Alcator C-Mod Weekly Highlights Feb 27, 2012 FY2012 weeks of research operations Target: 17 weeks Completed: 5.6 weeks Plasma Shots: 753 Operations ----------- Plasma operations continued at Alcator C-Mod last week. Four run days were scheduled and completed. A total of 105 plasma discharges were produced with a reliability of 87%. Runs supported experiments in the Transport physics, Diagnostic Development, and ICRF Physics areas. This week begins a scheduled maintenance interval. No plasma operation is scheduled. Physics ------- In a collaborative effort between MIT and GA, MIT researchers and graduate students have implemented "in-between shot" initial value linear stability analysis for the first time during a tokamak experiment using GA's GYRO code. The goal of this analysis was to assess the transition between ITG and TEM driven turbulence in C-Mod. The JRT 2012 experiment "MP689:Validation of gyrokinetic transport models across the ITG/TEM boundary in L-mode and I-mode plasmas" was run at Alcator C-Mod on Feb 21st (described below), and the linear GYRO runs were completed between shots using actual experimental density and temperature profiles from Thomson scattering and ion temperature profiles from imaging X-ray crystal spectroscopy. In order to run GYRO in between C-Mod shots, 160 processors on the local MIT Plasma Science and Fusion Center cluster, LOKI, were dedicated on this day to support the experiment. This is the first time that linear GYRO has been used to inform the running of a tokamak experiment (i.e. deciding what parameters to change for the next shot), and also the first time that dedicated experimental run time at a major facility was combined with large-scale dedicated computational resources in real-time. Operation details ----------------- Tuesday's experiment was devoted to MP#689, "Validation of gyrokinetic transport models across the ITG/TEM boundary in L-mode and I-mode plasmas". The goal was to produce high power L-mode and I-mode plasmas at low density, with dominant electron heating, in order to favor dominance of TEM instead of ITG. We collected a complete set of profile data of Te, Ti, Rotation, and density, as well as turbulence data (PCI, FTCI, reflectometer, polarimeter) for validation studies, in order to investigate how well GYRO and similar codes can reproduce experimental transport levels and turbulence in TEM vs ITG L-mode and I-mode plasmas. This experiment supports the 2012 Joint Research Target. Profile analysis and linear gyrokinetic stability analysis (GYRO) were performed between shots to assess when the boundary from ITG to TEM dominated plasmas was crossed. Datasets were obtained in steady discharges at high and low power with Ip=700kA, nebar=1e20/m^3 and at Ip=800kA, nebar=.8e20/m^3. In addition, using stepped and ramped RF power waveforms we obtained datasets at the L-mode to I-mode transition in several discharges. We also operated the spatially resolved soft Xray diagnostic HIREX_SR with high time resolution (~5msec) for a number of shots in order to resolve the evolution of Ti during the sawtooth cycle, which will be important for detailed transport analysis. Two experiments were carried out on Wednesday. In the morning we carried out MP#677 "Microproposal for Faraday Rotation Tests" to investigate sources of noise and disturbances affecting the Faraday rotation diagnostic. Single coil test shots of the TF and several of the poloidal field coils were carried out to evaluate the vacuum response of the Faraday diagnostic. This work comprises part of the thesis research of an MIT graduate student. No plasmas were produced for this experiment. The afternoon was devoted to MP#671 "Investigation of dilution effects on turbulence and transport in low density Ohmic plasmas". This MP comprises part of the thesis research of an MIT graduate student, and also supports the 2012 Joint Research Target. The purpose of this run was to investigate whether diluting the main ion species of the plasma with some lower Z ions results in reduced ion transport without having significant effects on electron transport, as suggested by previous analyses. The impurity content of unseeded plasmas was measured using the newly calibrated impurity diagnostics and plasmas were then seeded with nitrogen in order to dilute the main ions by at least 15-20% (the level at which we expect effects on transport). A previous run dedicated to this MP did not obtain profile data of sufficient quality for unambiguous transport analysis at all the desired conditions. On Wednesday we successfully completed a scan of density in seeded discharges, as well as obtaining profile data in the high density unseeded case. We also observed rotation reversals induced by the nitrogen puffing. On Thursday we carried out MP#673 "Study of L-H transition and H-mode at low plasma density with improved C-Mod diagnostics". This MP comprises part of the thesis research of an MIT graduate student. The primary objective of this experiment was to study the plasma density dependence of the divertor geometry effect on H-mode threshold power and plasma behaviors close to L-H transition. L-H transitions were documented over a broad density range 0.8 < nebar < 2.2e20/m^3 with the outer strike point either in the slot or on the vertical plate. The run