ECRH-assisted start-up in ITER

In ITER, the electric field applied for ionization and to ramp up the plasma current may be limited to approximate 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 strut-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 ECRH-assisted 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.