K. Sakurai

Observation of lower-hybrid-current drive in the JIPP T-II torus

Current driven by injecting lower hybrid waves has been observed in low-density plasmas in the JIPP T-II. It is confirmed that RF-driven current is generated by momentum transfer from lower hybrid waves to suprathermal electrons with an energy of 8–25 keV. The driving efficiencies in tokamak and stellarator configurations are 1 kA·kW−1 and 0.3 kA·kW−1, respectively, at a power level of 40 kW. Enhanced electron cyclotron emission due to pitch-angle scattering of RF-driven suprathermal electrons is observed, and spikes in loop voltage and X-ray bursts appear coincidentally. In a long RF-pulse, these rapid changes advance to relaxation oscillations. It is concluded that the pulsating changes originate in the instantaneous scattering of RF-driven suprathermal electrons by the unstable waves excited at an anomalous Doppler resonance.

Current density profile control by programming of gas puffing and plasma current waveform in the JIPP T-II tokamak

In the resistive-shell tokamak, JIPP T-II , a control of the current density profile has been attempted by programming both gas puffing and plasma current waveform. A stable high-density plasma has been obtained with the following parameters: the maximum line-average electron density is e = 8.5 × 1013cm−3, the minimum q(a)-value is 2.2, and the relative amplitude of the m/n = 2/l mode is suppressed to an extent less than 10−3. A derivation of the current density profile by solving the magnetic-diffusion equation on the basis of the experimental data shows that the current density profile favourable to the stability of low-m kink and tearing modes is realized by combining the effects of cooling through an increase in density and of heating by a current rise in the outer plasma region. The results of kink and tearing modes analysis agree well with the experimental observations. The criterion that the current density profile is successfully controlled by this method is derived as a function of the ratio of plasma current to electron density in the current-rise phase, i.e. 20 × 10−13 Ip/e 30 × 10−13 kA·cm3. The major disruption due to the density increase is completely suppressed by the method proposed in this paper. The major disruption due to a reduction of q(a) to less than 2.2 has, however, not yet been suppressed. In future, the current density profile should be maintained more precisely at its optimum shape by using a feedback-control technique and a control of the plasma boundary with titanium gettering, etc.

Pre-ionization and heating of stellarator plasma at electron cyclotron frequency in JIPP T-II

Experimental studies of electron cyclotron pre-ionization and heating have been carried out in the JIPP T-II torus by injecting a power of 36 kW at the frequency of 35.5 GHz. Pre-ionization effectively decreases the loop voltage at the initial stage and eliminates strong spikes in the signals of electron density and of light and hard-X-ray emission which is due to runaway electrons in the initial breakdown phase of the Joule heating. From the measurement of the central electron temperature only, it is seen that electron cyclotron heating of a stellarator plasma with ordinary-mode radiation shows a heating efficiency of 1.6 eVkW−1 and, from power balance considerations, the absorption rate of microwave power is estimated to be around 50%.

Thermal shock tests of tungsten by H+-beam bombardment

Cyclic thermal tests were performed on tungsten using the 120 keV Neutral Beam Test Stand at IPP, Nagoya University. A maximum heat flux of 8.8 kW/cm 2 was first applied for 56.7 ms to large samples for a total of 5 cycles. Many surface microcracks were observed after two thermal shocks, but only a few active cracks grew further in a direction almost perpendicular to the heated surface with an average growth rate of about 0.2 mm/cycle. Thermal shocks of 50.4 ms were next applied with an average heat flux of 8.2 kW/cm 2 for a total of 23 cycles. Active cracks grew in the plane lying about 3 mm below the heated surface. No visible cracks were produced in one of the four samples, but cracking occurred after cutting the sample into four identical blocks. The thermal shock cracking was restricted in the region where grain growth had occurred.

Effect of the filament‐induced magnetic field on the spatial distribution of primary electrons in magnetic multicusp source

The influence of the filament on the discharge characteristics of the multicusp source (bucket source) is studied experimentally and theoretically. It is found that the magnetic field which is produced by the filament itself plays an important role to determine the trapped region of primary electrons by coupling with the magnetic multicusp field. Experimentally, arc efficiency varies drastically when only the direction of the filament current is changed. The measurements of the spatial profiles of the electron density and the temperature show that the plasma production occurs at the periphery of the chamber in the case of poor arc efficiency. Theoretical calculation also shows that in this case primary electrons are trapped in the local magnetic mirror of the multicusp field. The trapping condition depends on the magnetic field geometry around the filament, and, therefore, it is sensitive to the location of filament in the magnetic multicusp field as well as the strength and the direction of filament current.

