Characterization and performance of a field aligned ion cyclotron range of frequency antenna in Alcator C-Mod

Abstract

Ion cyclotron range of frequency (ICRF) heating is expected to provide auxiliary heating for ITER and future fusion reactors where high Z metallic plasma facing components (PFCs) are being considered. Impurity contamination linked to ICRF antenna operation remains a major challenge particularly for devices with high Z metallic PFCs. Here, we report on an experimental investigation to test whether a field aligned (FA) antenna can reduce impurity contamination and impurity sources. We compare the modification of the scrape of layer (SOL) plasma potential of the FA antenna to a conventional, toroidally aligned (TA) antenna, in order to explore the underlying physics governing impurity contamination linked to ICRF heating. The FA antenna is a 4-strap ICRF antenna where the current straps and antenna enclosure sides are perpendicular to the total magnetic field while the Faraday screen rods are parallel to the total magnetic field. In principle, alignment with respect to the total magnetic field minimizes integrated E|| (electric field along a magnetic field line) via symmetry. A finite element method RF antenna model coupled to a cold plasma model verifies that the integrated E|| should be reduced for all antenna phases. Monopole phasing in particular is expected to have the lowest integrated E||. Consistent with expectations, we observed that the impurity contamination and impurity source at the FA antenna are reduced compared to the TA antenna. In both L and H-mode discharges, the radiated power is 20%-30% lower for a FA-antenna heated discharge than a discharge heated with the TA-antennas. However, inconsistent with expectations, we observe RF induced plasma potentials (via gas-puff imaging and emissive probes to be nearly identical for FA and TA antennas when operated in dipole phasing). Moreover, the highest levels of RF-induced plasma potentials are observed using monopole phasing with the FA antenna. Thus, while impurity contamination and sources are indeed reduced with the FA antenna configuration, the mechanism determining the SOL plasma potential in the presence of ICRF and its impact on impurity contamination and sources remains to be understood. V C 2013 AIP Publishing LLC. [http://dx.

Assessment of a field-aligned ICRF antenna

“…However, we have measured the plasma potentials associated with the FA and TA antennas using both GPI and emissive probes. [18] For dipole phasing, the maximum plasma potential (found on field lines nearest the radius of the side protection tiles) for the FA antenna is indistinguishable from that measured when the TA antenna is powered as shown in Figure 11. In hindsight, we may have expected this because previous plasma potential measurements of a toroidally aligned antenna at J port -TA@J (antenna replaced by the field aligned antenna) had lower plasma potential than the TA antenna also shown in Figure 11.…”

“…The measured plasma potentials when the FA antenna is operated in monopole phasing is much higher than when the FA antenna is operated in dipole. [18] However, experiments indicate that the elevated plasma potentials and poor heating efficiency is due to poor wave penetration to the plasma core. We performed a series of discharges where the heating scenario is mode conversion heating and the minority 3 He concentration is ~15%.…”

Development of slow and fast wave coupling and heating from the C-Stellarator to NSTX

“…Extensive efforts have been and are underway to quantify the near field and far field effects, and efforts are also underway to modify antenna designs to reduce these fields. Particularly noteworthy recent antenna advances include the field aligned antenna on C-mod [25] and the 3-strap antenna on ASDEX Upgrade [26]. …”

Major upgrades of the high frequency B-dot probe diagnostic suite on ASDEX Upgrade

“…Examples of tokamaks using ICRF systems are ASDEX Upgrade [1], Alcator C-Mod [2], Tore Supra/WEST [3], and JET [4]. ITER, the next generation tokamak [5], and DEMO, the first generation fusion power plant [6], are also going to rely on ICRF power for their operation.…”