Alcator C-Mod Weekly Highlights September 24, 2012 FY2012 weeks of research operations Target: 18 weeks Completed: 18.5 weeks Plasma Shots: 2571 Operations ----------- Plasma operations continued at Alcator C-Mod last week. Five run days were scheduled and four and a half were completed. A total of 136 plasma discharges were produced with a reliability of 93%. The runs supported research in the Transport Physics, Pedestal Physics, ICRF Physics, and Lower Hybrid Physics topical areas. A boronization was carried out on Wednesday evening. One and a half hours were lost on Tuesday morning due to a cryogenics system problem. About 2.5 hours of run time were lost on Wednesday in order to replace a capacitor in the EF1 commutation bank. Plasma operations are planned to continue this week. Operation Details ----------------- Run time on Monday and on Friday morning was devoted to MP#680, "Reduction of core turbulence and transport across the L-I transition". The overall goal of this experiment is to study changes in core transport and turbulence (core and edge) that occur across the L-I transition and to document as completely as possible the differences in turbulence/transport in the L-mode core and edge compared to I-mode. Past measurements at C-Mod have indicated that core and edge turbulence evolve differently prior to the L-I transition, with changes in the core turbulence preceding changes in edge turbulence. We obtained new data from the 8-channel tunable CECE system (providing the first local, quantitative measurements of turbulent Te fluctuations in the core of C-Mod plasmas) simultaneously with new HIREX data (allowing diagnosis of rotation, Er and transport of laser blow-off injected impurities using a single crystal) and CXRS and GPI data (to gather information of edge turbulence and edge electric field). This will provide us with a very complete set of Er profiles and turbulent fluctuations, covering the core to the edge, in L-mode and I-mode plasmas, and across the L-I transition. This data set will be used to constrain turbulent models of transport in unprecedented detail. This experiment contributes to the thesis research of several MIT graduate students. The experiment on Tuesday morning was devoted to MP#681, "Evaluate field-aligned antenna performance". This experiment comprises part of the research of an MIT graduate student. During this half-day run, we investigated the effects of low-Z seeding and He GPI puffing on RF enhanced potentials from the non-aligned D-port dipole antenna and the four-strap Field-Aligned J-port antenna. We were also able to obtain additional phasing data with the FA-J antenna for analysis. We found that low Z seeding had significant impacts on the RF enhanced potentials, and that D and FA-J responded differently to the seeding. More analysis is required to clarify the underlying physics. Tuesday afternoon's experiment was dedicated to MP#704, "Observation of current density profile change in LH discharges with polarimeter system". This experiment supports the thesis research of an MIT graduate student. A shot-to-shot scan of plasma current from 400 to 950kA with lower hybrid current drive was completed. We also carried out a scan of the LH phase at a current of 800kA. Effects of LHCD were clearly observed using the 3-chord polarimeter diagnostic. On Wednesday, we completed MP#702, "Cold pulse modulation experiments for the study of non-local heat transport and correlation with rotation", which comprises part of the thesis research of an MIT graduate student. Cold pulses were generated using repetitive laser blow-off (LBO) injections of CaF2 into ohmic target plasmas with plasma currents of 0.55, 0.8, and 1.1MA, covering a range of densities spanning the Linear Ohmic Confinement and Saturated Ohmic Confinement regimes. The amount of impurity injected by the LBO was adjusted to optimize the diagnostic response for each condition. Observations of diffusive and non-diffusive transport of the cold pulses were made, along with measurements of the plasma rotation, using the HIREX_SR spatially resolved X-ray spectroscopy diagnostic. On Thursday morning, we continued the experiments dedicated to MP#707, "Pedestal and SOL heat-flux width scaling studies in I-mode plasmas". These experiments support the thesis research of an MIT graduate student. Thursday's experiment produced data at n/nG~0.2 with Ip=1MA and nebar~1.3e20/m^3, and with Ip~800kA, nebar~1.05e20. Thursday afternoon was devoted to MP#694, "Transport of helium impurity in L-mode, H-mode, and I-mode". This experiment comprises part of the research of a University of Texas graduate student. Trace helium gas injected using a piezoelectric valve was monitored using the DNB-based CXRS system. During this half-day, data were obtained in I-mode and in ohmic plasmas. These data are being analyzed. Friday afternoon's experiment supported MP#648, "Direct boron density asymmetry measurements". This run focused on I-modes with optimized diagnostic setup for edge CXRS (local D2 gas puffs with the inner and outer NINJA capillary system, inner gap scans, and an upgraded set of deuterium light photo-diodes (measuring Dbeta)). Data were obtained in I-mode with Ip=1.1 and 1.3MA. Attempts to obtain a transition from I to H-mode for comparison were not successful, as the I-mode persisted with maximum available ICRF power, up to 5MW. ICRF Systems ------------ The new ARRA-funded Fast Ferrite Tuners (FFTs) were installed in the matching network for the E-port antenna last week. Shortly after we began operation, the FFTs began arcing at 300 kW forward power. We bypassed the FFT's and the E-port antenna was restored to service at nominal power levels on Thursday. Measurements indicate that the water vapor content of the FFT's had risen to unacceptable levels, suggesting an internal water leak form the cooling system. We are working to understand and fix this problem. Preparations were made last week for converting the J-port transmitters to 50MHz operation. Modifications to the J-Port matching network and re-tuning of the FMIT#3 and 4 transmitters were begun over the weekend and are expected to be completed in time for this week's operation. Lower Hybrid System ------------------- The LH system was successfully utilized during Tuesday's run. A faulty inlet cooling water temperature sensor transmitter was discovered on cart 2. The faulty transmitter was replaced with a unit from cart 4, and replacement components were ordered. Diagnostic Neutral Beam Systems ------------------------------- The DNB was operational at somewhat reduced parameters (45kV, 6 Amps) for runs early last week, but performance degraded during the week. On Thursday the operation was restricted to about 40kV and 5 Amps. The cathode emitter failed on Friday morning. On inspection, we found a short in the cathode assembly, and a temporary fix was implemented. Reconditioning of the source was begun on Friday and continued on the weekend. Diagnostics ----------- Glen Wurden (LANL) brought a new high resolution camera to C-Mod from Los Alamos, for testing last week. Eventually this camera will go to Germany for use on the W7X Stellarator, presently under construction in Greifswald, Germany. The infrared camera (3-5 micron wavelength) can record 1344x784 pixels, at 14-bit depth, and 124 frames per second at full frame size. The system was installed on top of the Alcator C-Mod igloo, using a 6" diameter silicon wafer acting as a beam splitter to get a view of the divertor via our existing infrared periscope. The system was aligned and focused on Monday, and during the following run, on Tuesday, we obtained shot data at a variety of speeds, both in free-running, and externally synched timing modes. We were able to accomplish all of our near-term goals for demonstrating this new diagnostic in the tokamak environment last week. Travel and Visitors -------------------- Glen Wurden (LANL) visited MIT last week to perform testing of the new high resolution camera described above. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly