A. Hirose

Rotating nonlinear magnetic islands in a tokamak plasma

The nonlinear dynamics of rotating low m (poloidal mode number) tearing modes in a tokamak with external resonant magnetic perturbations is examined. Nonlinear evolution equations for the island width and the toroidal rotation frequency are derived within the two‐fluid magnetohydrodynamic model, taking into account the plasma rotation and neoclassical parallel viscosity. The nonlinear stability of magnetic islands interacting with a static external magnetic perturbation is considered, and the critical magnetic field for the appearance of a locked mode is determined. It is shown that the coupling of the perpendicular and longitudinal plasma flow due to the neoclassical plasma viscosity enhances the amplitude of the critical magnetic field compared to the value obtained in a slab approximation. The perpendicular plasma viscosity causes a finite phase shift between the applied external field and the magnetic island, and further increases the value of the critical magnetic field required to induce a magnetic ...

Electromagnetic and kinetic effects on the ion temperature gradient mode

Effects of finite beta , ion kinetic damping and trapped electrons on the toroidal ion temperature gradient (ITG) mode have been investigated by two methods-a fully toroidal fluid analysis corrected for ion Landau damping, and an electromagnetic local kinetic dispersion relation. When trapped electrons are ignored, the ITG mode is stabilized at a beta value well below the critical beta for the ideal MHD ballooning mode (βMHD). Trapped electrons are destabilizing and increase the upper limit of beta to a level comparable with βMHD. Ion Landau damping increases the critical temperature gradient typically by a factor of two (LT/LB 0.18), and the growth rate remains smaller than the ion transit frequency, ωTi = k||vTi

Nonlinear evolution of Buneman instability

The nonlinear evolution of one‐dimensional electron‐ion two‐stream instability in a field‐free plasma is studied both analytically and numerically (computer simulation). The instability is dominated by the fastest growing mode and its harmonics, provided that the initial fluctuation level is sufficiently small. A nonlinear dispersion relation is derived and solved numerically, taking into account; (a) the frequency and growth rate modulation, (b) the electric field up to ‖Ek‖4, and (c) the ’’renormalized’’ particle distribution functions. The model can successfully explain the results of a computer simulation, particularly the presence of an algebraic growth stage following the breakdown of the exponential linear growth, the appearance of harmonics, and the final saturation level. The minimum conductivity found scales as (M/m)0.61wpe, where M/m is the ion/electron mass ratio.

Growth mechanism and orientation control of well-aligned carbon nanotubes

Abstract Carbon nanotube films have been synthesized on nickel coated silicon wafer substrate by hot filament chemical vapor deposition under various conditions in a CH 4 /H 2 gas mixture. Randomly oriented nanotubes have been obtained with no discharge or with dc glow discharge using the substrate holder as the anode. By initiating a dc glow discharge with the substrate holder as the cathode, well-aligned carbon nanotubes with various orientation angles have been achieved. The nanotubes located far enough from the sample edges are completely vertically aligned, while the nanotubes around the sample edges point away from the sample center and the alignment angle depends on the growth conditions and the distance from the sample edge. The alignment orientation of the nanotubes appears to be determined by the direction of the electric field lines on the sample surface.

Observation of hot electrons produced by second harmonic electron cyclotron heating in the axisymmetric tandem mirror GAMMA 10

Microwave power, PECH ≤ 140 kW, has been injected at 28 GHz into the axisymmetric plug/barrier cell in the axisymmetrized tandem mirror GAMMA 10. As observed by soft X-ray measurements, the microwaves generate a hot (50-60 keV) electron population, radially peaked on the magnetic axis, which results in the formation of a thermal barrier. The production mechanism of these hot electrons is found to be second harmonic electron cyclotron heating (ECH), corrected for the effects of the relativistic mass variation and the Doppler shift. This mechanism also explains the first experimental observation of a saturation of the single-component hot electron temperature Teh as being caused by the finite width of the incident microwave lobe. The dependence of the plasma parameters on the filling gas pressure, the plasma density and the ECH power is studied. It is found that the heating process can be interpreted as a competition between electron acceleration by the incident wave, electron deceleration by collisions, and the mirror trapping efficiency of the source electrons for hot electrons. The axial profile of the soft X-rays is investigated in relation to the mechanism of the second harmonic ECH. The heating process is discussed in terms of the electron pitch angle and the magnetic field intensity.

Ultrathin W-Al dual interlayer approach to depositing smooth and adherent nanocrystalline diamond films on stainless steel.

The adherence of diamond coated on steel is commonly low and needs to be strengthened with thick intermediate layers. In this paper, a nanoscale W-Al dual metal interlayer has been applied on SS304 substrates to facilitate deposition of continuous, adherent and smooth diamond thin films. During the microwave plasma-enhanced chemical vapor deposition process, the Al inner layer 30 nm thick diffuses into steel surface inhibiting carbon diffusion and graphitization. The W outer layer 20 nm thick is transformed into W carbides, both preventing carbon diffusion and enhancing diamond nucleation. The diamond films synthesized are of high purity and have smooth surfaces and dense structures. Indentation and shear deformation tests indicate high delaminating tolerance of the diamond films.

Stable AC tokamak discharges in the STOR-1M device

Stable and clean AC tokamak discharges have been achieved in the STOR-1M device. The plasma current is reversed from +4.1 kA to −4.0 kA within 1.9 ms. During the reversal, no disruptive behaviour is observed, the loop voltage changes smoothly from +1 V to −5 V without any spike, and impurities are not released. An electron density of (2–4) × 1012 cm−3 is maintained during current reversal. The possibility of continuous tokamak operation with a low frequency alternating plasma current is discussed.

Experimental demonstration of tokamak fuelling by compact toroid injection

The most promising concept for deep fuelling a reactor is by the injection of compact toroid (CT) plasmoids. The first results showing CT fuelling of a tokamak plasma, without any adverse perturbation to the tokamak discharge, are reported. The Compact Toroid Fueller (CTF) device was used to inject a CT-spheromak plasmoid into the TdeV tokamak. Following the CT penetration, the tokamak particle inventory increased by 16%, the loop voltage and the plasma current did not change, and there was no increase in magnetohydrodynamic (MHD) activity. The number of injected impurities was low and dominated by non-metallic elements. The plasma diamagnetic energy and the energy confinement time increased by more than 35%

Electrostatic instabilities driven by currents perpendicular to an external magnetic field

High-frequency (ω ωci) electrostatic instabilities driven by an electron-ion relative drift velocity across an external magnetic field are discussed for an inhomogeneous plasma. The presence of a density gradient perpendicular to the magnetic field has been found to greatly reduce critical velocities for the onset of several instabilities such as lower hybrid oscillations and ion cyclotron harmonic waves. It has been also shown that the lower hybrid instability can be caused by some positive dissipation mechanism such as Landau damping due to thermal ions and a finite electron collision rate. Some applications have been made to turbulent heating experiments in plasmas in which cross-field currents are present.

Expansion of an electron cloud

Abstract An analytical expression for the expansion of a thermalized electron cloud is obtained.