Alcator C-Mod Weekly Highlights January 14, 2008 FY2008 weeks of research operations: Target: TBD Completed: 1.7 weeks Operations ---------- Plasma operations continued last week, with three run days completed. A total of 69 plasma discharges were produced, with a startup reliability of 80%. Experiments proposed by the Lower Hybrid, MHD, H-mode Integrated Scenarios, and Operations groups were carried out. The first boronization of the FY08 Experimental Campaign was successfully carried out on Monday. A series of experiments using the Surface Science Station to document the ECDC plasma parameters and boron deposition rates were conducted over a period of several hours prior to the main boronization operation. Approximately 8g of boron was deposited using 10% B2D6 in 90% He gas. The cryopump was successfully operated during all plasma runs last week. The run originally scheduled for Friday had to be canceled due to an unplanned computer outage; this experiment has been rescheduled. Plasma operations are scheduled to continue this week. Operation Details ----------------- The first half of the run on Tuesday tested the proposed techniques of MP#496 "Development of Post-Boronization Discharges that Minimize Boron Erosion", employing LSN discharges that have the outer strikepoint on the horizontal target, i.e. a deep slot divertor during the requisite series of discharges for conditioning the ICRF antennas after boronization. This is motivated by the concern that flux surfaces connected to the ICRH antennas are causing enhanced erosion, first of the boron then the underlying molybdenum, via sheath rectification. After some tuning and conditioning, including about one hour of ICRF vacuum conditioning of the J-port antenna, the RF power came up on all the sources to a total of ~4.5 MW, with the RF power apparently limited by the presence of type-I ELMs, which were expected in this high triangularity shape. Radiated power was relatively well-controlled. Besides the type-I ELMs, which were readily obtained on each discharge, we also saw a broad fluctuation feature at ~800 kHz associated with these H-modes. These discharges may provide a good target for future ELM and H-mode studies. The remainder of Tuesday was devoted to MP#496 "High Performance Operation at Ip > 1.2 MA". While a number of 1.35MA discharges with elongation>1.8 were produced, the ICRF system had difficulty matching to these discharges at high power. The H-modes obtained were mainly ELM-free, and several persisted as ohmic H-modes after termination of the RF pulse. The cryopump was employed to provide density and neutral pressure control for these near double-null equilibria. Wednesday's run was devoted to MP#518, on the use of localized gas puffing to control the plasma density and wave coupling in front of the LH launcher. These experiments were carried out with upper null equilibria, and employed the cryopump to aid in maintaining the targeted low plasma density, nebar~6e19/m^3. Good coupling, i.e. low reflection coefficients, were obtained with outer gaps as large as 3.5cm, and the LH grill positioned 1.5mm behind its local guard limiter, i.e. over 4cm outside the LCFS. Edge densities of 4-5e17/m^3 measured by the LH Langmuir probes are consistent with the observed reflection coefficients of 20-30%. This result, obtained both with and without the use of local gas puffing at the launcher, was unexpected, because experiments during the 2007 campaign typically required much smaller outer gaps, <0.5cm, to obtain such good coupling. Clear decreases in the loop voltage, signatures of good current drive, were observed in most of these discharges, with ~500kW net lower hybrid power. Several plasmas exhibited locked modes, and in these cases there was essentially no current drive observed on the loop voltage, despite significant signs of fast electrons on the HXR and non-thermal ECE diagnostics. When the LH launcher position was moved out to 5mm behind the guard limiters, the reflection coefficients increased to 80-100% without a local gas puff; in this case the density measured at the front of the grill was much lower. Increasing the gas puff and reducing the plasma outer gap succeeded in raising the launcher density to values similar to those obtained earlier in the day, about twice the cutoff density, but the reflectivity improved only slightly, to ~50%. This result may indicate that the density gradient in the case of the more recessed position is less favorable. Thursday's run supported MP#480, "Damping rate of Toroidal Alfven Eigenmodes at the Threshold of Stability in the Presence of ICRF heating", which comprises part of the thesis research of an MIT graduate student. The experiment succeeded in obtaining a successful scan to high RF powers to determine the TAE stability threshold. L-mode was maintained in these USN configurations above the usual H mode threshold by injecting large amounts of argon to increase radiated power. Unstable TAEs were observed at RF powers above 3.5MW. The ICRF functioned very well and powers up to 5.5MW for 0.5s were achieved. All three channels of the Compact Neutral Particle Analyzer (CNPA) were operational, giving spatial and energy distributions of the energetic particles at each RF power level. The unstable modes, as well as the active MHD perturbations, were also observed by the Phase Contrast Imaging (PCI) diagnostic. Physics --------- Dennis Whyte presented calculations estimating the effect of neutron damage for tritium retention in refractory metal plasma facing components (PFC) such as Mo or W. A literature review showed that displacements from neutrons lead to production of traps (i.e. energy wells from which H cannot escape at low temperature) throughout the bulk of the PFC. Permeating D/T from the surface can access, and fall into, these traps throughout most of the thickness of the PFC armor due to the high diffusivity of D/T in refractory metals. The conclusion was that for ITER with an all-W divertor, the worst-case scenario would see the 350 g in-vessel tritium limit reached in ~ 500 shots. The low operational temperature of the ITER divertor precludes easy removal of the tritium from the volumetric traps in the W. ICRF Systems ------------- Recovery from the first boronization was similar to that of the last campaign: the D- and E-port antennas recovered within ~5 discharges while the J-port antenna voltage and power conditioning required a more lengthy procedure. After some vacuum conditioning and operation into plasma at current drive phasing, the J-port antenna condition improved sufficiently that we could couple 1.5 MW into an H-mode target within ~15 discharges. The antenna condition continued to improve with additional high power plasma discharges. We also began implementation of changes to the E antenna matching network. The addition of prematching components is designed to reduce the maximum voltage at the fast ferrite tuners to less than 30kV for high power operation into H-mode plasmas. Lower Hybrid System -------------------- Borescope inspections of the new circulators has been completed. Measurement of the S parameters has been carried out for one of the new units, and high power testing was begun. The design of the new four-way splitter prototype with no windows, to be used for high power testing, has been completed and passed on for fabrication. The lower hybrid system was successfully employed during the experiment on Wednesday. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly