Impact of off-axis beam and low-field side gas fuelling on safety factor, rotation and fast ions MAST-U

The off-axis beam in MAST-U is utilized to deposit heating power and current drive avoiding MHD instabilities, but is inherently less efficient than the on-axis beam. We performed a systematic scan in beam timing to validate the q profile evolution change from MSE due to the off-axis beam. As part of the TRANSP analysis, the effects of reduced resistivity and bootstrap current due to heating are distinguished from NBCD. Doppler back-scattering is used to perform rotation and turbulence measurements in the k_⊥ρ~1.5-15 range [1,2], in low density on and off-axis beam heated plasmas showing stronger rotation towards the edge as expected. With the off-axis beam, fast ions are located closer to the edge neutrals and is thus more subject to the influence of low-field side gas fuelling, producing charge exchange losses as well as damping the ExB and/or toroidal rotation. Deliberate flooding of gas into the low-field side region resulted in a slowing of the rotation, enhancement of passive FIDA (a proxy for CX losses [3]), reduction in the neutron rate and lowering electron temperature. Such a loss may be a result of loss of heating power or degradation or thermal conductivity from a decreased ExB shearing rate, supported by theory.

[1] T. L. Rhodes et al RSI 93 113549 (2022)

[2] P. Shi et al JINST 18 C11022 (2023)

[3] P Ollus et al PPCF 66 025009 (2024)