Alcator C-Mod Weekly Highlights July 16, 2012 FY2012 weeks of research operations Target: 18 weeks Completed: 10.2 weeks Plasma Shots: 1548 Operations ----------- Plasma operations continued at Alcator C-Mod last week. Four run days were scheduled and completed. A total of 98 plasma discharges were produced with a reliability of 88%. The runs supported research in the Lower Hybrid Physics, Transport, and Edge/Boundary Physics, and Advanced Scenarios (I-mode) topical areas. About two hours of run time were lost on Thursday when a gas leak at a nearby construction site required evacuation of a number of buildings on Albany Street, including the C-Mod facility. All C-Mod systems were put into a safe standby status, and operation resumed smoothly after the all-clear was given. A fresh boronization was carried out over Wednesday night in preparation for the experiments on Thursday and Friday. Plasma operations are planned to continue this week. Physics ------- A total of 63 abstracts based on C-Mod research were submitted for the 54th APS Division of Plasma Physics Meeting, including one Review talk, 9 invited oral presentations, 18 contributed orals (including 3 in the special ITER session), and 35 contributed posters. C-Mod contributions to the 2012 EPS meeting and the EFTSOMP2012 workshop can be found on-line at http://www.psfc.mit.edu/research/alcator/pubs/eps/eps2012/eps_2012.html and http://www.psfc.mit.edu/research/alcator/pubs/index.htm Operation Details ----------------- Two experiments were carried out on Tuesday. The goal of MP#664, "Study of Spectral Broadening of LH waves", is to determine if there is a link between broadening of the LH wave spectrum and the density limit observed in lower hybrid current drive experiments in Alcator C-Mod. This experiment, which comprises part of the thesis research of an MIT graduate student, aims to document both density and temperature profiles and turbulence (k and f-spectrum of turbulence from GPI, delta_n/n from probes and reflectometer) in the SOL before and during LHRF injection at different densities and LH power, and then to correlate the change of these parameters to the amount of spectral broadening. During Tuesday's experiment, data were obtained in lower and upper single null and inner wall-limited equilibria, at a range of target densities and LH powers. In LSN we observed strong sidebands (70MHz apart), comparable to the pump amplitude, when the density was raised over nebar >1.1e20/m^3. However, in USN topology the sidebands became weaker and the frequency spacing changed to ~35MHz; the measurements suggest that in this case there was not strong depletion of the pump wave. It was observed in the USN discharges that the density in front of the LH grill was a factor of 10 higher than in comparable LSN cases, and it was suspected that the higher local density was resulting in Parametric Decay Instability (PDI) at the outboard side. We increased the outer gap in order to reduce the local density at the launcher, and observed that the PDI level decreased and the edge non-thermal "temperature" increased. Under these conditions, loop voltage decreases of 0.5V (from the ohmic level of ~1V) were observed at a line average density of 1.3e20/m^3, which is above the usual density limit for effective current drive in C-Mod. These results are being analyzed in detail. The second experiment on Tuesday was devoted to MP#710, "Impurity transport in non-inductive plasmas". This experiment, which contributes to the thesis research of an MIT graduate student, was designed to study the evolution of a non-recycling impurity (calcium) introduced by laser blow-off, in lower hybrid current driven tokamak plasmas with reduced Ware pinch and no sawtooth activity, with the goal of analyzing the impurity particle transport in the context of theoretical models. A non-standard grating was installed in the spatially resolving high resolution X-ray spectrometer, HIREX-SR, in order to diagnose the Ca transport in the core. The MSE diagnostic, which had been intended to provide measurements of the magnetic shear in the non-inductive plasmas, was unavailable because of a problem with in the DNB system earlier in the day, resulting in a less than complete dataset for comparison with theoretical models. Nevertheless, the experiment succeeded in obtaining calcium transport data in a series of discharges with different levels of LHCD power, with the loop voltage sustained as low as 0.2 Volts, under conditions with and without m=2 MHD activity, as well as some data in the presence of a locked mode. The target plasma was a 500kA, upper null equilibrium, with nebar~6e19/m^3. Fully non-inductive operation (0 volt) was not achieved. Full kinetic profiles (Te, ne, Te) as well as fluctuation data, were obtained and will contribute to the analysis. The run on Wednesday was devoted to MP#706,"Fluctuation-induced heat and particle transport in the boundary plasma and connection to critical gradients". The goal of this experiment is to use the new Mirror Probe System on the A-port drive, in conjunction with Gas Puff Imaging (GPI), to record temperature, density and potential fluctuations and the resultant energy and particle transport fluxes. The mirror probe system has the unique capability of making simultaneous measurements of electron temperature, floating potential and ion saturation currents at each location of the four electrodes in a scanning probe (12 data signals total), all with ~1 MHz bandwidth. Datasets were obtained in 0.55 and 0.8MA ohmic discharges at 5.4T with a series of densities. These data are being analyzed. Following Wednesday's run the A-port probe assembly was vented and the probe head replaced with a magnetic sensor in preparation for Thursday's experiment, which was devoted to MP#697, "First operation of Shoelace Antenna: Survey of boundary plasma response". This experiment comprises part of the thesis research of an MIT graduate student. The primary goal of this run was to observe the interaction between the Shoelace antenna and the quasi-coherent mode. A secondary goal was to determine whether the Shoelace can drive a QCM in a plasma where the mode has been suppressed. The target plasmas for this experiment were ohmic EDA H-mode discharges, obtained by operating at low q95 and reduced, time-varying Bt. The first half of the experiment was carried out with the QCM (aka "Shoelace") antenna operating as a receiver. These shots confirmed the sensitivity of the antenna to the QCM. The antenna was then connected to the drive electronics and the active phase of the experiment was begun. Plasma response to the antenna drive was observed on a number of Mirnov coils, as well as on the ASP scanning probe with the magnetic sensor. Any interaction with the QC mode will have to be extracted from the data by analysis of the transfer function, cross-correlation techniques, and antenna on-off diagnostic signal binning. This data analysis is in progress. The run on Friday was dedicated to MP#643 "Optimization of I-mode in normal field, USN". A key focus of the experiment was to explore access to I-mode as a function of density. Density scans wee carried out at currents of 1.1 and 1.2MA over a range 1.5e20 < nebar < 2.8e20/m^3. ICRF power was above 4MW in all cases, and in some cases up to 5.5MW was coupled to the plasma. I-mode access was clearly sensitive to density over this range. Confinement in the I-mode discharges was typically H98~0.8. There were also a number of discharges that exhibited mildly enhanced H98 (above 0.5) and high central Te, but did not show clear signs of an edge temperature barrier or obvious changes to fluctuations. Raising the density under these conditions enabled transitions to I-mode. ICRF Systems ------------ The ICRF system ran in support of experimental runs last week. The builder of our Fast Ferrite Tuners (FFT) has indicated that the arcing in the original FFT system is likely due to failure of the tile bonding. As a result of cyclic thermal expansion with insufficient spacing between tiles, the bond has been repeatedly cycled and is beginning to fail at multiple locations in the FFT. The FFT in question is one of the original prototypes. We are in discussions with the vendor to decide on the best repair strategy. The first pair of new ARRA-funded FFTs is to be tested at the vendor's facility during the week of July 23rd and shipped via airfreight by the end of July. Lower Hybrid System ------------------- The LH system was used during plasma operations on Tuesday at net power levels up to 800 kW. Diagnostic Neutral Beam System ------------------------------ Early in the week it was discovered that the DNB would not condition up to full operating voltage, and several problems were also encountered with control of the arc current. The latter problems were traced back to high voltage faulting on the exterior of the beam source, due to a less-than-optimal design of some insulators. This was remedied after removal of the source and several nights of work. A more permanent, improved solution is being engineered. The beam operation is still limited at 41 kV, and the cathode heater and arc source are presently undergoing refurbishment. Diagnostics ----------- The core-cxrs poloidal optics were removed this week to diagnose and repair an apparent loss in transmission. The location of the components and the alignment were compared to records of those at the beginning of the campaign. Some optical components were cleaned. Following re-installation at the end of the week, the transmission was recovered to the nominal level. Travel and Visitors -------------------- Miklos Porkolab attended the EPS2012 satellite meeting in Stockholm, EFTSOMP2012 (Workshop on Electric Fields, Turbulence and Self Organization), July 9, 10, 2012, where he gave a 30 minute presentation: "Gyrokinetic Analysis of Turbulent Transport in Alcator C-Mod Ohmic Plasmas". This annual workshop was attended by about 30 scientists, representing Europe, the US, Japan and Russia. Stewart Zweben, PPPL, visited MIT last week to participate in two experiments, one on edge fluctuations and the other on the 'shoelace' antenna. Good data were obtained for both GPI cameras on both days; in particular, the best images ever obtained from the X-region GPI camera were taken at the end of the second day. Yijun Lin, Bruce Lipschultz, and Syunichi Shiraiwa attended the US-China Magnetic Fusion Collaboration Workshop in San Diego. Bo Lu, a scientist working on the EAST tokamak, visited the PSFC on Saturday and Sunday. He met with Ken Liao and Matt Reinke to discuss potential collaborations on charge exchange and x-ray crystal spectroscopy. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly