Energy confinement of beam-heated divertor and limiter discharges in Doublet III

Abstract

Observation of the intensity of the recycling particle flux at the main plasma edge for various limiter and divertor discharges indicates that the gross energy confinement of beam-heated discharges is closely related to the intensity of the edge particle flux. In limiter discharges, the global particle confinement time and the energy confinement time τE show many similarities: 1) linear Ip dependence at Ip < 600 kA, 2) no BT dependence, and 3) deterioration against injection power. Improvement of τE by increasing Ip, for example, is associated with high temperatures at the plasma edge region accompanied by reduced particle recycling. – Divertor discharges with low particle recycling around the main plasma show better energy confinement than limiter discharges at high plasma densities. The improvement of τE is primarily originated in the reduction of heat transport at the main plasma edge region, which is associated with the reduction of recycling particle flux at the main plasma edge. Under certain operation condition, for example, excessive cold-gas puffing, the discharge shows relatively high scrape-off plasma density and strong particle recycling between the main plasma and the limiter. The energy confinement time of these discharges degrades somewhat or reduces completely to that of the limiter discharge. – In low-recycling divertor discharges, the central electron and ion temperature is proportional to the injection power, and the plasma stored energy is proportional to n̄ePabs (scales as INTOR scaling). With ≈ 4 MW beam injection, high-temperature and high-density plasmas were obtained (stored energy up to 280 kJ, Te(0) ≈ Ti(0) ≈ 2.5–3.0 keV at n̄e ≈ (6–7) × 1013 cm−3, τE* ≈ 70 ms).

The history of research into improved confinement regimes

“…Doublet III followed shortly thereafter [34] confirming the lower power threshold in deuterium. In 1985, JET demonstrated that the H-mode could also be realised in the worldwide largest tokamak [35].…”

A review of energy confinement and local transport scaling results in neutral-beam-heated tokamaks

“…In their expanded boundary configuration, both the JAERI and GA teams on D-III attained the H-mode, 41 ' 59~61 but their details differed from those of ASDEX and PDX. For instance, no sharp transition into the H-mode was observed during NBI; improved energy confinement was observed from the outset of NBI, and even, in some cases, in the ohmic plasma.…”

Poloidal asymmetries in the scrape-off layer plasma of the Alcator C tokamak

“…Impurities found in the main plasma are derived from interaction of the SOL plasma with the limiter and other vacuum vessel components. There is also experimental evidence that plasma near the boundary (defined by the limiter or separatrix radius, r «* a) can strongly affect the global energy confinement time (H-mode) [1][2][3]. Furthermore, the parameters of the SOL plasma reflect the heat and particle fluxes originating from r ^ a, an area which is generally difficult to diagnose by direct means.…”