ECRH-assisted start-up in ITER

Abstract

In ITER, the electric field applied for ionization and to ramp up the plasma current may be limited to ≈0.3 V m −1 . In this case, based on established theories of the avalanche process, it is shown that ohmic breakdown in ITER is only possible over a narrow range of pressure and magnetic error field. Therefore, ECRH may be necessary to provide robust and reliable start-up. ECRH can ensure prompt breakdown over a wide range of prefill pressure and error field and can also give control over the initial time and location of breakdown. For ECRHassisted start-up in ITER, the power and pulse length requirements are essentially determined by the need to ensure burnthrough, i.e. complete ionization of hydrogen and the transition to high ionization states of impurities. A zero-dimensional (0D) code (with inclusion of some 1D effects) has been developed to analyse burnthrough in ITER. The 0D simulations indicate that control of the deuterium density is the key factor for ensuring successful start-up in ITER, where the effects of neutral screening and dynamic fuelling by the ex-plasma volume are also crucial. It is concluded that without ECRH, successful start-up will only be possible over a very restricted range of parameters but 3 MW of absorbed ECRH power will ensure reasonably robust start-up for a broad range of conditions with beryllium impurity. In the case of carbon impurity, even with an absorbed ECRH power of 5 MW one may be restricted to low prefill pressure and/or low carbon concentration for successful start-up.

Study of plasma start-up initiated by second harmonic electron cyclotron resonance heating on WEGA experiment

“…Therefore, the plasma start-up requires an external heating source in a stellarator, to compensate the lack of internal ohmic heating. In ITER, pre-ionization and assisted start-up with ECRH (or ICRH) will be necessary to achieve plasma breakdown because of the low toroidal electric field (≤ 0.3 V/m) compared to the large volume of the vessel [2].…”

Second Harmonic 110 GHz ECH-assisted Start-up in KSTAR

“…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 addition, in-vessel structure generates considerable in-vessel poloidal magnetic stray fields [2].…”

Studies on absorption of EC waves in assisted startup experiment on FTU

“…A key aspect is a correct evaluation of the EC absorbed power and its localization, assuming that it is more effective to deposit it in the magnetic null. Absorption is generally assumed high, and total after several reflections at the wall [3] including a favourable polarization conversion (from OM to XM) occurring at wall reflection in case of oblique injection. Nevertheless no detailed experimental data or computational analysis, on power absorption in the initial phase of tokamak plasma, are available up to now.…”

“…In this work a comparison of absorption between perpendicular and oblique injection is presented on the basis of FTU data [4], as a first attempt to explain the observed differences between the two cases and in view of an extrapolation to the ITER case. [3] have been performed varying filling pressure, toroidal electric field and to start preliminary comparison between perpendicular and oblique injection of EC waves [4]. In the experiments the power was switched on before the start of the electric field, raised from zero to the set value.…”