Progress of long pulse discharges by ECH in LHD

Abstract

Using ion cyclotron heating and electron cyclotron heating (ECH), or solo ECH, trials of steady state plasma sustainment have been conducted in the superconducting helical/stellarator, large helical device (LHD) (Ida K et al 2015 Nucl. Fusion 55 104018). In recent years, the ECH system has been upgraded by applying newly developed 77 and 154 GHz gyrotrons. A new gas fueling system applied to the steady state operations in the LHD realized precise feedback control of the line average electron density even when the wall condition varied during long pulse discharges. Owing to these improvements in the ECH and the gas fueling systems, a stable 39 min discharge with a line average electron density n e_ave of 1.1 × 1019 m−3, a central electron temperature T e0 of over 2.5 keV, and a central ion temperature T i0 of 1.0 keV was successfully performed with ~350 kW EC-waves. The parameters are much improved from the previous 65 min discharge with n e_ave of 0.15 × 1019 m−3 and T e0 of 1.7 keV, and the 30 min discharge with n e_ave of 0.7 × 1019 m−3 and T e0 of 1.7 keV.

Development of Thomson Scattering Measurement System for Long Duration Discharges on the QUEST Spherical Tokamak

“…Thus, long duration discharge experiments are necessary for the fusion research. Long duration discharges have been performed in several devices, such as Tore Supra [1], TRIAM-1M [2], LHD [3,4], and EAST [5], all of which have reported that plasmas are sustained for durations sufficiently longer than the characteristic time scales of transport in core plasmas, such as the energy confinement time and the current diffusion. However, TRIAM-1M reports that the discharges are terminated because the density becomes uncontrollable due to the increase in the particle recycling from the plasma-facing components.…”

Stable sustainment of plasmas with electron internal transport barrier by ECH in the LHD

“…As a reference, T e and n e profiles without e-ITB (black) of another long pulse (~41 min) ECH discharge #131054 (R ax = 3.6 m, B t = 2.75 T) [15] are plotted in Fig. 3 (c).…”

“…On the other hand, as a stellarator, the LHD plasma is excellent at T p of up to 54 min, and the plasma parameter shows significant improvements in recent years. In addition, long pulse discharges sustained only with the ECH have been investigated in LHD [12][13][14][15], because ECH is considered to be unique and the most reliable heating technique for the future fusion reactors. The highest record of the long pulse plasma sustained with ECH in LHD is n e_ave =1.1×10 19 m −3 , T e0 ∼2.7 keV, and T i0 =1 keV for T p =39 min [15].…”

“…In addition, long pulse discharges sustained only with the ECH have been investigated in LHD [12][13][14][15], because ECH is considered to be unique and the most reliable heating technique for the future fusion reactors. The highest record of the long pulse plasma sustained with ECH in LHD is n e_ave =1.1×10 19 m −3 , T e0 ∼2.7 keV, and T i0 =1 keV for T p =39 min [15].…”

Investigation of dust shielding effect of intrinsic ergodic magnetic field line structures in the peripheral plasma in the large helical device

“…One is the divertor region, and the other is the surface on the vacuum vessel (including helical coil cans). While some long pulse discharges were terminated by carbon dust emission from the divertor region, which were caused by the exfoliation of carbon‐rich mixed material deposition layers accumulated near the divertor plates (isotropic graphite), some long pulse discharges were interrupted by the iron dust emission from the surface on helical coil cans (stainless steel) just after the appearance of sparks (arcing). An observation with a stereoscopic fast framing camera revealed that iron dust released from the arcing point penetrated into the main plasma confinement region, causing the radiation collapse …”