Generation of initial closed flux surfaces by ECH at a conventional aspect ratio of R/a ∼ 3: experiments on the LATE device and JT-60U tokamak

Abstract

Generation of initial closed flux surfaces by electron cyclotron heating (ECH) at a conventional aspect ratio of R/a ∼ 3 is investigated on the Low Aspect ratio Torus Experiment (LATE) device and further tested on the Japan Atomic Energy Research Institute Tokamak-60 Upgrade (JT-60U) tokamak. In both experiments, a plasma current is initiated and increased by ECH under steady external fields composed of a toroidal field and a weak vertical field. In the LATE experiments, a movable inboard limiter is used to change the aspect ratio of plasmas up to R/a ∼ 3. The results show that the formation of closed flux surfaces is still possible up to R/a ∼ 3 while higher decay indices of vertical field are required as the aspect ratio increases. Similar current start-up discharges are performed on JT-60U, and a plasma current is initiated and increased up to 20 kA under a vertical field having a high decay index.

Open field equilibrium current and cross-field passing electrons as an initiator of a closed flux surface in EC-heated toroidal plasmas

“…We need quite a large power ≈ 140 kW to keep this small CFP current in the present case since the loss via the pitch angle diffusion is very fast due to the large hole of lost area even in the mid-plane segment, much larger loss in off mid-plane segments. This power is ≈ 10% of the power injected in the experiments [11,12] and seems to be rather reasonable when we take into account the same reasons as those mentioned in the previous section.…”

“…Suppose that B V is increased twice, for example, in figure 14. Local equilibrium equation (12) as well as the generalized Shafranov formula (15) predicts that when the pressure is increased four times, the current density increases twice and the same field lines are obtained.…”

“…Here, it is noted that as far as B ϕ >> B V , magnitude of B ϕ does not affect all results presented in figures 8-13, where B ϕ = 1285 Gauss is nominally employed. This is because characteristics of drift motion described by equations (16)- (24) and equilibrium equation (12) essentially do not depend on the magnitude of B ϕ as far as B ϕ >> B V . Especially, we employ B ϕ = 480 Gauss in figure 6 in order to see explicitly electron Larmor motions.…”

“…We examine the model described in sections 2-4 for experiments in large tokamaks of DIII-D and JT-60U [11,12].…”

Noninductive plasma initiation and startup in the DIII-D tokamak

“…Higher aspect ratio tokamaks have also observed noninductively driven EC currents during startup, e.g. T-IIU [5], WT-3 [6], DIII-D [7], JT-60U [8] and KSTAR [9]. In the experiments reported here, up to 33 kA has been driven with 2 MW of second harmonic EC power (X-mode).…”