Y. Uesugi

Static and dynamic behaviour of plasma detachment in the divertor simulator experiment NAGDIS-II

A comprehensive investigation has been performed of the static and dynamic behaviour of detached recombining plasmas in the linear divertor plasma simulator NAGDIS-II. For stationary plasma detachment, the transition from electron-ion recombination (EIR) to molecular activated recombination (MAR) has been observed by injecting hydrogen gas into high density helium plasmas. The particle loss rate due to MAR is found to be comparable to that of EIR. Experiments have also been performed by the injection of a plasma heat pulse produced by RF heating into the detached helium plasma to demonstrate the dynamic behaviour of volumetric plasma recombination. Negative spikes in the Balmer series line emission were observed and found to be similar to the so called negative ELM observed in tokamak divertors. Observed Balmer spectra were analysed in detail using the collisional-radiative model. A rapid increase of the ion flux to the target plate was observed associated with the re-ionization of the highly excited atoms generated by EIR.

Generation and characteristics of detached recombining plasma and its dynamic behaviour—a bridge between fusion plasmas and low-temperature ionized gases

First, the role of a magnetic divertor in fusion devices is introduced, and it is shown that the energy balance in burning International Thermonuclear Experimental Reactor indicates a strong necessity for reduction of plasma heat load on the divertor target plate. Next, the plasma heat flow to a material surface through sheaths is generally given in terms of the energy transmission factor, in which the energy deposition based on the surface recombination is essential in high-density plasmas. The heat load is independent of any cooling of the plasmas. Then, a variety of interesting characteristics of detached recombining plasmas, which are important in reducing the plasma heat flow, are discussed: generation and structure of plasma detachment, light emission originating from a series of highly excited Rydberg states of atoms, contribution of molecular activated recombination (MAR), transition between conventional electron–ion recombination (EIR) and MAR, relation to dust coagulation processes and the experimental verification of plasma detachment in tokamak fusion devices. Finally, the dynamic behaviour of the detached EIR plasma obtained in the linear divertor plasma simulator NAGDIS-II against the ELM-like heat pulse is shown.

Transition from electrostatic-to-electromagnetic mode in a radio-frequency Ar inductively coupled plasma in atmospheric pressure

The dynamics of mode transition from the electrostatic (E) to electromagnetic (H) mode of a radio-frequency argon inductively coupled plasma (ICP) in atmospheric pressure is investigated, both experimentally and theoretically. High-speed (4500 f/s) camera imaging is performed to investigate the dynamics of E–H discharge mode transition. The temporal plasma loading impedance of Ar inductive discharges at the E–H transition stage is also observed to investigate the transition dynamics. The experimental observations reveal that the formation of the multiple streamerlike electrostatic discharge paths followed by the strong ring-shaped azimuthal discharges leads to ignite the high-pressure (around 1 atm or more) ICPs. The time scale of E–H discharge mode transition is estimated by using proposed models and the estimated results are compared with that of the experimental one. It is found that the estimated results agree well with that of the experimental one.

Characteristics of detached plasmas associated with electron- ion and molecular assisted recombinations in NAGDIS-II

Recombination in a helium and helium-hydrogen plasma was experimentally investigated in the linear divertor simulator NAGDIS-II. To identify the physical mechanisms, electron-ion recombination (EIR) and molecular assisted recombination (MAR), the plasma parameters in the recombining region have been measured. We used spectroscopic techniques on the basis of continuum emission and line emission from highly excited levels due to EIR and Langmuir probes. The dependence of the plasma parameters on the neutral pressure in a pure helium plasma and in a helium-hydrogen mixture shows clearly that recombination is caused by different processes in the two cases. In a pure helium plasma a roll-over and a following decrease of the electron density as well as a monotonic decrease of the electron temperature down to 0.1 eV were observed when increasing the helium pressure. Here EIR is the recombination mechanism. Conversely, when hydrogen is added to the helium plasma, the electron density decreases monotonically while the central electron temperature remains above 1 eV. This particular behaviour in the helium-hydrogen plasma can only be explained by the dominant action of MAR.

