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The electrical insulation of the Megavolt ITER Injector and Concept Advancement (MITICA) beam source (BS) at 1 MV in vacuum is a challenging issue, which could not be fully addressed so far on the basis of experimental results and of theoretical models available in literature. A specific high-voltage (HV) test campaign is being prepared to validate and optimize the voltage holding capability of the BS insulation under realistic conditions, using full-size mockup electrodes reproducing in detail the geometry of the BS and accelerator. The proposed test strategy will address both the single-gap and the multistage insulation, so as to obtain a verification of voltage holding at 1 MV before the installation of the real components. This approach is intended to reduce the risk related to the HV insulation at 1 MV and, if necessary, to allow the development of effective corrections. In this article, the test motivations and requirements are defined, and the electrode implementation and diagnostic setup are described. Finally, the test configurations and the experimental procedure are discussed.

Dynamics of a fast-flowing plasma through a magnetic mirror field was investigated. A highly-ionized, high-density, He plasma produced by a quasi-steady MPD arcjet (MPDA) was injected into a magnetic mirror. In a uniform magnetic field region, ion acoustic Mach number (Mi) was almost unity, while in a diverging field region the Mach number increased up to 2-3. When the supersonic plasma flows into a converging field region, a shock-like structure was formed. The subsonic flow downstream of the shock was re-accelerated up to Miof 2-3. The sonic condition (Mi=1) is satisfied at the magnetic mirror throat as in a conventional Laval nozzle. The adiabatic exponent of ions was evaluated by comparing measured spatial profiles with the prediction from 1D isentropic model.

A higher specific impulse and a larger thrust are required for a manned interplanetary space thruster. Until the realization of a fusion-plasma thruster, a magneto-plasma-dynamic arcjet (MPDA) powered by a fission reactor is one of the promising candidates for a manned Mars space thruster. The MPDA plasma is accelerated axially by a self-induced j× Bforce. Thrust performance of the MPDA is expected to increase by applying a magnetic nozzle instead of a solid nozzle. In order to get a much higher thruster performance, two methods have been investigated in the HITOP device, Tohoku University. One is to use a magnetic Laval nozzle in the vicinity of the MPDA muzzle for converting the high ion thermal energy to the axial flow energy. The other is to heat ions by use of an ICRF antenna in the divergent magnetic nozzle. It is found that by use of a small-sized Laval-type magnetic nozzle, the subsonic flow near the muzzle is converted to be supersonic through the magnetic Laval nozzle. A fast-flowing plasma is successfully heated by use of an ICRF antenna in the magnetic beach configuration.

A supersonic plasma is produced quasi-steadily by use of a magneto-plasma-dynamic arcjet (MPDA) in various shapes of an external magnetic field configuration. An ion acoustic Mach number Miof the plasma flow is limited to be nearly unity in a uniform magnetic field configuration, while it increases up to almost 3 in a divergent magnetic nozzle configuration. Spatial variations of Miis well predicted by an isentropic model for a compressible gas. The Mach number decreases in the far downstream region due to charge-exchange collisions between flowing ions and neutral atoms which are produced through surface-recombination on the end wall. Ion heating of the fast flowing plasma has been successfully demonstrated for the first time. This success is mainly due to the plasma density is high enough to reduce the penetration of neutral gases which cause the charge-exchange energy loss. It is found that an asymmetric RF wave with an azimuthal mode number m= ± 1 is most effective to heat the ions.

At both end cells of GAMMA 10, two of Magneto-Plasma-Dynamic Arcjet (MPDA) are installed. MPDAs consists of two coaxial electrodes (tungsten stick and molybdic cylinder), and can produce quasi-stationary hydrogen plasmas. In GAMMA 10, the plasma generated by the MPDAs are used as the seed plasma. The main plasma is produced by ion cyclotron range of frequency (ICRF) waves in combination with the hydrogen gas injection. In the center at r=0 near the mirror throat of plug/barrier cell, the flow velocity and the mach number are measured with a mach probe in cases of several discharge voltages. The flow velocity and the mach number increase with the discharge voltage. The mach number of the seed plasma and the main plasma (from the confined region) is also measured in the radial direction. The flow velocity and the mach number of the seed plasma become small in the core region than in the peripheral region.

Calibration of an up-down type Mach probe is performed using a fast-flowing plasma produced by a magneto-plasma-dynamic arcjet. Mach probe data are compared with ion acoustic Mach numbers Mi, which are calculated using a plasma flow velocity Upand an ion temperature Timeasured by spectroscopy and electron temperature Teby Langmuir probe. The obtained data are also compared with Hutchinson’s PIC simulation results in an unmagnetized plasma and are in good agreement with each other. First attempts to measure plasma flow field using a Mach probe are performed at the open-end section in GAMMA10. It is found that Miat r=0 is more than 2, namely a supersonic plasma flow is formed in the end-cell region.

Macroscopic behavior of a high-beta and supersonic plasma flow produced by a magneto-plasma-dynamic arcjet (MPDA) was investigated in the HITOP device. A macroscopic instability was observed as a bulk rotation of the plasma plume eccentrically around the center axis of the vacuum vessel. We investigated whether it was caused by current-driven or pressure-driven instability. A plasma current circulating in the plasma plume was controlled by changing discharge current, magnetic field configuration and mass flow rate and by inserting a copper-mesh grid. It was found that the plasma plume was macroscopically stable when the current extending into the plasma plume was low and the safety factor was more than unity. This indicates that the observed macroscopic instability was mainly caused by a current-driven one.