Akio Komori

HELICON WAVES AND EFFICIENT PLASMA PRODUCTION

Helicon waves generated by radio‐frequency (rf) waves are experimentally demonstrated to have the characteristics of Landau damping, as predicted theoretically, and fully ionized plasmas are realized by this efficient coupling of rf powers to plasmas. Excited waves are identified as a helicon wave by measuring wavelengths in the plasma along the magnetic field and comparing with the dispersion relation. Good agreement is found between experimental and theoretical results.

Chaotic behavior of current-carrying plasmas in external periodic oscillations

A set of cascading bifurcations and a chaotic state in the presence of an external periodic oscillation are experimentally investigated in a current-carrying plasma. The measured bifurcation sequence leading to chaos, which is controlled by changing plasma densities and the frequencies of external oscillations, is in qualitative agreement with a theory which describes anharmonic systems in periodic fields.

Instabilities associated with a negative rf resistance in current‐carrying ion sheaths

Coherent instabilities on the order of the ion plasma frequency are generated by applying a dc voltage between a meshed grid and a disk target in an unmagnetized plasma. The instability occurs in the ion‐rich current‐carrying sheath on the negatively charged grid when some ions are reflected from the grid to the sheath edge by the potential difference between two plasmas divided by the grid. The exciting mechanism of the instability has been identified as a negative rf resistance associated with the ion inertia in the ion‐rich sheath, coupled to an ion resonance caused by a positive feedback due to the reflection of ions in the sheath region. The frequency of the instability, which is proportional to the plasma density, is basically determined by the ion transit time through the sheath, and thus, is proportional to the product of inverse sheath thickness and average ion velocity related to the applied voltage.

Production of a Large-Diameter Uniform ECR Plasma with a Lisitano Coil

A large-diameter uniform plasma was demonstrated to be produced by electron cyclotron resonance heating with a slotted Lisitano coil of 40 cm diameter. The diameter of the realized uniform plasma was found to be almost equal to that of the Lisitano coil. It was also suggested that the microwave is propagated in the whole region inside the Lisitano coil, and produces the uniform plasma.

In situ calibration of neutral beam port-through power and estimation of neutral beam deposition on LHD

The neutral beam (NB) shine-through profile is routinely monitored on the Large Helical Device (LHD) both to calibrate the port-through power of the NB and to evaluate the NB-deposition power to LHD plasmas. The profile is measured with a calorimeter (CM) array on an armor plate of the NB counter wall inside the LHD vacuum vessel. An infrared camera is also used to check the beam profile where CMs are not located, and measures a temperature increase of the armor plate due to the NB heat load. The measured beam profile is compared to the calculated NB profile at the armor plate. The measurement indicates that the beam is not uniform at the exit of the ion source and that the steering angle of the beam in the horizontal direction is not the same as the designed value. It is found that the monitoring of the NB shine-through profile is important to estimate the NB port-through power and the NB deposition power, especially when the neutral beam injector (NBI) is based on a large negative-hydrogen ion source.

Bifurcations and chaos in a current-carrying ion sheath

Cascading bifurcations to chaos are investigated experimentally and theoretically in a current‐carrying stable plasma. A dc plasma current is required to produce an electron‐depleted thick sheath on a grid, which obeys the Child–Langmuir law of space‐charge‐limited current in a diode. Bifurcation cascade and chaotic behavior are exhibited when an external periodic oscillation is applied to the grid, and are in good agreement for the first time with a theory, which describes ion dynamics in the Child–Langmuir sheath and is represented by the differential equation with three independent variables. A fractal dimension predicted by the theory is verified by the experiment.

Chaotic behavior driven by an external periodic oscillation in a current‐carrying unstable ion sheath

A quasiperiodic route to chaos is investigated experimentally and theoretically in a current‐carrying unstable ion sheath. A quasiperiodic regime with two frequencies is realized by a coherent instability and an external periodic oscillation. Chaos appears directly from the quasiperiodic regime when the amplitude of the external oscillation is increased, making nonlinear effects more important. A correlation dimension and the largest Lyapunov exponent, obtained experimentally in the chaotic regime, confirm that the observed chaos is deterministic and the attractor is strange. A period‐doubling route to chaos is also observed after the instability is quenched with the external oscillation by increasing the amplitude of the external oscillation further. These experimental results are in fair agreement with a theory that describes ion dynamics in the ion sheath.

A theory of bifurcations and chaos observed in an ion sheath

A theory of cascading bifurcations to chaos observed in a current‐carrying ion sheath [Komori et al., Bull. Am. Phys. Soc. 36, 2408 (1991)] is developed. The ion sheath on both sides of a grid forms a potential well in which ions oscillate to give a primary motion, being subject to cascading bifurcations to chaos when an external oscillation field is applied to the grid. A model equation is derived to describe ion dynamics in the ion sheath which obeys the Child–Langmuir law for a large potential drop. The numerical solutions are shown to recover the observations.

Cascading Bifurcations to Chaos in a Current-Carrying Ion Sheath

A theory is developed to explain cascading bifurcations to chaos observed in a current-carrying ion sheath. The basic equation is derived to describe ion dynamics in an ion sheath which obeys the Child-Langmuir law for a large potential drop and is numerically solved to show the same bifurcation sequences as those in the experiments.

Initial results of local island divertor experiments in the large helical device

Abstract A local island divertor (LID) experiment has begun in the Large Helical Device (LHD) to demonstrate improved plasma confinement, and fundamental LID functions were demonstrated in the sixth experimental campaign in 2002–2003. It was clearly shown that when an m/n = 1/1 island is generated by adding a resonant perturbation field to the LHD magnetic configuration, the particle flow is guided along the island separatrix to the backside of the island, where carbon plates are located on a divertor head. The particles recycled there are pumped out efficiently so that the line-averaged core plasma density is reduced by a factor of ~2 at the same gas puff rate, compared with non-LID discharges. Obvious improvement of the global plasma confinement was, however, not observed yet, because the discharge could not be optimized, due to a large amount of outgas from the divertor head to the core plasma. The size of the divertor head was found to be larger than the optimum one; hence, the core plasma impacted slightly on the core plasma-facing portion of the divertor head with which the core plasma was not expected to collide.