Effect of rotating helical magnetic field on the turbulence fractal structure and transport in the tokamak edge plasma

Fractal structure of the edge turbulence and enhanced turbulent transport have been studied in the tokamak HYBTOK-II with a variation of the rotating helical magnetic field (RHF) frequency in the range 5–30u2009kHz. Edge fluctuations have non-Gaussian statistics caused by intermittent bursts with a time scale of 40–100u2009µs. The variation in the RHF frequency has a selective effect on the fractal structure of edge turbulence and the turbulent flux, demonstrating a selective control of the transport process. A delayed synchronization control of resonant drift wave modes by the RHF is considered as a candidate mechanism to explain the dependence of the effect on the RHF frequency.

Ion temperature measurement using an ion sensitive probe in the LHD divertor plasma

Abstract The first reliable measurement of ion temperature in the divertor plasma of the Large Helical Device has been done by using an ion sensitive probe. The satisfactory current–voltage characteristics of the ion collector for evaluating the ion temperature were obtained at the outer part of the divertor leg. Furthermore, simultaneous ion and electron temperature measurements were successfully done in this part. The results show that the ion temperature is higher than the electron temperature in the part. There is a possibility that the profiles of the evaluated ion temperature which shows relatively higher than the electron temperature at the outside of divertor leg are qualitatively explained by particle’s orbits around the edge and divertor region.

Dynamic effects of rotating helical magnetic field on tokamak HYBTOK-II

Penetration processes of rotating helical magnetic perturbation (RHMP) into tokamak plasmas have been studied on a small tokamak device HYBTOK-II for dynamic ergodic divertor experiment preparing in TEXTOR. The radial profiles of RHMP in the plasma have been measured by inserting small magnetic probes into the plasma. The penetration of RHMP has been discussed by using the observed radial profile of RHMP in the plasma based on the growth of magnetic islands (i.e. tearing mode). In the case of the low Doppler-shifted frequency observed from the plasma, the amplification of RHMP in the plasma due to a spatial modification of plasma current has been found. A large Doppler-shifted frequency produces an expected attenuation of RHMP near the magnetic resonance surface due to screening current flow. A decrease of the amplification of RHMP originated from the suppression of the growth of magnetic islands due to a rapid spatial scan of magnetic resonance layer has been found by plasma current oscillation.

Measurement of peripheral electron temperature by electron cyclotron emission during the H-mode transition in JFT-2M tokamak

Time evolution and profile of peripheral electron temperature during the H-mode like transition in a tokamak plasma is measured using the second and third harmonic of electron cyclotron emission (ECE). The so called “ H-mode ” state which has good particle/energy confinement is characterized by sudden decrease in the spectral line intensity of deuterium molecule. Such a sudden decrease in the line intensity of D α with good energy confinement is found not only in divertor discharges, but also in limiter dischargs in JFT-2M tokamak. It is found by the measurement of ECE that the peripheral electron temperature suddenly increases in both of such phases. The relalion between H-transition and the peripheral electron temperature or its profile is investigated.

Application of high performance inverter circuit to plasma generation and heating in fusion devices

We have constructed a rf power source using static induction transistor (SIT) inverters to generate high heat flux plasmas in the diverter plasma simulator, NAGDIS-II. Since the SIT can enlarge the operating frequency range of the high power inverter above 1 MHz with output power of several tens of kW, SIT inverters can be used to generate rf plasmas and to heat plasmas by exciting low frequency plasma waves, such as Alfven waves and ion cyclotron waves in plasma devices. The rf conversion efficiency of a SIT inverter with matched load is >85% for frequencies up to 1.2 MHz. The third harmonic antenna current is found to be less than −30 dB. Using four sets of inverter power supplies, we can generate more than 56 kW in continuous rf power or 80 kW in 1 s pulses to produce high heat flux plasmas in the diverter plasma simulator.

Sheath formation on carbon and refractory metals in a plasma with energetic electrons

The sheath formation at the surface of several materials in a plasma with an energetic electron component is experimentally investigated to show whether the sheath voltage is reduced by the secondary electron emission as expected from theory. The sheath voltage was measured using a Langmuir probe to find its dependence on the energetic electron temperature. The experimental results of the sheath voltage on several materials, such as carbon, molybdenum, tungsten and gold, allow the understanding of the effect of the secondary electron emission on sheath formation. The suppression of sheath voltage is observed when the energetic electron temperature increases. These results agree quantitatively with theoretical estimations, except for carbon.