Positional Stability of a Current-Carrying Plasma in an l=2 Stellarator Field

In order to investigate the positional stability of a current-carrying plasma in an l =2 stellarator field, we estimate the decay index of the vertical magnetic field when the stellarator field is applied. Experimental results show that the positional stability of a current-carrying plasma in the stellarator field is better than that of a tokamak plasma. Although the value of the decay index is negative, the plasma column is also positionally stable in the vertical direction, A following relationship \(\delta B_{\text{v}}/\delta\varDelta{\simeq}\imath_{\text{h}}B_{\text{t}}/2\pi R\) is experimentally confirmed, where δ B v is a change of the vertical magnetic field, δ Δ is the displacement of the plasma column, \(\imath_{\text{h}}\) is the rotational transform angle of the helical field, B t is the toroidal magnetic field and R is the major radius of the plasma column.

Development of a large bucket plasma source

A large bucket-type plasma source has been developed for a neutral beam injector. An ion current density of 0.25 A/cm2 in a hydrogen plasma was achieved with a density uniformity of better than ~10% over an area of 15×60 cm2 at about 100 kW of arc power in pulsed operation for 1s. Plasma density profiles and the discharge voltage-current characteristics, etc., were studied by varying the geometrical parameters of a tungsten filament i.e., the disposition, number, diameter, and shape. The profile was affected by the filament disposition and the gas pressure. A measure of the gap length between filaments to generate a dense uniform (< 10%) plasma was roughly 14-22 cm. An experimental scaling relation of the arc power is discussed; the arc power for the same plasma density appears to increase with an increase in the total loss-area in various sources.

Equilibrium in an octopole Tokamak without conducting shell

A numerical method is described for solving free boundary problems in an axially symmetric toroidal configuration. Approximate solutions for equilibrium plasmas are given for a fixed external magnetic field. The equilibrium plasma in the multipole configuration consisting of a uniform vertical magnetic field and a shaping field is discussed. A shift of the equilibrium position leads to a change of the cross-section shape from rectangular to a D- or a tear-drop type. The plasma equilibria with the D-shaped cross section are horizontally unstable and an off-centre equilibrium is obtained. The safety factor and the well depth are increased by adding the shaping magnetic field.

Heat flux testing of carbon materials with an H + ion beam

Abstract The 10 MW Neutral Beam Injection Test Stand of the Institute of Plasma Physics of Nagoya University was used to subject fine grain graphites (13 types), and 2-d (6 types) and 4-d (2 types) carbon/carbon composites to heat fluxes of 16–106 MW/m 2 for pulse durations of 50–950 ms. Graphites with a low coefficient of thermal expansion and a coarse microstructure had good resistance to crack initiation but worse erosion compared to fine microstructure graphites. The 2-d carbon/carbon materials suffered extensive erosion under heat fluxes incident perpendicular to their weave orientation, while moderate erosion was detected when the heat flux was incident parallel to the weave orientation. The 4-d carbon/carbon materials showed little erosion in any orientation. After testing, no fatal cracks or fractures were observed in any of the carbon/carbon composites.

Plasma Diagnostics Using Charge Exchange Spectroscopy on the JIPP T-IIU Tokamak

The ion temperature profile and the density distribution for the beam penetration in the plasma have been measured and evaluated using emissions produced by the charge exchange reaction between a heating neutral hydrogen beam and fully ionized impurities on the JIPP T-IIU tokamak. A 1 m spectrometer with a multichannel detector is used to detect ultraviolet and visible emissions from this reaction. The ion temperature profile during neutral beam heating (or plus ion cyclotron range of frequency heating) is obtained from the Doppler broadening of emission spectra. The central ion temperature agrees with those from other diagnostics. The beam density profile is also in good agreement with the simulational result. The availabilities are demonstrated on this technique to measure the ion temperature and the beam density profile which are necessary to discuss beam heating efficiency and thermal transport.