Alcator C-Mod Weekly Highlights May 19, 2008 FY2008 weeks of research operations: Target: 15 weeks Completed: 14.8 weeks Operations ---------- Research operations resumed last week at Alcator C-Mod. Three and a half run days (Tuesday-Friday) supported five experiments, from the Lower Hybrid Physics, Advanced Scenarios, and Boundary Physics research efforts. Operation originally planned for Monday afternoon was curtailed due to a ruptured gas line at a nearby construction site (not related to PSFC activities), which required temporary evacuation of the facility. All systems were placed in a safe and secure condition following established procedures. Personnel access to the C-Mod site was restored after about two hours. Rather than resume plasma operations, the rest of the afternoon was used for maintenance activities, and the run schedule for the remainder of the week was adjusted. Plasma operations are scheduled to continue this week. Operation Details ------------------ A five hour experiment was carried out on Tuesday to complete the dataset for MP#551 "Penetration of Accessible LH Waves". This experiment is designed to investigate the location and intensity of hard xray (HXR) emission (as a proxy for LH current drive) as a function of line averaged density, toroidal field, and plasma current. Data were obtained at 75 and 90 degree phasing for multiple conditions over the line average density range from 0.9 to 1.5e20 m^-3. The low current scans matched well with data from the previous experiments, extending the curve of HXR emission vs line averaged density. No significant variations was seen with phase or magnetic field. HXR emission at 1.1MA was significantly higher (by about a factor of 2) than for 800kA shots, across a range of densities from nebar=0.8 to 1.5e20/m^3. This is consistent with results from the '06 campaign when we observed the best current drive at higher current. Several possible explanations for this effect are under consideration. The run on Wednesday and an additional two hours on Friday were devoted to MP#501 "Fluctuations in ITB plasmas". Internal transport barrier discharges were produced using two-frequency ICRF heating (80 and 70 MHz), with the barrier being induced by heating off-axis and the subsequent density peaking controlled by application of on-axis heating. The Phase Contrast Imaging (PCI) diagnostic was used to monitor fluctuations during ITB discharges with and without central heating. This data, and comparison with predictions of transport code synthetic diagnostics, will comprise part of the thesis research of an MIT graduate student. In addition, analysis of the peaking of boron density profiles observed using the CXRS diagnostic will contribute to the thesis research of a University of Texas graduate student. Two experiments were carried out on Thursday. The first supported MP#543 "Exploration of LHCD effects on pedestal and core of H-mode plasmas". The goal of this experiment was to document and understand the conditions under which LHCD causes changes in H-mode pedestal and core. The "pump-out" effect of plasma density observed in earlier H-mode experiments during LHCD, resulting in reduced pedestal collisionality, higher Te, and reduced impurities, was reproduced and extended to lower plasma current (Ip > 450kA) and a range of LH phasing, 1.9 < n_parallel < 2.8 . These data will be important inputs for LH modelling, and may suggest a direct effect of waves on the edge plasma. The second experiment on Thursday was devoted to continuation of MP# 547, "High-field side SOL flow momentum coupling experiments". The goal was to restrict flows into the high field side scrape off layer by scanning the LCFS of an USN diverted discharge close to and then grazing the inner divertor nose. The entire suite of toroidal rotation diagnostics was used to evaluate whether the HFS flows are the causal agent for a change in v_tor. We also successfully obtained LSN and DN plasmas for comparison. Data were obtained from the high resolution x-ray spectroscopy as well as from probes and inner-wall CXRS. This work comprises part of the thesis research of an MIT graduate student. The bulk of Friday's run was in support of MP#454a "Low Ip H+ITB Plasmas, Their Modulation and Interaction with LH". Chuck Kessel (PPPL) served as Session Leader using remote participation videoconferencing tools, assisted by Amanda Hubbard. The goal of this experiment is to further investigate the dynamics of ITB control using on- and off-axis ICRF heating, and to study the effect of injecting LHCD into these stabilized target plasmas. ITB discharges were produced at Ip=450kA, Bt=5.4T, and with low density n_ped~1.2e20/m^3 using off-axis ICRF heating at 70MHz. These low current ITB's were stabilized without requiring injection of on-axis heating; addition of on-axis heating at 80MHz had little effect on the density or ITB formation, but did help maintain the central electron temperature. Sawteeth were suppressed during the later stages of the ITB plasma, possibly as a result of enhanced bootstrap current. Lower hybrid injection into the ITB discharges was accomplished on a few discharges, but no positive effects were apparent. DNB Systems ----------- The leak in the DNB arc source was successfully repaired over the weekend and the DNB was returned to service for the experiments on Tuesday and Wednesday, with near-nominal beam parameters. During operation on Thursday the cathode heater failed. A replacement cathode is in preparation, and we expect the DNB to be available for experiments this week. ICRF Systems ------------- Problems with the HV systems for FMIT transmitters #3 and 4 delayed conditioning and matching activities for J-port at the 70MHz frequency. After overcoming some transmitter instability issues, operation at 70 MHz resumed on Wednesday afternoon, and all four ICRF transmitters were successfully employed in experiments for the remainder of the week. Transmitters #3 and 4 are now being retuned to 50MHz in support of experiments planned for this week. We are continuing to investigate end of life issues associated with the high power tube, 4CM2500KG. We have leak checked two of the anodes and have found both to have leaked away from obvious melted regions. We are seeking the anodes from our last three tubes, currently at the vendor for rebuilding, for additional information. Travel and Visitors -------------------- Earl Marmar visited Princeton on Wednesday, May 14, where he presented the PPPL Colloquium. His talk was entitled "Alcator C-Mod Research Highlights and Plans". While at PPPL, he also had discussions with Dale Meade, Masa Ono, Michael Bell, Stan Kaye, Stefan Gerhardt, Mike Zarnstorff, Hutch Neilson, and Sam Cohen. Alex Ince-Cushman, Matt Reinke, Bill Rowan, Igor Bespamyatnov, Eric Edlund, Jinseok Ko, and Greg Hanson (ORNL) presented papers based on C-Mod reesearch at the 17th Topical Conference on High Temperature Plasma Diagnostics in Albugqurque, NM, May 11-15. Nat Fisch gave the MIT Physics Department Colloquium on May 15th with the title: "Manipulating Phase Space in Plasma with Waves". On May 13 and 14th he visited several members of the PSFC, including M. Porkolab, R. Parker, P.T. Bonoli, E. Marmar, P Catto, and discussed the latest C-Mod RF and Lower hybrid experimental results and full wave modeling, well as explored the possibility of carrying out model experiments for alpha channeling on C-Mod. Chuck Kessel (PPPL) visited MIT on Tuesday. This visit was to participate as session leader for his experiment MP#454a, which had to be rescheduled for Friday due to problems encountered with the ICRF system on Monday. While at MIT, Chuck discussed TSC simulations with several C-Mod physicists, running TRANSP on C-Mod discharges with C. Fiore, equilibrium reconstructions with S. Shiraiwa, and reviewed the experimental plan for his run. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly