Thermal helium buildup and control in a fusing tokamak

The effect of thermal helium "ash" accumulation on the ignition burn parameter has been examined using a global model. The sensitivity of ash build-up to changes in transport coefficients was explored using a 1-1/2-D transport code for an ARIES-I type plasma. The effect of helium edge recycling was also examined using the code. In addition, the influence of controlled sawteeth on helium exhaust was explored. It is shown that ash accumulation potentially stops ignition of tokamaks, such as TIBER II/ETR-type, if ash concentration exceeds $\sim$10%. It is also shown that ash buildup is sensitive to the ratio of helium to fuel ion transport coefficient, and to the magnitude of the helium inward pinch velocity. Sawteeth are useful for removing the ash from the center of the plasma to the outer region. Outer region transport enhancement is found essential to help in removing the ash from there. It is found that the optimal sawtooth period to give minimum ash level (or maximum fusion power) is several times larger than the fast alpha slowing down time for the ARIES-I type driven plasma. With sawtooth period $\sim$5 seconds and helium transport selectively enhanced by an order of magnitude, the fusion power increases $\sim$30% compared to the case without sawteeth/without enhancement. It is shown that ash buildup effect can seriously degrade the plasma performance, and sawteeth combined with outer region transport enhancement have the potential to advantageously reduce the helium concentration.U of I OnlyETDs are only available to UIUC Users without author permissio