Alcator C-Mod Weekly Highlights January 28, 2008 FY2008 weeks of research operations: Target: TBD Completed: 3.4 weeks Operations ---------- Plasma operations continued last week, with four run days completed. A total of 94 plasma discharges were produced, with a reliability of 89%. Experiments proposed by the Lower Hybrid, Transport, and Operations groups were carried out. Between-shot boronization was carried out during the run on Tuesday, and an overnight boronization was performed before Friday's run. Four hours of run time were lost due to RF and computer data acquisition delays last week. Plasma operations are scheduled to continue this week. Operation Details ----------------- The run on Tuesday was devoted to MP#484, which is designed to test the use of "atypical" equilibria during post-boronization RF conditioning to minimize erosion of boron films in the locations which dominate the molybdenum source in standard lower null plasmas. Between-shot boronization was employed for these tests, with the Surface Science Station (S^3) used to monitor deposition rates and localization. A high-triangularity shape (JFT2-M shape) with the outer separatrix leg located on the divertor floor rather than the vertical plate was employed as the trial equilibrium. This shape was not successful in preventing erosion of boron films in the important locations, as evidenced by high molybdenum radiation in subsequent LSN discharges that was similar to levels in the control cases. Both inner wall limited and upper divertor configurations still need to be explored. On Wednesday the lower hybrid system was operated and the launcher was scanned forward relative to the local LH guard limiter location, with the goal of improving wave coupling and increasing the net power delivered to the plasma. Net sustained LH power of 875 kW was achieved. The launcher operated successfully with grill-to-limiter separations of 1mm, 0.5mm, and finally with the grill flush with the local limiter surface. The reflection coefficients decreased as the launcher was moved forward, as expected. No hot spots or injections from the launcher were observed even at the most forward position. The run on Thursday was devoted to MP#493 "Ohmic L-H Threshold Experiments", which has the goal to investigate in detail the plasma conditions near the last-closed flux surface (pressure gradients, collisionality, plasma flows) that are associated with ohmic L-H transitions. Ohmic H-modes were produced, using the usual technique of reducing the toroidal field and then ramping it back up following the L-H transition, at plasma currents of 0.6, 0.8, and 1.2 MA. High quality profile data were obtained, including well-resolved measurements of the velocity shear layer in L- and H-mode. Analysis of these data is underway. Friday's experiment supported MP#514 "Influence of magnetic topology on the H-mode pedestal", and followed up on earlier experiments examining the modification of the pedestal parameters as the magnetic balance is dynamically changed from lower (SSEP=-5mm) to double to upper null (SSEP=+5mm) configurations. These experiments also made use of the upper divertor cryopump. We reproduced the previously observed pump-out of the density pedestal as magnetic balance shifts toward USN, and saw that this trend continued as we went to higher q discharges (Ip=600kA). We had difficulties producing steady EDA-like H-modes at lower q (Ip=1MA), which is consistent with previous experience, particularly when approaching unfavorable ion grad-B drift direction. Despite the good pump-out for SSEP>0, the collisionality was generally lower for SSEP between about -4mm and -2mm. Our best confinement was achieved in this range of magnetic balance, with relatively high edge beta_pol, and the appearance of high-frequency small ELMs. Physics --------- David Mikkelsen (PPPL) worked with Jeff Candy and Ron Waltz at GA on GYRO simulations of peaked density H-mode plasmas in C-Mod and discussed the availability of codes that could be used for validating the TGLF transport model. Lower Hybrid System -------------------- The lower hybrid system ran at up to 900 kW of coupled power last week. Faulting in external feedline components (jungle gym) limited performance beyond this power level. Inspections and high power testing of these components will be performed this week. David Terry and Pat MacGibbon were at the klystron vendor last week for acceptance tests of the #117 klystron rebuild. It had been conditioned up to full power at low duty cycle when they arrived. However, during removal of the tube from the socket, a vacuum leak occurred that will require some reprocessing of the tube. David and Pat also witnessed testing of our #106 klystron. The rf performance of this tube is very good, but it will require a new cathode/filament assembly. Diagnostics ----------- Steve Scott and Manfred Bitter (PPPL), and Jinseok Ko (PSFC/MIT) are examining design options for a new in-vessel optical system for the C-Mod Motional Stark Effect (MSE) diagnostic. There appear to be some designs using spherical mirrors positioned at an angle of 46-68 degrees that would provide sufficient focusing and etendue and which would not interfere with other in-vessel hardware. These designs would remove all in-vessel lenses and would therefore eliminate stress-induced birefringence. The FRCECE high resolution heterodyne radiometer electron cyclotron emission diagnostic is undergoing repair. Inspection of the low frequency mixer revealed contamination between the Gunn diode and the choke contact surface, and repairs are underway. The high frequency mixer is undergoing evaluation at the vendor. Education --------- On Tuesday, as part of the MIT Plasma Science and Fusion Center participation in the MIT Independent Activities Period, C-Mod scientists Catherine Fiore and Amanda Hubbard presented lectures entitled "A Hitchhiker?s Guide to Fusion Basics" and "Advanced Scenario Research on C-Mod: Towards a Better Tokamak Reactor", respectively. Summaries of these presentations and other lectures from the IAP@PSFC Series can be found at http://www.psfc.mit.edu/news/iap08.html . Travel and Visitors -------------------- Randy Wilson (PPPL) visited MIT on Thursday and Friday last week for discussions regarding the TORIC RF-code. Bruce Lipschultz traveled to PPPL for the NSTX PAC meeting from 1/22-124. NSTX plans for the periods through 2010 and 2013 were discussed. Yijun Lin was invited to visit the Institute of Plasma Physics of Chinese Academy of Sciences (ASIPP) in Hefei, China, from 1/10 to 1/12 and to give a series of lectures on ICRF physics and applications on tokamaks at a plasma winter school sponsored by the Chinese Academy of Sciences. On 1/12, he discussed potential collaborations between ASIPP (EAST and HT-7) and C-Mod on ICRF, LH, divertor tungsten tile fabrication, long pulse MDSplus and other issues. Jim Terry spent the week of Jan. 15-18 at GA. He served on DIII-D's Program Advisory Committee, which held a program review Jan. 15-17. He also had discussions with Tom Osborne (GA) about edge profile fitting and analysis codes and with Rejean Boivin (GA) and Steve Allen (LLNL) about diagnostics. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly