Non-inductive Current Start-up Research Articles

Experimental study of non-inductive current start-up using electron cyclotron wave on EXL-50 spherical torus

Although experiments have demonstrated a feasibility of the electron-cyclotron-wave (ECW) non-inductive current start-up, the physical mechanism of the current start-up remains unclear, particularly the role of the preferential confinement of collisionless tail electron currents ( and vertical charge-separation currents (. Therefore, and were observed using limiter probes during the ECW non-inductive current start-up on an ENN XunLong-50 (EXL-50) spherical torus to understand the significance of these values. It was found that increased and decreased during start-up, indicating a dominant role played by in this process. Furthermore, it was found that the product of the gas pressure and vertical magnetic field has a limit for the ECW non-inductive current start-up. Finally, we found that the ECW non-inductive current start-up can be realized only for sufficiently low vertical fields and gas pressures in EXL-50.

Design Details of the Transient CHI Plasma Start-up System on NSTX-U

Elimination of the central solenoid would simplify the engineering design of a fusion nuclear science facility and tokamak based devices. The method of transient coaxial helicity injection (CHI) has successfully demonstrated formation of a high-quality closed flux plasma in NSTX and will be used as the front end of the start-up method for a full demonstration of noninductive current start-up, followed by noninductive current ramp-up using neutral beams in the NSTX-U device that is now under construction at the Princeton Plasma Physics Laboratory. CHI is implemented by driving current along open field lines that connect the lower inner and outer divertor plates of a spherical torus. The engineering system requirements and the design of the CHI system on NSTX-U are described.

Investigation of Non-inductive Plasma Current Start-up by RF on QUEST

Formations of a closed flux surface (CFS) on QUEST are achieved by fully non-inductive current start-up driven by RF, which is 8.2GHz in frequency and more than 40kW in power. It found that appropriate magnetic configuration with positive n-index and reduction of particle recycling was crucial to achieve the non-inductive plasma current start-up (PCS) successfully. Especially the controllability of particle recycling should be improved by wall conditioning based on successive plasma production and wall cleaning with electron cyclotron resonance heating (ECR) plasmas induced by RF in frequency of 2.45GHz.

Non-inductive current start-up assisted by energetic electrons in Q-shu University experiment with steady-state spherical tokamak

After intensive discharge cleaning of the chamber wall, non-inductive current start-up experiments have been successfully performed in QUEST in moderate vertical fields of about 1.0–1.5 mT with positive n-index. Simultaneously, with increasing plasma current, an asymmetric toroidal flow of energetic electrons was observed and direct measurements of current driven by this asymmetric flow were taken with a newly developed Langmuir probe technique. A numerical study of the energetic electron orbits indicates that the total current is enough to play a dominant role in the formation of a closed flux surface in QUEST.

Analysis of PWI footprint traces and material damage on the first walls of the spherical tokamak QUEST

After several non-inductive current startup experimental campaigns in the spherical tokamak QUEST, its metallic first walls have revealed various kinds of damages as a signature of strong plasma wall interaction (PWI). Several types of footprint traces, namely colored regions formed due to material erosion/redeposition, melting of plasma facing components (PFCs) and numerous arc tracks on the chamber walls are recognized. Analysis of the re-deposited materials on collector probes is carried out using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS). Redeposition of several impurity materials such as carbon, oxygen and tungsten is identified. The footprint traces are majorly formed on the lower side PFCs, showing a large up/down asymmetry. Both toroidally symmetric and asymmetric footprint traces are formed on the bottoms side divertor plate and the lower part of the outboard side walls, respectively. Localized melting occurred on the outboard side limiters is attributed to the loss of energetic electrons produced via electron cyclotron resonance (ECR) heating. The observed damages are discussed in view of localized PWI, loss of energetic electrons, particle drifts, sputtering, arcing and redeposition of eroded materials. Material analysis and numerically calculated guiding center orbits of the charge particles are used to discuss these damages.

Analysis of the footprint traces on the first walls of the compact plasma wall interaction device (CPD) using surface analysis and electron orbit calculations

After the non-inductive current startup experimental campaign on the spherical tokamak compact plasma wall interaction device (CPD), various localized damage tracks or footprint traces have been observed on plasma facing components (PFCs), such as the chamber walls and the ‘stiffeners’ that support them. Although the magnetic field configuration is mainly open, the footprint traces are classified as (1) radially distributed traces, (2) toroidal imperfect circular traces with small gaps, (3) arc-shaped traces and (4) vertically distributed traces. The surface analysis of the samples attached near the traces has been carried out by scanning electron microscopy and x-ray photoelectron spectroscopy. They suggest thin deposition of impurity materials (C, Cu, Ti, Fe and their oxides) over the traces. These footprint traces are analysed in view of the localized plasma wall interaction and the loss of energetic electrons using orbit calculations. Radially distributed traces correspond to the loss of co- and counter-moving passing electrons mainly escaping along the magnetic field lines. The imperfect circular traces are found corresponding to lost orbits of the energetic trapped electrons largely crossing the magnetic field lines. Other traces are also discussed from a viewpoint of loss along the magnetic field lines and impurity deposition.

Physics and Technological Issues for Steady-State Tokamak Operation on TRIAM-1M

Subjects on TRIAM-1M are presented from the viewpoint of plasma physics and reactor technology associated with ‘steady state operation’ (SSO). For a future fusion power plant, for burning plasma, complete steady state operation is required. Control of the density under the complicated plasma-wall interaction, non-inductive current start-up, and sustainment of high performance are key areas in the present and future investigations. The following experimental results are reviewed. First, it is shown that the co-deposition of the metallic impurity and oxygen plays an important role in the temporary change in the wall pumping rate, and a model of the co-deposition probability agreed with the observation. It was also noticed that the thermal release of the hydrogen from the plasma-facing components affects the steady state density operation in the ultra long discharge. It was found that enhanced influx of metal impurities from the hot spot affect the steady state operation of the high performance plasma. Second, helium effects on microscopic damage on metals were studied in helium/hydrogen mixture discharges. A large quantity of dislocation loops and dense fine bubbles were observed by means of TEM even for exposure only 125 seconds in duration. From TDS for the specimens, the amount of retained helium was evaluated to be 3.9 · 10 20 He/m 2 . Third, the physics understanding for the enhanced current drive (ECD) mode around the threshold power level progressed from the viewpoint of transition probability. The forward transition frequency from a non-ECD plasma state to the ECD state was precisely determined under fixed LHCD power. Thus, a statistical probability for ECD transition was determined; that is, the transition behavior around the threshold power could be described in a statistical manner. Transition frequency showed a strong power dependence. Fourth, the current ramp-up scenario without using centre solenoid coils was reinvestigated at higher density, and controllability of the current ramp-up rate was studied. The plasma was initiated by ECH (fundamental O-mode at 170 GHz with 200 kW) at B = 6.7 T, and a ramp-up rate below the technical limit of 150 kA/s for ITER could be achieved by choosing LH power. A model to describe the ramp-up is proposed.