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 c ase, based on established theories of the avalanche process, it is sho wn that ohmic breakdown in ITER is only possible over a narrow range o f pressure and magnetic error field. Therefore, ECRH may be necessary to provide robust and reliable strut-up. ECRH can ensure prompt breakd own over a wide range of prefill pressure and error field and can also give control over the initial time and location of breakdown. For ECR H-assisted start-up in ITER, the power and pulse length requirements a re essentially determined by the need to ensure burnthrough, i.e. comp lete ionization of hydrogen and the transition to high ionization stat es 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 al so crucial. It is concluded that without ECRH, successful start-up wil l 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 b road range of conditions with beryllium impurity. In the case of carbo n impurity, even with an absorbed ECRH power of 5 MW one may be restri cted to low prefill pressure and/or low carbon concentration for succe ssful start-up.