Taiichi Shikama

Practical formula for Mach number probe diagnostics in weakly magnetized plasmas

A practical formula for Mach number probe diagnostics in unmagnetized and weakly magnetized plasmas is proposed. The unmagnetized theoretical models are modified so as to make them applicable to the different shapes of Mach number probes by including the broadening of the collection angle and are extended to the weakly magnetized case. The calculated Mach numbers based on the modified expression show good agreement with each other regardless of the probe shape geometry, and are revealed to be consistent with the spectroscopic results. Applicability of the modified expression to the supersonic flow when using the Hutchinson’s model and in the existence of the multidirectional flows is numerically examined.

Molecular Zeeman spectroscopy for H2 Fulcher-α band spectra as a local measurement of rovibrational structures

A diagnostic technique for the local measurement of emissions from the molecular hydrogen has been developed. In the presence of an external magnetic field, the emission position can be deduced from the Zeeman patterns in the spectral line shape, and with the aid of the coronal model the rovibronic emission intensity provides the local rovibrational temperatures. In order to evaluate the Zeeman effect on the hydrogen molecule, a quantum-mechanical calculation is performed. The experimentally observed Fulcher-α rovibronic band spectra under a magnetic field strength of about 7T are well represented by the calculation. The temporal evolution of the emission position as well as the rovibrational temperatures are measured in the 8.2GHz lower hybrid current drive discharges of the TRIAM-1M tokamak [H. Zushi et al., Nucl. Fusion 45, S142 (2005)]. The measured emission is found to originate from the region where the plasma comes in contact with the limiter, and the observed increase in the rotational temperature...

Application of the Zeeman patterns to the measurement of local neutral behaviour in the edge plasma of TRIAM-1M tokamak

Neutral particle dynamics is measured in the edge plasma of the TRIAM-1M tokamak by means of the Zeeman patterns in the spectral shape. This measurement can provide the emission position with high spatial resolution, as well as the local drift velocity and temperature. In the TRIAM-1M, the inward neutral flow velocity driven by the radial neutral pressure gradient is observed. The temporal evolution of the flow velocity suggests that the distance between the last closed flux surface and the first wall changes the recycling flux. Thereby, the neutral pressure gradient and the neutral flow velocity are varied. This technique can be an effective way for the measurement of the local neutral behaviour in the edge region.

A study on temperature effects on hydrogen recycling and molybdenum impurity emission from a movable limiter in TRIAM-1M Tokamak

In order to investigate the surface temperature effects on plasma fuel recycling and impurity release from the plasma facing components, plasma discharges have been performed under selected plasma–wall interaction (PWI) conditions in the high-field superconducting tokamak, TRIAM-1M. By moving a water-cooled molybdenum movable limiter (ML) beyond the last closed flux surface, as defined by poloidal limiters, the surface temperature profile on it is varied. Hot spots have been observed on the ML surface in such conditions. The release behaviour of fuel as well as impurity particles from the ML surface has been studied as a function of hot spot temperature (Thot) by means of wide range spectroscopy (200–1600 nm). A critical Thot is found to be ~2100 K above which the emission of both hydrogen and impurity particles enhances significantly. This is indicative of some thermally activated process playing an important role in PWIs between the limiter and the edge plasma. With the rise in hot spot temperature localized PWI at the ML is found to dominate the global recycling even when external fuelling is stopped.

Application of the Zeeman patterns in Ov and Hα spectra to the local plasma diagnostics of the TRIAM-1M tokamak

The positions of emission of berylliumlike oxygen ions in the core region as well as the hydrogen atoms in the boundary region of the limiter shadow have been measured by means of the difference of the Zeeman patterns in the spectral shape in the poloidal section of the TRIAM-1M super conducting tokamak [H. Zushi et al., Nucl. Fusion 43, 1600 (2003)]. For determining magnetic field strength, the σ components of the Zeeman spectra are resolved by a linear polarizer. In addition to the emission region, the local hydrogen neutral temperature and the recycling flow velocity are measured in the boundary region, and the bulk ion temperature is measured in the core region.

Measurements of hydrogen negative ion and its comparison with the molecular hydrogen spectra in divertor simulator MAP-II

Abstract Negative ion density and vibrationally excited molecular hydrogen were compared in divertor simulator MAP-II. Laser photo-detachment technique was used for the negative ion measurement, and Fulcher- α band ( d 3 Π u → a 3 Σ g + ) spectra were used to determine the vibrational distribution of excited hydrogen molecules in the electronic ground state. As the hydrogen gas pressure increased, the negative ion density increased at first, then started to decrease as the hydrogen pressure exceeded 10 mTorr. On the other hand, the vibrational temperature measured in the same condition decreased when the hydrogen gas pressure became higher than 10 mTorr. We found that the decrease in the vibrational temperature resulted in the decrease in the negative ion density.

Experimental Study of the Atomic and Molecular Processes Related to Plasma Detachment in Steady-State Divertor Simulator MAP-II

Atomic and molecular processes relevant to the volumetric recombination phenomena were investigated in a linear divertor plasma simulator MAP-II. Volumetric recombination is induced in He plasma by puffing of He or H2. In the He puffing case, the reduction of the ion flux is dominated by the electron-ion recombination. In the H2 puffing case, however, it is dominated by the molecule-assisted recombination (MAR), which is characterized by the disappearance of the Helium Rydberg spectra and by the existence of the hydrogen negative ions. Current achievement and the future prospect are described.

The Effect of Superthermal Electrons on Mach Probe Diagnostics

For the analysis of plasma flow velocity, the Mach probe is widely used. Depending on the existence of superthermal electrons particularly at energies higher than the probe biasing voltage, collected ion saturation current can be reduced. This phenomenon can cause considerable errors in flow velocity estimation. The effect of superthermal electrons has been investigated in the steady-state linear divertor plasma simulator MAP-II. The directional measurement of electron energy distribution function (EEDF) suggests the energy of superthermal electrons and their generation mechanisms. The directional dependence of superthermal electrons is clearly observed from the angular distribution of ion saturation current measured using the Mach probe and a directional Langmuir probe (DLP). The error in flow velocity estimation under the conditions of a weakly magnetized plasma (ap/ρi~1) is calculated.

Calculation of a magnetic field effect on emission spectra of light diatomic molecules for diagnostic application to fusion edge plasmas

A scheme for computation of emission spectra of light diatomic molecules under external magnetic and electric fields is presented. As model species in fusion edge plasmas, the scheme is applied to polarization-resolved emission spectra of H2, CH, C2, BH and BeH molecules. The possibility of performing spatially resolved measurements of these spectra is examined.

Spectroscopic measurement of the degree of ionization in a helium electron cyclotron resonance discharge in a simple cusp field

For an electron cyclotron resonance (ECR) discharge, a simple cusp field can improve electron confinement and enhance the degree of ionization (DOI) without sacrificing accessibility to the plasma. In this study, the spatial distribution of the DOI is experimentally revealed in a helium plasma produced with widely used 2.45 GHz and 800 W microwaves. The DOI is evaluated from the electron density and ground state atom density measured using HeI emission line intensities and by collisional-radiative model analysis. It is found that the DOI increases to more than 15% within a reasonably large volume surrounded by the ECR surface and locally reaches as high as 25%.