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A Surface Science Station (S[superscript 3]) on the Alcator C-Mod tokamak is used to study and optimize the location and rate of boron film deposition in situ during electron cyclotron (EC) discharge plasmas using 2.45 GHz radio-frequency (RF) heating and a mixture of helium and diborane (B[subscript 2]D[subscript 6]) gasses. The radial profile of boron deposition is measured with a pair of quartz microbalances (QMB) on S[superscript 3], the faces of which can be rotated 360° including orientations parallel and perpendicular to the toroidal magnetic field B[subscript T] ~0.1 T. The plasma electron density is measured with a Langmuir probe, also on S[superscript 3] in the vicinity of the QMBs, and typical values are ~1 × 10[superscript 16] m[superscript −3]. A maximum boron deposition rate of 0.82 μg/cm[superscript 2]/min is obtained, which corresponds to 3.5 nm/min if the film density is that of solid boron. These deposition rates are sufficient for boron film applications between tokamak discharges. However the deposition does not peak at the EC resonance as previously assumed. Rather, deposition peaks near the upper hybrid (UH) resonance, ∼5 cm outboard of the EC resonance. This has implications for RF absorption, with the RF waves being no longer damped on the electrons at the EC resonance. The previously inferred radial locations of critical erosion zones in Alcator C-Mod also need to be re-evaluated. The boron deposition profile versus major radius follows the ion flux/density profile, implying that the boron deposition is primarily ionic. The application of a vertical magnetic field (B[subscript V] ~0.01 T) was found to narrow the plasma density and boron deposition profiles near the UH resonance, thus better localizing the deposition. A Monte Carlo simulation is developed to model the boron deposition on the different QMB/tokamak surfaces. The model requires a relatively high boron ion gyroradius of ~5 mm, indicating a B[superscript +1] ion temperature of ~2 eV, to match the deposition on QMB surfaces with different orientation to B[subscript T]. Additionally, the boron ion trajectories become de-magnetized at high neutral gas throughput (~0.5 Pa m[superscript 3] s[superscript −1]) and pressure (~2 Pa) when the largest absolute deposition rates are measured, resulting in deposition patterns, which are independent of surface orientation to B[subscript T] in optimized conditions., United States. Dept. of Energy (Award DE-FC02-99ER54512)

Minimizing impurity production associated with ion cyclotron range of frequency (ICRF) operation to an acceptable level in H-mode, particularly with metallic plasma facing components (PFC), is challenging. To identify important erosion and impurity source locations, we have vacuum plasma sprayed ~100 μm of boron (B) onto molybdenum (Mo) tiles. For ICRF heated H-modes, the core molybdenum levels have been significantly reduced and remained at low levels for increased injected RF energy. The core Mo level also no longer scales with RF power in L-mode. With boronization and impurity seeding (typically nitrogen or neon), the plasma and ICRF antenna performance were improved. Surprisingly, impurity seeding did not result in increased core Mo levels and also suppressed antenna faults. Spectroscopic monitoring of the plasma limiter found that the impurity profile at the limiter was centered near the plasma mid-plane and the profile did not change shape with plasma current. From post campaign inspection, the B coating was not significantly eroded except in locations where melting occurred or where it peeled. Trace material analysis also found that the B surface was contaminated with Mo and tungsten (W). Improved performance with impurity seeding, the lack of erosion, and metallic contamination of the B coating suggest ICRF impurity generation is related localized heat loads (that can lead to melting) rather than entirely a result of sputtering., United States. Dept. of Energy (Cooperative Agreement No. DE-FC02-99ER54512)

Radio frequency (RF) sheath rectification is a leading mechanism suspected of causing anomalously high erosion of plasma facing materials in RF-heated plasmas on Alcator C-Mod. An extensive experimental survey of the plasma potential (Φ[subscript P]) in RF-heated discharges on C-Mod reveals that significant Φ[subscript P] enhancement (>100 V) is found on outboard limiter surfaces, both mapped and not mapped to active RF antennas. Surfaces that magnetically map to active RF antennas show Φ[subscript P] enhancement that is, in part, consistent with the recently proposed slow wave rectification mechanism. Surfaces that do not map to active RF antennas also experience significant Φ[subscript P] enhancement, which strongly correlates with the local fast wave intensity. In this case, fast wave rectification is a leading candidate mechanism responsible for the observed enhancement., United States. Dept. of Energy (DE-FC02-99ER54512)

Impurity contamination and localized heat loads associated with ion cyclotron range of frequency (ICRF) antenna operation are among the most challenging issues for ICRF utilization.. Another challenge is maintaining maximum coupled power through plasma variations including edge localized modes (ELMs) and confinement transitions. Here, we report on an experimental assessment of a field aligned (FA) antenna with respect to impurity contamination, impurity sources, RF enhanced heat flux and load tolerance. In addition, 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 and 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. 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. Further we observe that the fraction of RF energy deposited upon the antenna is less than 0.4 % of the total injected RF energy in dipole phasing. The total deposited energy increases significantly when the FA antenna is operated in monopole phasing. The FA antenna also exhibits an unexpected load tolerance for ELMs and confinement transitions compared to the TA antennas. However, inconsistent with expectations, we observe RF induced plasma potentials to be nearly identical for FA and TA antennas when operated in dipole phasing. In monopole phasing, the FA antenna has the highest plasma potentials and poor heating efficiency despite calculations indicating low integrated E∥. In mode conversion heating scenario, no core waves were detected in the plasma core indicating poor wave penetration. For monopole phasing, simulations suggest the antenna spectrum is peaked at very short wavelength and full wave simulations show the short wavelength has poor wave penetration to the plasma core., United States. Dept. of Energy (DOE award DE-FC02-99ER54512), United States. Dept. of Energy (Fusion Energy Postdoctoral Research Program administered by ORISE)