Experimental Study of RF Sheath Formation on a Fast Wave Antenna and Limiter in the LAPD

Ion cyclotron resonance heating (ICRH) will be an essential component of heating power in ITER. During ICRH, radio frequency (RF) sheaths may form both at the exciting antenna and further away, e.g. in the divertor region, and may cause wall material sputtering and decreased RF power coupling to the plasma. It is important to do detailed laboratory experiments that fully diagnose the sheaths and wave fields. This is not possible in fusion devices. A new RF system has recently been constructed for performing such studies in the LAPD plasma column ($n_{e} \sim 10^{12}-10^{13}cm^{-3},$ $T_{e} \sim 1-10eV, \quad B_{0} \sim 400-2000G,$ diameter $\sim 60cm,$ length $\sim 18m)$. The RF system is capable of pulsing at the 1 Hz rep. rate of the LAPD plasma and operating between 2 -- 6 MHz (1$^{\mathrm{st}}$ -- 9$^{\mathrm{th}}$ harmonic of f$_{\mathrm{ci}}$ in H) with a power output of 200 kW. First results of this system driving a single-strap fast wave antenna will be presented. Emissive and Langmuir probe measurements in the vicinity of both the antenna and a remote limiter and wave coupling measured by magnetic pickup loops will be presented.