No abstract

The internal transport barrier (ITB) has been obtained in ELMy H-mode plasmas by neutron beam injection and lower hybrid wave heating on the Experimental Advanced Superconducting Tokamak (EAST). The ITB structure has been observed in profiles of ion temperature, electron temperature, and electron density within ρ<0.5. It was also observed that the ITB formation is stepwise. Due to the ITB formation, the confinement quality H 98y2 increases from 1 to 1.1 and the normalized beta, β N , increases from 1.5 to near 2. The fishbone activity observed during the ITB phase suggests the central safety factor q(0)∼1. Transport coefficients are calculated by particle balance and power balance analysis, showing an obvious reduction after the ITB formation.

A multichannel far-infrared laser-based POlarimeter-INTerferometer (POINT) system utilizing the three-wave technique is under development for current density and electron density profile measurements in the EAST tokamak. Novel molybdenum retro-reflectors are mounted in the inside wall for the double-pass optical arrangement. A Digital Phase Detector with 250 kHz bandwidth, which will provide real-time Faraday rotation angle and density phase shift output, have been developed for use on the POINT system. Initial calibration indicates the electron line-integrated density resolution is less than 5 × 10(16) m(-2) (∼2°), and the Faraday rotation angle rms phase noise is <0.1°.

The boundary and divertor plasma of a large spherical tokamak (ST), with significant auxilliary heating, is explored in detail for the first time. The extreme geometry of the ST is found to play a key role, giving rise to strong asymmetries in the transport of heat and particle fluxes (including those during ELMs) towards the outer divertor targets. Preliminary modelling reveals the very significant contribution of magnetic flux expansion to the particle flux balance of the ST scrape-off layer. Divertor detachment at the inboard targets is observed in L-mode at moderate core densities for plasmas with up to 1 MW of auxilliary heating.

Mitigation of type-I edge-localized modes (ELMs) was observed with the application of an n = 2 field in H-mode plasmas on the JET tokamak with the ITER-like wall (ILW). Several new findings with the ILW were identified and contrasted to the previous carbon wall (C-wall) results for comparable conditions. Previous results for high collisionality plasmas with the C-wall saw little or no influence of either n = 1 or n = 2 fields on the ELMs. However, recent observations with the ILW show large type-I ELMs with a frequency of ∼45 Hz were replaced by high-frequency (∼200 Hz) small ELMs during the application of the n = 2 field. With the ILW, splitting of the outer strike point was observed for the first time during the strong mitigation of the type-I ELMs. The maximal surface temperature (Tmax) on the outer divertor plate reached a stationary state and has only small variations of a few degrees due to the small mitigated ELMs. In moderate collisionality H-mode plasmas, similar to previous results with the C-wall, both an increase in the ELM frequency and density pump-out were observed during the application of the n = 2 field. There are two new observations compared with the C-wall results. Firstly, the effect of ELM mitigation with the n = 2 field was seen to saturate so that the ELM frequency did not further increase above a certain level of n = 2 magnetic perturbations. Secondly splitting of the outer strike point during the ELM crash was seen, resulting in mitigation of the maximal ELM peak heat fluxes on the divertor region.

A double-pass, radially-viewing, multichannel far-infrared (FIR) polarimeter/interferometer system is under development for current density profile and electron density profile measurements in the EAST tokamak. The system utilizes three 432.5 µm CW formic acid FIR lasers pumped by three CO 2 lasers. Each of the three FIR lasers can generate high output power of more than 30 mW per cavity. Two lasers, with slight frequency offset (∼ 1 MHz), will be made collinear with counter-rotating circular polarization in order to determine the Faraday effect by measuring their phase difference. The third laser also frequency offset, will be used as a reference providing local oscillator (LO) power to each mixer so that one can obtain the phase shift caused by the plasma electron density. Novel molybdenic retro-reflectors with shutter protection have been designed and will be mounted on the inner vessel wall in EAST. The retro-reflectors can withstand baking temperature up to 350 • C and discharge duration more than 1000 s. Vibrations and path length changes due to thermal expansion will be compensated using a He-Ne interferometer as the second color. VDI planar-diode Integrated Conical Horn Fundamental Mixers optimized for high sensitivity, typical 750 V/W, will be used. Initially a five-chord system will be installed in 2013 and an eleven-chord system will be implemented on the core region of EAST plasmas. MHz frequency response allows system to resolve fast MHD events such as tearing/neoclassical tearing, disruptions and fast-particle modes. Preliminary design will be presented.

EAST has been equipped with two high power lower hybrid current drive (LHCD) systems with operating frequencies of 2.45 GHz and 4.6 GHz. Comparative LHCD experiments with the two different frequencies were performed in the same conditions of plasma for the first time. It was found that current drive (CD) efficiency and plasma heating effect are much better for 4.6 GHz LH waves than for the one with 2.45 GHz. High confinement mode (H-mode) discharges with 4.6 GHz LHCD as the sole auxiliary heating source have been obtained in EAST and the confinement is higher with respect to that produced previously by 2.45 GHz. A combination of ray-tracing and Fokker-Planck calculations by using the C3PO/LUKE codes was performed in order to explain the different experimental observations between the two waves. In addition, the frequency spectral broadening of the two LH wave operating frequencies was surveyed by using a radio frequency probe.

A new technique for wall conditioning that will be especially useful for future larger superconducting tokamaks, such as ITER, has been successfully developed and encouraging results have been obtained. Solid carborane powder, which is non-toxic and non-explosive, was used. Pulsed RF plasma was produced by a non-Faraday shielding RF antenna with RF power of 10 kW. The ion temperature was about 2 keV with a toroidal magnetic field of 1.8 T and a pressure of 3 × 10-1 Pa. Energetic ions broke up the carborane molecules, and the resulting boron ions struck and were deposited on the first wall. In comparison with glow discharge cleaning boronization, the B/C coating film shows higher adhesion, more uniformity and longer lifetime during plasma discharges. The plasma performance was improved after ICRF boronization.