Divertor Plasma Simulator NAGDIS-II Research Articles

Evaluation of axial decay length of plasma pressure in detached plasma

Abstract This study proposes a method for evaluating the axial decay length of the plasma pressure in detached helium plasmas with a fixed-point laser Thomson scattering and spectroscopic measurements in the linear divertor plasma simulator NAGDIS-II. By controlling the neutral gas pressure, the highly volume-recombining region called the “recombination front” was moved along the magnetic field. The differential terms for the electron pressure and the recombination-front position with respect to the neutral pressure were obtained. By using these two terms, the decay length of the plasma pressure was qualitatively calculated. In addition, from the movable Langmuir probe measurement, difference between the locally estimated decay length around the recombination front and the decay lengths over a wide axial range was discussed. The proposed method can be applied even in the huge heat flux condition where direct-contact measurements such as the Langmuir probe are not available.

Tritium retention in nanostructured tungsten with large effective surface area

Abstract Tungsten specimens with fiberform nanostructure (Nano-W) were prepared by exposure to helium plasma in the divertor plasma simulator NAGDIS-II with different amounts of helium fluence. For comparison, tungsten specimens with smooth surface (Polished-W) were also prepared. Surface area of Nano-W was measured by using Brunauer, Emmet and Teller (B.E.T.) method. Tritium retention of Nano-W and Polished-W was investigated by an imaging plate (IP) and β-ray induced X-ray spectrometry (BIXS) technique exposure to mixture gas of deuterium and tritium. It was found that surface area of Nano-W was significantly larger than that of Polished-W and increased in proportion to the amount of helium irradiation. On the other hand, tritium retention showed a saturation trend when the helium fluence was higher than 5.0 × 1025 m−2.

In situ reflectivity of tungsten mirrors under helium plasma exposure

The formation of micron-sized helium bubbles/holes on tungsten surface due to helium ion irradiation has been investigated in the linear divertor plasma simulator NAGDIS-II [1]. In order to investigate the surface variation during the helium plasma exposure, the optical reflectivity is measured by using a He–Ne laser and a photodiode. When the incident helium ion energy is 15 eV, a great amount of bubbles and holes were formed by the exposure with the ion fluence of 1.8 × 10 27 m −2. The degradation of the reflectivity is enhanced when increasing the incident ion energy. When the incident ion energy is 30–50 eV, the surface is covered with fiberform nanostructured tungsten. The reflectivity recovery effect by a ruby laser with a long pulse duration of ∼0.6 ms is demonstrated. The reflectivity recovers to the initial value when the laser pulse energy exceeds ∼1.2 MJ m −2.

Formation mechanism of bubbles and holes on tungsten surface with low-energy and high-flux helium plasma irradiation in NAGDIS-II

A systematic study on the formation mechanism of micron-sized He bubbles and holes in powder metallurgy tungsten due to helium ion irradiation with an ion energy below 30 eV and a particle flux above 10 22 m −2 s −1 has been performed in the linear divertor plasma simulator NAGDIS-II. Holes are formed with incident helium ion energy above 5 eV, which could be related to the surface barrier potential energy for He penetrating into tungsten. Tungsten surface temperature strongly influences the number and size of hole. Above 1600 K, bubbles and/or holes with several hundreds nano-meter diameter appear on the tungsten surface. Single crystal tungsten, which has much fewer intrinsic defects than powder metallurgy tungsten, was also irradiated by He plasmas. There is no qualitative difference in the hole formation between the two grades of tungsten. Bubble and hole formation mechanisms are discussed based on the experimental results.

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.

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

A comprehensive investigation has been performed of the static and dynamicbehaviour of detached recombining plasmas in the linear divertor plasmasimulator NAGDIS-II. For stationary plasma detachment, the transitionfrom electron-ion recombination (EIR) to molecular activatedrecombination (MAR) has been observed by injecting hydrogen gas intohigh density helium plasmas. The particle loss rate due to MAR is found tobe comparable to that of EIR. Experiments have also been performed bythe injection of a plasma heat pulse produced by RF heating into thedetached helium plasma to demonstrate the dynamic behaviour of volumetricplasma recombination. Negative spikes in the Balmer series lineemission were observed and found to be similar to the so callednegative ELM observed in tokamak divertors. Observed Balmer spectrawere analysed in detail using the collisional-radiative model. A rapidincrease of the ion flux to the target plate was observed associatedwith the re-ionization of the highly excited atoms generated byEIR.

Dynamic behavior of detached recombining plasmas during ELM-like plasma heat pulses in the divertor plasma simulator NAGDIS-II

It has been recognized that the ELMs associated with a good confinement at the edge, such as H-mode, must bring an enormous energy to the divertor target plate through SOL and detached plasmas. The understanding of the ELM energy transport through SOL to the divertor target is rather poor at the moment, which leads to an ambiguous estimation of the deposited heat load on the divertor target in ITER. In the present work the ELM-like plasma heat pulse is generated by rf heating in a linear divertor plasma simulator. Energetic electrons with an energy range 10–40 eV are effectively generated by rf heating in low temperature plasmas with (T e )< ∼1 eV. It is observed experimentally that the energetic electrons ionize the highly excited Rydberg atoms quickly, bringing a rapid increase of the ion particle flux to the target, and make the detached plasmas attached to the target. Detailed physical processes about the interaction between the heat pulse with conduction and convection, and detached recombining plasmas are discussed.

Generation of high heat flux plasmas by high power rf heating in the divertor plasma simulator NAGDIS-II

High-power rf heating in the ion cyclotron range of frequency (ICRF) has been carried out in a divertor plasma simulator NAGDIS-II. The main objective of the present work is to produce high heat flux plasmas for fundamental studies of the divertor plasmas in magnetically confined fusion devices. Waves are excited by phased-loop antenna array in a frequency range between 0.5 and 1.5 MHz. Inverter power supplies using static induction transistors (SIT) are employed to excite ion cyclotron slow waves. In NAGDIS-II a target plasma for rf heating is produced by TP-D-type dc discharges, where the electron density is n e<6×10 19 m −3, and the electron and ion temperature are T e=5–10 eV and Ti=1–10 eV in helium plasmas, respectively. By injecting about 14 kW rf power we have obtained high heat flux plasmas with n e=2×10 19 m −3, T e∼20 eV and T i∼18 eV so far. The plasma heat flux into the end plate is measured by calorimetric method and the power balance of the rf heated plasmas is studied experimentally.

Contribution of molecular activated recombination to hydrogen plasma detachment in the divertor plasma simulator NAGDIS-II

We have performed experiments on detached hydrogen plasmas associated with molecular activated recombination (MAR) in a linear divertor plasma simulator. Reductions of the ion particle flux and heat load to the target plate were clearly observed in hydrogen plasmas with the hydrogen gas puff. Detailed analysis of Balmer series spectra with the Collisional-Radiative Atomic-Molecular data (CRAMD) code shows that MAR mainly appears as a weak dependence of the Balmer series emission intensities on experimental conditions in detached hydrogen plasmas. The plasma conditions necessary to obtain plasma detachment through MAR or EIR (electron ion recombination including the radiative and three-body recombination) in tokamak divertors are also discussed.

Study of the radio-frequency driven sheath in the ion cyclotron slow wave antennas

Formation of the radio-frequency driven sheath and resulting parasitic antenna loading in the ion cyclotron slow wave antennas are studied experimentally in the linear divertor plasma simulator NAGDIS-II. A phased loop antenna array with a poloidal mode of m=0 is used in the present ICRF heating experiment. A large DC voltage drop of about several hundreds volts is induced on the loop antennas with and without Faraday screen during high power RF heating and causes the additional power dissipation ( P sh) due to the heat flux to the antenna current strap of ions accelerated by the RF driven DC sheath potential. This parasitic antenna loading is measured by calorimetric method and compared with that obtained from the conventional measurement of the antenna voltage and current. When a Faraday screen is employed to reduce the antenna–plasma interaction, P sh becomes much smaller than the radiated RF power. The net antenna loading for ICRF slow wave excitation is evaluated, taking account of the RF sheath dissipation.