reproduced the earlier observation of H-mode threshold reduction in the slot configuration at moderate to high density, while at low density the threshold power is similar in the slot and vertical plate geometry. Detailed analysis of these data is in progress. The majority of the run on Friday was devoted to MP#679, "Investigation of zonal flows and interaction with energetic particle induced geodesic acoustic modes". The goal of this experiment was to conclusively identify any relationship between zonal flows and geodesic acoustic modes (GAMs), including the possibility of an energetic particle-induced GAM. The plan was to reproduce the unique GAM feature observed in a previous experiment, and then determine its stability through modification of the ion cyclotron resonance heating (ICRH) tail ion distribution. Low frequency modes (f < 6kHz) were observed on numerous fluctuation diagnostics for discharges over a range of plasma parameters, with the toroidal field varied from 4.6T to 6.1T. The divertor probes and reflectometer observe a coherent feature that changes in frequency across RF pulses and L-H transitions. The Gas Puff Imaging (GPI) camera acquired data across multiple L-H transitions. These data are undergoing analysis. The last hour on Friday was devoted to MP#651 "Investigation of limiter heat loads during ICRF". Moderate to low current (800 and 600kA) discharges with nebar < 1.1e20/m^3 and 3.9MW of ICRF were produced, resulting in significant heat loads to midplane tiles of the outer (GH) limiter. ICRF System ----------- The ICRF system operated in support of experiments on Tuesday, Thursday, and Friday last week. All three antenna and transmitter systems performed nominally. Lower Hybrid System ------------------- The lower hybrid system was not required for C-Mod experiments last week. Work continued on the control system feedback loop test setup. Assembly of the 4th cart Transmitter Protection System (TPS) equipment rack continued. Diagnostics ----------- The fast ion loss detector (FILD) diagnostic was successfully commissioned during three experiments this week at Alcator C-Mod. The FILD is a scintillator-based detector that resolves the pitch angle and energy of RF-accelerated ions that reach its position at the outer midplane. Preliminary results include the observation of an increasing fast ion loss flux prior to large amplitude sawtooth crashes. An Ohmic experiment (MP671) allowed for the characterization of any stray light sources and the noise level, both of which were found to be acceptably low. Analysis of this initial data set is in progress, with a focus on identifying optimizations that may be applied during the Spring vent to improve resolution of the interaction between plasma facing components and fast ions. Outreach -------- Martin Greenwald participated with Mike Zarnstorff of PPPL in a Science Magazine "Live Chat" on the Fate of Fusion. Six hundred people around the country (and world) listened in and submitted several hundred questions. A transcript is available at http://news.sciencemag.org/sciencenow/2012/02/live-chat-the-fate-of-fusion.html Travel and Visitors -------------------- Stewart Zweben (PPPL) visited C-Mod last week to participate in two experiments, one on L-H transitions and the other on edge zonal flows/EGAMs. Very good GPI camera data was obtained using both the midplane and X-region GPI systems at 400,000 frames/sec. David Pace (GA) visited last week to participate in the commissioning of the fast ion loss detector (FILD) and to serve as a co-Session Leader for MP679 along with Stewart Zweben (PPPL) and Jim Terry (MIT). While on-site, he met with research staff and students concerning the application of new diagnostic capabilities, e.g., the polarimeter and fast interferometer, in the area of energetic ion instabilities. On February 21-22, 2012 Paul Bonoli and Earl Marmar were in Gaithersberg, MD for the US Paper Selection Committee Meeting for the upcoming IAEA Fusion Energy Conference that will be held in October, 2012 in San Diego, CA. Ron Parker and Greg Wallace participated in the 2012 KSTAR conference in Muju, Korea last week. Miklos Porkolab, AAAS Fellow, attended the AAAS-2012 meeting in Vancouver, BC. Among other activities, he attended the business meetings of the Engineering Section and that of the Physics Section, and presented a proposal for a Symposium on fusion at the AAAS-2013 meeting in Boston. The symposium would consist of 6 half hour talks, three on MFE and three on ICF/IFE, including leadership talks on ITER and NIF/NIC. The proposal was well received and will be further reviewed by the AAAS program committee at their April 2012 meeting. On Wednesday through Friday, Geoff Olynyk was in Boulder, CO, to participate in the NIMROD "boot camp" held at Tech-X Corporation. This is a two- or three-times-per-year event in which the NIMROD developers get together and collaboratively work on the code. NIMROD is a 3-D time-dependent resistive MHD code, which uses finite elements in the poloidal plane and Fourier components in the toroidal direction. Geoff will be attempting to port NIMROD to the PSFC parallel computing cluster LOKI, in order to use the code to support his thesis work on disruption mitigation studies. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly