Energetic beam ion collisions with the main limiter can be a significant power loss process under certain operating conditions in Doublet III. Furthermore, these collisions may cause measurable damage to the limiter itself.Under low current and low toroidal field conditions (e.g., Ip = 290 kA and BT = 6.3 kG), 20–38% of the inferred absorbed beam power may be deposited directly on the ion drift side of the limiter by the beam ions. However, for higher plasma current and toroidal fields (e.g., Ip = 290 kA and BT = 15 kG), the fraction of inferred absorbed beam power deposited on the limiter is reduced to < 10%. Monte Carlo code simulations show that this loss of beam power is primarily a result of the large poloidal and toroidal gyro-orbits of the energetic beam ions. Other factors which may enhance beam ion losses to the limiter are (1) large separation distances between the primary limiter and the (outboard) vacuum vessel wall, and (2) plasma density buildup near the plasma edge during high gas puff operation.In addition, our data suggests enhanced plasma density and recycling near the limiter. This localized density can cause appreciable premature ionizations of the incoming beam neutrals and thus reduce the effective plasma heating of the beamline which is immediately upcurrent of the limiter. The prematurely-ionized beam particles from this adjacent beamline are responsible for much of the damage to the ion drift side of the limiter. We have found that under certain operating conditions (1) the direct beam heating of the limiter is 50% greater and (2) the stored plasma energy is 10% less when the beamline immediately upcurrent of the limiter heats the plasma. Thus, the relative positions of the limiters to the beamlines are important in designing future tokamaks.
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