Abstract. ECRH assisted plasma break down is foreseen with full and half magnetic field in ITER. As reported earlier, the corresponding O1-and X2-schemes have been successfully used for pre-ionisation and breakdown assist in present day devices. This contribution reports on common experiments studying the effect of toroidal inclination of the ECR beam, which is ≥ 20 • in ITER. All devices could demonstrate successful breakdown assist also for this case, although in some experiments the necessary power was almost a factor of two higher compared to perpendicular launch. Differences between the devices with regard to the required power and vertical field are discussed and analysed. In contrast to most of these experiments, ITER will build up loop voltage prior to the formation of the field null due to the strong shielding by the vessel. Possible consequences of this difference are discussed.
Plasma start-up and sustainment without an inductive field have been studied in the TST-2@K spherical tokamak using high power RF sources (8.2 GHz/up to 170 kW). Steady state discharges with a plasma current of 4 kA were achieved. The line integrated density was about 3 × 1017 m−2 and the electron temperature was 160 eV. A truncated equilibrium was introduced to reproduce magnetic measurements. It was found that a positive Pfirsch–Schlüter current in the open field line region at the outboard boundary makes a significant contribution to the current. Insensitivity of the current to variations in the vertical field and RF power variation was also found.
Recycling and wall pumping have been studied comparing low (~10 18 m-3) and high (~10 19 m-3) density long duration plasmas in TRIAM-1M. The recycling coefficient of each plasma increases with time. There exist two time constants in the temporal evolution of the recycling coefficient. One is a few seconds and the other is about 30 s. They may relate with characteristic times during which the physical adsorption and absorption due to the CX neutrals reach the equilibrium state, respectively. The wall pumping rates of low and high density plasmas are evaluated to be ~1.5×10 16 atoms m-2 s-1 and ~4×10 17 atoms m-2 s-1 , respectively. The difference is caused by the difference of the total amount of the CX neutral flux with the energy of <0.7 keV. In the ultra-long discharge (~70 min), the recycling coefficient becomes unity or more and again decreases below unity, i.e. the wall repeats a process of being saturated and refreshed. This refreshment of the wall seems to be caused by the co-deposition of Mo, which is a material of the limiter and divertor plates. In the high power and high density experiments, the wall saturation phenomenon has been observed. The discharge duration limited by the wall saturation decreases with increase in the density.
In the last few years, long-pulse H-mode plasma discharges (with small edge-localized modes and normalized beta, β N ~ 1) have been realized at the Experimental Advanced Superconducting Tokamak (EAST). This paper reports on high-β N (>1.5) discharges in the 2015 EAST campaign. The characteristics of these H-mode plasmas have been presented in a database. Analysis of the experimental limit of β N has revealed several main features of typical discharges. Firstly, efficient, stable high heating power is required. Secondly, control of impurity radiation (partly due to interaction between the plasma and the in-vessel components) is also a critical issue for the maintenance of high-β N discharges. In addition an internal transport barrier (ITB) has recently been observed in EAST, introducing further improvement in confinement surpassing H-mode plasmas. ITB dynamics is another key issue for high-β N plasmas in EAST. Each of these features is discussed in this paper. Study and improvement of these issues could be considered as the key to achieving long-pulse high-β N operation with EAST.
The first equilibrium reconstruction of EAST current-density profile based on internal Faraday rotation measurements provided by the POlarimeter-INTerferometer (POINT) diagnostic is demonstrated using the EFIT equilibrium reconstruction code. EFIT incorporates 11 simultaneous line-integrated density and Faraday effect measurements from POINT to self-consistently reconstruct the equilibrium toroidal current density profile using a Faraday rotation reconstruction algorithm. It is shown that the POINT measurements can be applied to improve the accuracy of core plasma current density and q profile on EAST. Comparisons of magnetic surfaces and the q profile reconstructed using external magnetic data against those using magnetic and POINT data are presented. Equilibrium reconstructions using POINT data are found to be consistent with sawtooth phenomena. The sensitivity of equilibrium reconstruction to POINT measurements indicates Faraday rotation provides important constraints for determining the current profile.
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