Alcator C-Mod Weekly Highlights April 22, 2008 FY2008 weeks of research operations: Target: 15 weeks Completed: 12.2 weeks Operations ---------- Research operations continued at Alcator C-Mod last week. Three run days were scheduled and completed. A total of 82 plasma discharges were produced with a reliability of 84%. The three runs supported experiments from the Integrated Scenarios and Transport research programs. Overnight boronizations were carried out prior to each run day. No plasma operations were scheduled on Thursday or Friday so that the ICRF Group could begin the process of modifying the frequency of the FMIT#3 and 4 transmitters (J-port antenna) from 78 MHz to 50 MHz, in preparation for experiments scheduled to begin this week. Plasma operations are planned to continue this week. Operation Details ----------------- Monday's experiment was devoted to MP#533 "Producing the Hybrid Scenario on C-Mod with LHCD". A target 600kA discharge with X-point formation before 0.2 sec was established. Lower hybrid current drive (LHCD) was applied during the current rise (0.15sec and 0.2sec), resulting in delayed onset of sawteeth from 0.25sec to until well after the beginning of the flattop phase (0.3sec). The longest delay of the sawteeth was to 0.57sec. Simultaneous application of 1MW of ICRF and LHCD during the current raise resulted in earlier appearance of sawtooth activity. The timing of the start of high power (3MW) ICRF heating was varied from 0.3 to 0.7sec. Later start times resulted in higher stored energy. The optimum timing was between 0.5 and 0.6sec, with H98 > 1 obtained. The LHCD was observed to change the initial rise of the plasma stored energy and the way the discharge enters H-mode. Continuation of LHCD into the flattop during the ICRF heating phase also modified the evolution of the stored energy. Tuesday's run continued research under MP#538 "E_r profile studies", which comprises part of the thesis research of an MIT graduate student. Experiments were conducted in LSN EDA discharges and in USN H-mode (ELM-free and EDA) and Enhanced L-mode discharges. Enhanced L-mode operation in the USN configuration was investigated on Wednesday, which was primarily devoted to MP#539a "Development of enhanced L-mode scenarios". A portion of this extended run day was also devoted to investigating USN enhanced L-mode discharges at higher current, as part of MP#496 "High performance operation at Ip > 1.2MA". Variation in the triangularity and the distance of the x-point from the upper divertor ceiling, as well as variations in target plasma current and density were found not to significantly affect the L-H threshold in terms of the net input power, although the radiated power fraction was affected. In a discharge in which the input power was held just below the H-mode threshold, a Quasi-Coherent mode, normally a characteristic of EDA H-modes, appeared and lasted for hundreds of msec. Passive CXRS measurements at the outer edge showed variations in the poloidal velocity profile before the H-transition with an E_r well starting to form. In the high current discharges very high poloidal velocities were measured (30 km/s). Shots with high triangularity and power just above the L-H threshold transitioned into EDA H-modes at relatively low density, and in some cases exhibited ELMs in addition to the QC mode. At the highest currents explored (1.35MA) the confinement in L-mode was high (H98 > 1), impurities were well controlled and radiated power did not increase. These discharges had stored energy Wmhd > 200 kJ, volume average pressure above 1.5 atmosphere, and pedestal temperatures approaching 1 keV. ICRF Systems ------------- The ICRF Group reconfigured J antenna for 50 MHz operation and retuned the FMIT#3 and #4 transmitters to 50 MHz. The transmitters were tested to ~2 MW into dummy load and initial vacuum tests of the antenna were performed. First plasma operation at 50 MHz for this campaign is scheduled to commence next week. We also investigated the final power amplifier frequency spectrum up to 2 GHz for parasitic oscillations. One possible cause of variable lifetime of the final power amplifier 4CM2500KG tubes is parasitic oscillations, which result in localized electron beaming and excessive localized heating. Measurements indicate that one of our FPA tubes had no parasitic oscillations above 1 GHz but had a significant 850 MHz oscillation. The other two tubes had significant modes in the range of 1-2 GHz. Data Acquisition and Computer Systems -------------------------------------- A 12 TB disk array was installed to provide an off site second copy of the C-Mod data archive. The transfer of the file copies is almost complete. We intend to expand this backup storage, in parallel with expansion of the storage for the primary data archive. The C-Mod iSCSI disk network was reconfigured with redundant paths to all of the equipment. This will allow for automatic fail-over in the event of a large class of hardware failures. At the same time, the general network cabling for the infrastructure computers was re-organized and re-labeled. Travel and Visitors -------------------- Paul Bonoli hosted the 13th Steady State Operation ITPA Topical Group Meeting at the PSFC-MIT on April 15-17, 2008. The meeting was attended by approximately 25 physicists from the US, EU, Russia, Japan, and Korea. Presentations and discussions were made on issues related to integrated scenario modeling and actuator benchmarking for different operating scenarios in ITER including the ELMY H-mode Scenario 2, the Hybrid Scenario, and the Steady State Scenario 4. Results and plans related to joint experiments were also made. Presentations from MIT were made by Dr. Amanda Hubbard on "Recent Lower Hybrid Current Drive Results from Alcator C-Mod" and by Paul Bonoli on "Physics Research in The SciDAC Center for Simulation of Wave-Plasma Interactions". George Sips (IPP-Garching), chair of the SSO ITPA Topical Group, visited C-Mod prior to the meeting and served as co-Session Leader for the experiment on Monday, April 14. On Monday, April 14, 2008, Dr. Gerardo Giruzzi (Tore Supra) and Dr. Angelo Tucillo (Frascati) visited the PSFC and had discussions with Professor Ron Parker, Dr. Paul Bonoli, Dr. Stephen Wukitch, and Greg Wallace on possible collaborations in the areas of ICRF and LHRF physics. On Tuesday Brian LaBombard traveled to Dartmouth College and gave a Plasma Seminar at the Department of Physics and Astronomy entitled "Plasma Physics at the Edge of a Tokamak Fusion Reactor". He met with Barrett Rogers to discuss edge transport physics and to make plans for simulating C-Mod's boundary region with an electromagnetic fluid turbulence code. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly