Alcator C-Mod Weekly Highlights August 9, 2010 FY2010 weeks of research operations Base Target: 13 weeks Base Completed: 11.8 weeks ARRA Target: 5 weeks ARRA Completed: 5.0 weeks Plasma Shots: 2142 Operations ----------- Plasma operations continued at Alcator C-Mod last week. Four run days were scheduled and completed. A total of plasma 101 discharges were produced with a startup reliability of 86%. Runs supported research in the Boundary Science, Transport and Lower Hybrid Physics topical areas. The first boronization of the Summer 2010 Experimental Campaign was successfully completed over Tuesday night. Plasma operations are planned to continue this week. Physics ------- Determination of the hydrogen fraction (nH/ne) based on PCI measurements of the localization of the mode conversion layer in ICRF discharges has been found to be in relatively good agreement with edge spectroscopic measurements of nH/(nH+nD), subject to reasonable assumptions about Zeff and impurity ion charge. The technique was employed during the cleanup and conditioning phase of the current campaign, in discharges with spectroscopic ratios up to 50%. Operations Details ------------------ Tuesday's experiment was devoted to MP#570, "Boundary layer heat transport experiments in L-mode plasmas", which supports the 2010 SOL Heat Flux Joint Research Target. Experiments were carried out with ohmic L-mode plasmas at B=4T, Ip=800kA, and at B=5.4T, Ip=1.05MA. Density scans were completed for each set of conditions. The diagnostic set included the IR camera, divertor probes and thermocouples, and scanning probes to map the upstream SOL profiles. The strike point was dynamically swept across the outer divertor plate so that the embedded probes sampled the heat flux footprints. Gas puff imaging was used to observe the upstream turbulence spectrum. Wednesday morning was spent reconditioning the ICRF antennas following overnight boronization. The hydrogen fraction following boronization was reduced to <5%, suitable for the minority proton heating scenario, and remained low throughout the day. The D- and E-port dipole antennas conditioned to over 1MW each. The four-strap J-port antenna required additional time but reached the 1MW level required for the planned afternoon experiment after about 16 discharges (1:00PM). On Wednesday afternoon and Thursday morning we performed an experiment (MP#614) to examine the effects on H-mode pedestal transport induced by launching lower hybrid waves. Naoyuki Oyama of JAEA was on-site to participate in this research, which contributes to an ITPA joint experiment on pedestal and ELM control using electron heating and current drive (PEP-22). We successfully reproduced the density pump-out previously observed with LH in a target 600kA EDA H-mode, but this time with a lower wave refractive index than before. The H-mode core plasma is now clearly inaccessible to the launched LH waves, better supporting the conjecture that the pedestal modification is the effect of a local edge interaction. A LH power scan revealed that some H-mode density modification can be observed down to the 200kW level. Subsequent attempts in 800kA H-mode resulted in transient ELM-free H-modes, with reduced rate of density rise following the H-mode formation when LH was applied. The experiment on Thursday afternoon was in support of MP#468, "LH-driven fast electron diffusion time measurement", which comprises part of the thesis research of an MIT graduate student. The goal of this experiment was to extend the database of hard X-ray spectra obtained during 2008 by using the new LH2 coupler to obtain data at a phasing of 146 degrees (corresponding to n-parallel of 3.1, with target plasmas at nebar~8e19/m^3 and plasma current of 0.5 and 1.0MA, and somewhat higher density (nebar~1.25e20) at Ip=800kA. This experiment requires modulation of the LH power with a 50% duty cycle and 25 msec period. Difficulties were encountered with coupling at the desired n-parallel, and also with using the previously developed modulation technique due to modifications to the CPS (Coupler Protection System). This experiment has been rescheduled. Friday's run continued work on MP#612, "Expanded Exploration of the LH density limit".The goal of the day was to both confirm the data on the density limit taken during the 2008 campaign and examine the role of plasma topology and fueling on the density limit. Ramped density scans were peformed in single and double null, and also in inner wall limited discharges. Single density scans show agreement in non-thermal electron bremsstrahlung from the HXR camera as compared to results from 2008. Preliminary analysis shows a small increase in HXR emissivity at high density in double null as compared to single null. HXR emission appears to decrease like 1/ne in inner wall limited discharges up to line averaged densities of 1.6x10^20 m^-3. ARRA Activities --------------- Four FIR detectors developed under ARRA funding to be used in the C-Mod Polarimeter arrived last week, and preparations are being made to test them. ICRF Systems ------------ All three ICRF antennas were successfully reconditioned to moderate power levels following boronization on Tuesday night. E-port and J-port antennas were successfully employed, together with the LH system, in support of MP#614 on Wednesday and Thursday. Lower Hybrid System -------------------- The LH system was used in support of physics runs on Wednesday, Thursday, and Friday. The Tuesday night boronization did not effect high power operation of the LH launcher on Wednesday morning, and the LH and ICRF antennas (J- and E-port) operated in unison with minimal difficulty Wednesday afternoon and Thursday morning. The LH and ICRF antennas had not successfully operated together before the boronization. The center four columns of the LH launcher were operated at 120 kW/klystron, corresponding to a forward power density of 55MW/m^2 and a net power density of 40MW/m^2. The LH system also operated effectively in EDA H-mode discharges with ICRF and produced several ELM-free H-modes without ICRF. Progress was made towards a better understanding of the LHCD density limit on C-Mod with new data taken at a wide range of densities in double null and limited plasma configurations. Travel and Visitors ------------------- Martin Greenwald traveled to Washington D.C. to attend a "NERSC requirements workshop" for FES. He also attended a focus group, convened by undersecretary Koonin, to comment on the DOE 2010 strategic plan currently under development. Rajesh Maingi of ORNL was on site on Monday to meet with Brian LaBombard and Jim Terry. They discussed actions needed to complete the FY10 FES Joint Facilities Target on scrape-off layer heat flux characterization. While here he gave a talk entitled "Triggered confinement enhancement and pedestal expansion in NSTX: the Enhanced Pedestal H-mode" Naoyuki Oyama of JAEA visited MIT Aug. 2--6 in order to participate in experiments, discussed above, involving the application of lower hybrid waves into H-mode plasmas and modification of the edge pedestal. Yuri Podpaly attended the ASOS (Atomic Spectra and Oscillator Strength) 2010 meeting in Berkeley, CA from August 3rd through August 7th and presented an invited talk titled "Tungsten Measurements on Alcator C-Mod and EBIT for Future Fusion Reactors." All conference presentations will be available at http://sprg.ssl.berkeley.edu/labastro/ASOS10/. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly