Alcator C-Mod Weekly Highlights May 21, 2012 FY2012 weeks of research operations Target: 18 weeks Completed: 5.8 weeks Plasma Shots: 955 Operations ----------- Plasma operations continued at Alcator C-Mod last week. Four run days were scheduled and three completed. Experiments were carried out on Monday, and Wednesday-Friday, with a maintenance day on Tuesday. A total of 73 plasma discharges were produced with a reliability of 82%. In addition to tokamak cleanup and diagnostic commissioning, the runs supported research in the Lower Hybrid Physics topical area. Plasma operations are planned to continue this week. Physics ------- Naoto Tsujii successfully defended his doctoral thesis, "Studies of ICRF Mode Conversion with Phase Contrast Imaging and Full-Wave Simulations in Alcator C-Mod", submitted to the MIT Department of Physics. Radio frequency (rf) waves in the ion cyclotron range of frequencies (ICRF) are widely used for heating fusion plasmas. In a multi-ion-species plasma, the launched fast waves convert to ion cyclotron waves and ion Bernstein waves around the two-ion hybrid resonances that exist between the cyclotron resonances. The mode converted waves are of interest as actuators to optimise plasma performance through current drive and flow drive. Numerical simulations are essential to describe these processes in a realistic tokamak geometry, and it is important that these simulation codes be validated experimentally. The phase contrast imaging (PCI) technique has been used on Alcator C-Mod to measure directly the rf waves. The measurements are compared to predictions of full-wave simulations using a synthetic diagnostic method. The measured mode converted wave intensity was found to be a factor of 50 weaker than what was expected from the linear wave theory in a strong mode conversion regime. The agreement improved at weaker wave intensity, which is a possible indication of nonlinear physics being involved. Operation Details ----------------- The runs on Monday afternoon and Wednesday morning were devoted to MP#605, "LH2 lower hybrid launcher checkout and commissioning". Despite some delays due to data acquisition and alternator power system issues on Monday, good progress was made in bringing up the LH power to over 800kW, and up to 1 sec pulse lengths at lower power. The run on Wednesday afternoon was devoted to MP#693, "Validating the inter-shot calibration technique for MSE diagnostics". The experiment entailed evaluation of the Faraday rotation in the MSE optics, using ohmic C-Mod pulses with only poloidal fields energized (no plasma), as well as specially designed plasma discharges including a dynamic plasma size ramp and plasma current ramps at different sizes. These discharges were used to obtain the polarization-pitch angle offset for the outer MSE channels and polarization pitch angle dependence for several channels. On Thursday we carried out MP#662, "HiReX Sr startup MP". The experiment was used to verify proper alignment and operation of the High Resolution X-ray Spectrometer diagnostic in preparation for the start of physics operation. Reproducible Ohmic plasmas were run while adjusting HIREXSR internally (focusing/masking/etc) on a shot-to-shot basis. The focus of both the H- and He-like systems was demonstrated. The repeated discharges were also used for diagnostic checkout by a number of other systems. The run on Friday was dedicated to MP#553,"Creation of reversed shear current profile using LH on low Ip plasmas". LHCD pulses with P=800kW and pulse length of 0.75 sec were employed. Good coupling was obtained for target plasmas with nebar~0.5e20/m^3 and plasma current of 500kA. The loop voltage was reduced to 0.16 Volts at these parameters. On several shots strong low frequency MHD activity was observed, with apparently deleterious effects on current drive efficiency. Analysis of these modes is ongoing. ICRF Systems ------------ During the maintenance day on Tuesday, we removed the damaged stub tuner, located in the power room, from the field aligned antenna matching network. The tuner's push-pull G10 rod showed evidence of arcing and the damaged section will be replaced with lower loss G11 material. In addition, analysis of new antenna S-parameter data indicates an error in previous measurements may have been responsible for higher than normal rf voltages at the stub tuner location. We are modifying transmission line components to address this issue. Lower Hybrid System ------------------- The LH system was used during plasma operations on Monday, Wednesday, and Friday. Net power levels of up to 350 kW for 1.0s and 800 kW for 0.75 s were achieved. Zero loop voltage was attained transiently at 500 kA, with steady loop voltage of < 0.15 V during the LH pulse. The engineer from our Transmitter Protection System (TPS) SBIR vendor visited the PSFC on Tuesday to discuss performance of the TPS. Diagnostics ----------- A power supply for one of the Thomson scattering lasers was returned to service, restoring dual laser operation of all Thomson diagnostics. Travel and Visitors -------------------- Professor Sadayoshi Murakami visited the Plasma Science and Fusion Center and the Alcator C-Mod Group from May 10-18, 2012 as a Visiting Scientist. His visit was hosted by Paul Bonoli and they continued an effort that was started in May, 2011 to simulate the toroidal rotation that has been measured on Alcator C-Mod during ICRF heating. Professor Murakami has been using his GNET Monte Carlo orbit code coupled with an RF kick operator that employs model RF fields to do these simulations. During his visit, the primary computational activity was a successful mapping of the MHD equilibria from C-Mod discharges computed by EFIT and stored in EQDSK files to the Boozer coordinates used in GNET. He also made significant progress in mapping the ICRF fields from TORIC into the format/coordinates needed in GNET. During his visit Professor Murakami also had extensive discussions with Dr. Catherine Fiore, Dr. John Rice, and graduate student Chi Gao concerning the rotation data that has been measured in Alcator discharges in order to determine cases that would be of interest for simulation. Jeehyun Kim from POSTECH University and Seungil Park from Korea's National Fusion Research Institute are visiting the PSFC during the period 14-24 May. Their visit is part of an ongoing collaboration between POSTECH and the PFSC. Drs. Kim and Park work on KSTAR and specialize in preparing the KSTAR lower hybrid program. While at the PSFC they will participate in LH experiments on Alcator C-Mod and become familiar with the LH simulation codes GENRAY/CQL3D and LHEAF. Sanjeev Varshney from ITER-India visited C-Mod from Wednesday-Friday. He is part of the group responsible for the ITER edge crystal spectrometer which will be viewing argon soft x-ray emission in the pedestal region of ITER. Dr Varshney met with John Rice, Matt Reinke and Chi Gao to learn how HIREXSR and our analysis routines work, as well as discuss possible future collaborations, such as the development of a prototypes for testing on C-Mod and future use on SST. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly