PFC / JA - 94 - 02 First Ohmic H - modes in Alcator C - MOD

Ohmic H-modes have been achieved in Alcator C-Mod with toroidal fields up to BT5.25 T and line averaged densities up to n e 1.4 x 1020 m-3 with less than 1 MW of input power. In ELM-free cases at lower toroidal field, the energy confinement increases by at least 60% and the particle confinement increases by more than a factor of two. The ELM-free H-modes appear to fit the threshold power scaling observed on other tokamaks, P /S (MW / m2 ) = 4.4 x n,eBT (1019 m3 T), up to power densities of about 0.15 MW/M2. At higher toroidal field, ELMy H-modes were observed after the injection of lithium pellets at about half of the scaled threshold power density. All of these results were obtained with molybdenum plasma facing components and the ELM-free H-modes were obtained without boronization or other wall coating. Particle and energy confinement in a tokamak plasma have been observed to improve spontaneously in a large number of experiments throughout the world when certain edge conditions are achieved and a threshold input power is exceeded. Such an improved or high confinement regime, known as an H-mode, was first observed on ASDEX [1] when sufficient neutral beam power was injected into a well conditioned diverted discharge. With high neutral beam power (PNB > 18 MW) and substantial wall conditioning, H-modes have been achieved on JT-60U up to BT = 4.2 T with n e ! 3 x 1019 m-3 [2]. On several tokamaks, through extensive wall conditioning techniques to reduce impurity influxes, H-modes have also been achieved with ohmic power alone up to BT 5 2.1 T and ne 8 x 1019 m-3 [3-5]. The threshold input power necessary to achieve H-mode was found to increase with increasing toroidal field [2-6] and electron density [4,5], which led to the conclusion that high field, high density tokamaks would find it difficult or impossible to operate in the H-mode [7]. The initial results from Alcator CMod [8] show that ohmic H-modes can be achieved at toroidal fields at least as high as BT = 5.25 T and line averaged densities up to 1.4 x 1020 m-3 with all molybdenum plasma facing components. The first indications of ohmic H-modes on Alcator C-Mod (R = 0.67 m, a = 0.21 m, Ic = 1.6) were observed on the first day of operation with the ion VB drift direction toward the lower divertor in a lower single null X point magnetic configuration (Fig. 1). This field configuration was chosen because it was found to have a lower H-mode power threshold on other tokamaks. The toroidal field was ramped down at a rate of about 2 T/s with the plasma current held constant at Ip = 0.65 MA. Typically, ELMy H-mode transitions were observed as the toroidal