical triple probe and double probe methods are verified experimentally. The asymmetrical triple probe method has a merit that a disturbance, which is inevitable in the single probe method, can be eliminated in this method. The electron energy distribution obtained by the new probe method is not always in accord with that obtained by the usual single probe method. The hump often observed at high energy range in the single probe method does not appear in the new method, illustrating that the latter is free from erroneous estimation due to the disturbance.

On the Floating Probe Method for the Measurement of Ionized Gas

Several problems connected with the floating probe method are discussed and solved. Analysing the variation of ion current due to sheath expansion, the authors obtain the criterion for applicability of the usual double probe method and also the general expression available for the estimation of electron temperature. From the relation between probe characteristic and electron energy distribution, a floating triple method is newly proposed. This method is useful for the measuring of energy distribution in electrodeless or h.f. discharges. By special use of the floating double or triple probe, the measurement of potential distribution in such a quasi-plasma as ion sheath and the transition region between plasma and ion sheath can be attained. As one example, the floating potential becomes measurable. The floating, or insulating, potential is discussed in detail and expression is given showing good agreement with measured values.

Electric and magnetic structure of an edge plasma in a tokamak with a helical magnetic limiter

A strong modification of plasma potential was observed at the plasma surface with the application of a helical field produced by a set of local helical coils. The structure of magnetic field lines was computed by a field line tracing code for a tokamak plasma. Such a field structure was examined in the experiment using electron beam probing. The potential formation and the magnetic structure are discussed referring to the scheme of an ergodic magnetic limiter.

In-situ boron coating and its removal by glow discharge processes

Abstract Low-Z thin films of boron/carbon were deposited on the entire inner surface of a toroidal vessel by a dc glow discharge in B2H6/CH4 mixtures. The Auger analysis of the film revealed that the B/C atomic ratio is readily controlled by adjusting the gas mixing ratio. The hydrogen content of the film can be reduced to an extremely low value ( 10 sccm) of diborane. A simulation experiment of hydrogen recycling in the boron-coated toroidal vessel was performed. A strong wall pumping was observed after the boron coated wall had been conditioned with a helium glow discharge. The removal of the pure boron film with a hydrogen glow discharge was demonstrated. The measurement of total sputtering yield as a function of temperature gave an activation energy of 0.12 eV, which is comparable to that of amorphous carbon films. The residual gas analysis during the hydrogen discharge suggests the chemical process of diborane formation enhanced by energetic hydrogen ions.

Control of hydrogen content of boron thin films produced in a dc toroidal discharge

Hydrogenated amorphous boron films have been produced by a toroidal glow discharge of B2H6/He mixtures at low pressures (10−2 Torr). Chemically stable pure films with a low hydrogen content can be formed at a moderate substrate temperature (∼200 °C) when the diborane flow rate is raised above 10 sccm with the discharge power lowered below 100 W. The controllability of H content is potentially interesting for applications in the coating of nuclear fusion devices, as well as for use in electronic devices.

Two-Photon Ionization of Cesium and Sodium Vapors

The breakdown and the growth of plasma of cesium and sodium vapors by an intense laser beam are investigated. The minimum intensity which is observable by a double probe is found to be inversely proportional to the root of the neutral atom density for either vapor. This result confirms that the two-photon ionization is a dominant process in the stage of breakdown. Further increase of the intensity of laser beam leads to a steep rise of the plasma density proportional to the square of the intensity followed by a slow rise and then the second steep rise. The first steep rise is explained by the two-photon ionization process too. The electron temperatures are estimated to be about 10 4 °K under the condition of the present work, by the probe and optical methods.

Electric and magnetic structure of tokamak edge plasma with static and rotating helical magnetic limiter

Abstract Modification of tokamak edge plasma by an ergodic magnetic limiter is studied. Field line structures disturbed by a series of local helical coils are precisely investigated to make clear the presence of a distinct structure with the periodicity of the local helical coil near the wall as well as the presence of a chaotic region somewhat away from the wall. The enhancement of recycling due to the increase in plasma transport at the edge was confirmed with a two-dimensional profile of Lyα emission obtained by computed tomography. Poloidal modulations of the edge plasma parameters were detected with a Langmuir probe, and compared with the field line structure. It was found that such a nonuniform wall loading could be avoided by using a rotating helical field with a pair of local helical coils driven by an orthogonal ac current.

Ergodic magnetic limiter experiments in the HYBTOK-II tokamak

Abstract The effects of the ergodic magnetic limiter (EML) on edge modification, hydrogen recycling and impurity behavior are investigated in the HYBTOK-II tokamak. A plasma modification at the edge with the EML is confirmed by measuring the edge plasma parameters. An enhancement of recycling with the EML is demonstrated using computed tomography. Emissions from metal ions were reduced, while those from metal atoms and light impurities were increased with the EML. The reasons of this different effect on impurity behavior are discussed, based on an impurity control modeling in the EML, the magnetic field structure and a simple calculation on the penetration of neutral atoms into plasma.

Experimental observation of electrostatic ion cyclotron waves modified by minority ions

Abstract Electrostatic ion cyclotron waves in the presence of light minority-species ions are experimentally investigated. The observations reveal that already a small fraction (∼1%) of the minority concentratios leads to a new cutoff and resonance near the minority-cyclotron-resonance frequency.