Alcator C-Mod Weekly Highlights May 14, 2012 FY2012 weeks of research operations Target: 18 weeks Completed: 5.6 weeks Plasma Shots: 882 Operations ----------- Plasma operations continued with clean-up and conditioning runs at Alcator C-Mod last week. Three run days were scheduled and completed. A total of 56 plasma discharges were produced with a reliability of 62%. In addition to tokamak cleanup, the runs contributed to Diagnostic Development and ICRF conditioning. Diagnostics continue to be brought back on-line. During the maintenance day on Monday, a filter in the LN2 cooling line for one of the vertical TF legs was replaced in order to improve the flow to this leg, which had exhibited slower than normal cooling rates during the previous week's operation. The replacement was successful, and the between shot cooling rate returned to normal during last week's operation. Plasma operations are planned to continue this week. Physics -------- The paper "H-mode power threshold reduction in a slot divertor configuration on the Alcator C-Mod tokamak" by Y Ma, et al. has been accepted for publication by Plasma Physics and Controlled Fusion on 9 May 2012. The letter "Tungsten nano-tendril growth in the Alcator C-Mod divertor", by G. M. Wright, et al., has been published in Nuclear Fusion 52, 042003 (2012), and was featured on the cover of the print edition. Program Planning ---------------- A meeting of the Experimental Program Committee was held to discuss machine status, schedule and priorities for the balance of the FY12 campaign. An initial set of high priority runs has been developed - however it still requests more run time than is likely available. Given the current budget situation, priority will be given to experiments that provide students with data that will allow them to complete their thesis research. Topical group leaders have been asked to look for opportunities to combine experiments. Allocations for different groups will be developed. The rest of the meeting was spent reviewing a set of recently submitted experimental proposals. Operation Details ----------------- The run on Tuesday was intended to support MP#355, "ICRF Conditioning", but was hampered by poor startup reliability, and little progress was made on the conditioning. The cause of the startup difficulty was unclear, and did not appear to be related to machine cleanliness. On Wednesday, improved reliability was obtained after small adjustments were made to the feed-forward programming during the first 20msec after discharge initiation. The run on Wednesday was devoted to MP#700, "Edge CXRS dalpha diagnostic cross-calibration". The primary purpose of these experiments was to cross-calibrate the low field side and high-field side photodiode systems used to measure Dalpha for the edge CXRS system. This work supports the thesis research of an MIT graduate student. The experiments were successful in completing all of the planned Dalpha diode calibrations. A scan of the inner gap was also obtained. Thursday's run was devoted to MP#355, "ICRF Conditioning". The field-aligned (FA) J-port antenna was conditioned to a maximum power of 2.4 MW. Later in the day, the external transmission line feeding the J-port antenna began to experience faulting, which prevented further operation. This transmission line will be opened and repaired this week. The D- and E-port dipole antennas were each conditioned up to 1.5 MW. Because of the low absorption associated with the high H/D fraction, simultaneous operation of these two antennas at high power incurred some operational challenges. Most of the day's operation was carried out with LSN equilibria, but several upper null discharges were produced at the end of the day in order to begin conditioning the upper divertor strike surfaces. ICRF Systems ------------ During operations, we continued to increase the power on all systems. Some H-modes were achieved despite high H fraction and the antennas reached 2.4 MW for FA J antenna, 1.5 MW each for D and E antennas. For the FA-J antenna, we operated at voltages approaching 50 kV, which is about 25% higher than previous J antenna operation. After a period of high-power operation, the FA J antenna system developed an arcing issue that limited the power to <200 kW. Inspection and testing with a network analyzer determined that the arcing is not associated with the antenna itself, or the vacuum feedthroughs. The 6" transmission line network was also inspected and determined not to be implicated in the arcing. The arc location has been isolated to the long 9" coaxial transmission line connected to the FMIT#3 transmitter. Further work to localize the arc site will be carried out during maintenance this week. During operation on Thursday, one of the fast ferrite tuners (FFT) stopped working. We are evaluating this issue. Lower Hybrid System ------------------- An updated version of the transmitter protection system (TPS) firmware was installed to allow for higher values of the collector heat capacity. The higher values are necessary for the collector over temperature (COTS) calculation to agree with thermocouple measurements. The system was re-tested and is ready to resume operation. Diagnostic Neutral Beam System ------------------------------- The DNB LHe cryopumping is now active, and the DNB pulse length has been extended accordingly to 1 second (modulated) at full performance (50 kV, 7 amps). On Wednesday the DNB was used in a measurements involving MSE and CXRS. Work is underway to replace the DNB CAMAC-based digitizers with a CPCI-based system in order to increase data acquisition reliability. Diagnostics ----------- The Mirror Langmuir Probe electronics system has been connected to the A-port scanning probe, and is operational. Langmuir probe arrays on the lower inner and outer divertor, the lower hybrid launcher, and the upper divertor are operational and ready for use. A- and F-port scanning probes are operational. The 60, 75, 88 (sin), 112, and 140 GHz and variable frequency reflectometers are connected and turned on. The Thomson scattering system is operating with one 50 Hz laser. Precise calibration of the density is still required. Travel and Visitors -------------------- Will Bergerson, Matt Reinke, S. G. Baek, Luis Delgado-Aparicio, C. Kasten, C. Lau, R. Mumgaard, N. Pablant, Bill Rowan, John Rice, and C. Sung < and others? > participated in the High Temperature Plasma Diagnostics Conference in Monterey, CA, May 6-10. Copies of the talks and posters are available on the web at http://www.psfc.mit.edu/research/alcator/pubs/HTPD/HTPD_2012.htm . Matt Reinke, who presented an invited talk at the HTPD on analysis of the C-Mod Imaging X-Ray Crystal Spectrometer (HIREX_SR), also met with the EAST crystal spectrometer team to discuss the status of their instrumentation and their analysis routines to help inform possible collaboration work. The EC17 conference (17th Joint Workshop on Electron Cyclotron Emission and Electron Cyclotron Resonance Heating) was held Monday 5/7/12 to Thursday 5/10/12 in Duerne, Netherlands. From MIT/PSFC, Abhay Ram and Anne White attended. Abhay presented an invited theory talk on the interaction of edge turbulence and RF waves "Scattering of ECRF waves by edge density blobs and fluctuations in tokamak plasmas" and Anne presented an ECE experimental talk on the recent C-Mod results on the TS-ECE discrepancy, "Investigation of the Thomson Scattering-ECE Discrepancy in ICRF Heated Plasmas at Alcator C-Mod". The full conference programme is available here: http://www.differ.nl/ec17/ On Friday, May 11th, Anne White visited the University of York, in York, UK and gave a seminar at the York Plasma Institute, part of the Department of Physics, titled "Evolution of core turbulence and transport across the L-I transition in Alcator C-Mod". The abstract is available here: http://www.york.ac.uk/physics/news/events/groups/plasma-fusion/anne-white-seminar/ _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly