Alcator C-Mod Weekly Highlights December 10, 2007 FY2007 weeks of research operations: Target: TBD Completed: 0.13 weeks Operations ---------- Plasma startup and conditioning operations continued at Alcator C-Mod last week. Four run days were scheduled and completed. A total of 87 plasma discharges were produced with a startup reliability over 80%. Upper and lower single null equilibria were produced with plasma currents up to 1MA and toroidal field of 5.4T. Initial operation of the ICRF system, LH system, and cryopump were successfully accomplished. One half-day of research plasma operations, dealing with the efficacy of the cryopump in modifying the relative concentration of hydrogenic isotopes in the discharge, was completed. All tokamak systems performed nominally during the week, with the exception of a minor voltage regulation issue in the OH1 power supply, which was compensated using the DPCS feedback control, and replacement of a capacitor in the OH1 commutation bank, which resulted in a two hour delay during Friday's run. Plasma Operations are scheduled to continue this week. Operation Details ----------------- Following a high temperature (120C) bake and discharge cleaning over the weekend, plasma operations resumed on Tuesday. Full length (~2 sec) upper and lower null discharges were produced, and initial ICRF operation using the J-port four-strap antenna was obtained. ICRF testing and conditioning continued on Wednesday, using a standard 1MA lower null target plasma. All three antenna systems (four transmitters) were exercised. The fast ferrite tuner (FFT) matching system was employed on the E-port antenna. The D-port antenna was conditioned up to the nominal 2MW power level. The addition of prematching transformers in the D network reduced the voltage in the 9" coax section by ~30% and likely contributed to the high power obtained. The E-port antenna conditioned to 1.75MW; some communications issues with the FFT control computer were identified and are being addressed. The four-strap J-port antenna was conditioned up to 1.5MW. Power the J-Port antenna was limited by trips generated by the rf protection system in response to arcs we believe are associated with moisture in the external transmission line. A dry nitrogen flow has been instituted to dry out the line prior to the next ICRF operation. H/D ratios on Wednesday were too high for efficient minority heating, as is typical during early runs following personnel-access vacuum openings. On Thursday we resumed operation of the C-Mod upper divertor cryopump system. The primary goal of the experiment was to see if we could exert some control over the H/(H+D) concentration in a discharge by a combination of strong gas puffing and cryopumping. In addition we wanted to see if such puff-and-pump scenarios might help speed up the removal of hydrogen from the vessel. Upper null discharges were run both with and without pumping and the H/(H+D) behavior during the discharge monitored, along with the quantity of hydrogen recovered in the gas phase after the shot, keeping the gate valves closed during pump regeneration. 800kA discharges with line-averaged density of ~1.6e20/m^3 were employed for most of the experiment. With the cryopump on, these shots required a total of about 130 Torr-l of D2 injected and recovered, compared to about 30 Torr-l in the unpumped cases. With the pump active, the H/(H+D) levels in these discharges were as low as 4%, compared to 10-15% with the pump warm. Quantitative analysis of the recovered gas is underway to determine whether the rate of net hydrogen removal was enhanced using this technique. Friday marked the first day of lower hybrid operation with plasma for the current campaign. The primary goal was to verify system operation and condition the launcher. All elements of the LH system performed nominally during the run. The power was increased shot-by-shot in small increments up to a maximum net power of 700kW, which is comparable to the maximum power level achieved during the 2007 campaign. At this level an arc in the external waveguide system (jungle gym) detected by the coupler protection system generated a trip and limited the pulse to 475 msec of the nominal 0.5sec pulse that was programmed. Some density rise was observed during high power LH pulses and additional conditioning will be required. Run Planning ------------- The C-Mod Experimental Program Committee met on Monday, Dec 3. The main item of business was reviewing new miniproposals for the early period of the FY08 campaign. Three MP's, two in the MHD/Control topic and one from the Lower Hybrid Physics group, were approved for near-term scheduling. Run planning for the physics phase of the 2008 C-Mod experimental campaign is underway. Most task groups held proposal prioritization meetings or carried out planning for their efforts using e-mail during the week. A project-wide general meeting is planned for this week to review the plans. Lower Hybrid System ------------------- A successful test was carried out on the first of the repaired circulator units on the test stand. The circulator now operates properly at high power into a matched load. Additional testing into a shorted output is planned. A second repaired circulator is being prepared for testing. Diagnostic Neutral Beam System ------------------------------ Late in the week installation began on a refurbished DNB cathode assembly to replace the old cathode unit, which is nearing the end of its life after 1.5 years of heavy use. Hydrogen fueling and water cooling have been reconnected, and the new cathode assembly is now pumped down. Electrical connections will be finished on the next maintenance day, after which testing and conditioning of the new cathode will commence. Travel and Visitors -------------------- Randy Wilson (PPPL) visited MIT on Thursday and Friday. He chaired the Lower Hybrid Group run planning session on Thursday and participated in the Lower Hybrid experiment on Friday. Stewart Zweben (PPPL) was at C-Mod last week to work on the calibration of the Phantom camera and upgrades to the GPI diagnostic. Martin Greenwald attended the Fusion Power Associates meeting in Oak Ridge to present a summary of the FESAC report "Priorities, Gaps and Opportunities: Towards A Long-Range Strategic Plan For Magnetic Fusion Energy". Miklos Porkolab also attended the annual Fusion Power Associates meeting on Dec. 4-5 in Oak Ridge, TN and made a presentation entitled "Research Highlights and Plans for Alcator C-Mod" on behalf of Earl Marmar and the entire C-Mod Team. The previous week, on November 29, Miklos attended the Annual Meeting of the Hungarian Nuclear Society in Budapest, Hungary, and gave a presentation with the title : "Plasma Physics and Controlled Nuclear Fusion Research". He was also awarded the Karoly Simmony Plaque and Prize, the newly established Hungarian prize for outstanding achievements in plasma physics and fusion research. Paul Bonoli attended a workshop on RF/MHD coupling related to the SWIM Prototype FSP Project. The workshop was held at the University of Wisconsin, Madison from December 4-5, 2007. He gave a talk at the workshop titled "Physics Issues Related to RF Models in the Ion Cyclotron and Lower Hybrid Range of Frequencies". _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly