Simulations of lower-hybrid coupling in the Madison Symmetric Torus

An ana\-lysis will be presented of radio-frequency (RF) coupling with the inter-digital line slow-wave antenna used for lower-hybrid (LH) heating and current drive at 800~MHz in the Madison Symmetric Torus (MST) reversed-field pinch (RFP). The primary simulation tool was the VORPAL code, but MicroWave Studio and RANT3D/AORSA1D-H were also used. Due to the special requirements of the RFP configuration (tight-fitting conducting shell in which only minimal portholes are acceptable to maintain MHD stability), the unusual inter-digital line antenna was chosen. Accessibility in MST requires a very large parallel wave number $k_\parallel$, with $N_\parallel = c k_\parallel / \omega > 7.5$. A blind V\&V exercise done in vacuum showed excellent agreement for the phase difference between the antenna rods, with VORPAL and measurement differing by only $1.0^\circ$, but with MWS deviating more. Unfortunately the phasing excites a wave with $N_\parallel$ approximately 10\% too small. With plasma, VORPAL gives $N_\parallel$ around 15\% below the accessibility limit. VORPAL simulations performed on ANL Intrepid to investigate antenna modifications to increase $N_\parallel$ will also be presented.