Abstract
In KSTAR device, a 110 GHz ECH system has been a useful heating device for a stable plasma start-up because a pure ohmic discharge scenario with a limited loop voltage of about 4 V was sometimes not successful for burn-through and plasma current ramp-up due to inconsistent wall conditioning and density control. Even though a pure ohmic discharge also was successful, the application of X2-mode ECH could reduce the flux consumption of poloidal field coils, leading to long pulse discharges. The ECH power was injected at the time of the field null formation after the onset of the toroidal electric field in which the electron temperature significantly increased up to 100 eV so that burn-through is overcome. The ECH heating enabled the formation of close flux surfaces earlier, leading to the reliable plasma current ramp-up, but, it caused outward plasma movement and failure of the plasma control, resulting in loss of the discharge. Moreover, impurities from the plasma facing components caused by not fully absorbed ECH power had a detrimental effect on the H-mode transition. In ECH-assisted start-up in the ramp-up phase, ECH power was mainly used for central electron heating, leading to reduction of the flux consumption of the central coils by increasing the electron temperature along with a density increase in the ramp-up phase. When X2-mode ECH power of 350 kW was injected at the mid-plane with a toroidal angle of 10 degree for 1 sec after the onset of the loop voltage, the flux consumption was reduced by about 30% in comparison with the pure ohmic discharges.
Highlights
Present tokamaks and future reactor devices with superconducting poloidal field coils and thick vacuum vessel well have a limited toroidal electric field which is too low reliably to achieve ohmic start-up [1]
The ECH power was injected at the time of the field null formation after the onset of the toroidal electric field in which the electron temperature significantly increased up to 100 eV so that burn-through is overcome
When X2-mode ECH power of 350 kW was injected at the mid-plane with a toroidal angle of 10 degree for 1 sec after the onset of the loop voltage, the flux consumption was reduced by about 30% in comparison with the pure ohmic discharges
Summary
Present tokamaks and future reactor devices with superconducting poloidal field coils and thick vacuum vessel well have a limited toroidal electric field which is too low reliably to achieve ohmic start-up [1]. In Townsend avalanche breakdown theory this stray field makes it difficult to initiate the plasma in the limited toroidal electric field. ECH-assisted start-up, especially ECH preionization, has been carried out for the past few years and was successfully achieved at a low toroidal electric field (Et ≤ 0.3 V/m). These results were reviewed in references [3, 4] and were shown that ECH can reduce the minimum toroidal electric field to breakdown the plasma as ECH assist can provide an additional margin in the operation range of pressure and magnetic stray field.
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