Alcator C-Mod Weekly Highlights Feb 13, 2012 FY2012 weeks of research operations Target: 17 weeks Completed: 3.8 weeks Plasma Shots: 544 Operations ----------- Plasma operations continued at Alcator C-Mod last week. Four run days were scheduled, but only three were accomplished. Time was lost due to a vacuum leak on Wednesday and a power system fault on Thursday. A total of 80 plasma discharges were produced with a reliability of 78%. Runs supported experiments in the Transport Physics and Lower Hybrid Physics topical areas. Plasma operations are planned to continue this week. Operation details ----------------- Tuesday's run was dedicated to MP#653b, "Hidden variables in neo-Alcator scaling". The aim of this experiment is to clarify the physics behind the linear density scaling in the neo-Alcator or linear Ohmic Confinement (LOC) regime. While density explicitly enters the gyrokinetic equations only through the collisionality, co-varying quantities including Ti/Te, main ion dilution, density profile shape, and rotation complicate the physical interpretation. The experiment on Tuesday aimed to clarify the role of collisionality, and separate the effects of Te/Ti and density by employing (H minority) ICRF heating to scan Te/Ti at constant density and to maintain constant Te/Ti during a density scan; we also dynamically ramped the density at constant heating power over a range of powers to investigate the effect of electron heating and associated transitions from ITG to TEM turbulence on rotation reversals in the LOC-SOC transition range. Initial results show that rotation reversals (which are known to occur at the LOC-SOC critical density) occurred at much higher density when heating power was applied, suggesting a collisionality dependence. Extensive profile and fluctuation data, including initial Correlation ECE data, PCI, and GPI, along with low noise, 50 Hz Thomson scattering data from the two new lasers, were obtained for comparison with gyrokinetic simulations. Because the electrons and ions are decoupled at low density, and heating power goes primarily to electrons, electron transport dominates the LOC regime. The critical density is thought to be associated with the transition from TEM to ITG turbulence. This work supports the 2012 Joint Research Target on electron transport. On Wednesday, we carried out MP683 "Operations with both ICRF and LHRF systems: effects on LH coupling" to explore the differences between ICRF+LH and LH operations with respect to LH coupling (reflection coefficients and faulting). During this run day, the LH launcher location was moved from 1-4mm behind the main limiter, outer gaps (from the LCFS to the limiter) ranged from 6mm to 2cm, plasma current from 600-1000kA, and nebar from 7e19 to 1.2e20/m^3. All three ICRF antennas were employed in sequence, with the order changed from shot to shot. The LH power was kept constant at ~200kW for all discharges, except for one shot at higher power. The ICRF power was also kept constant at ~500kW throughout the day. For the wide range of plasma parameters explored, the effect of the ICRF on LH coupling was quite consistent throughout the day. Compared to LH only operations, reflection coefficients increased considerably when the D-port antenna (adjacent to the LH launcher) was used, increased moderately during operation of the E-port antenna (~72 degrees away toroidally from the launcher), and showed little effect during J-port operation (~144 degrees away from the launcher). Further analysis is underway to understand these results. Thursday's run was devoted to MP#679, "Investigation of zonal flows and interaction with energetic particle induced geodesic acoustic modes". The goal of this experiment is to conclusively identify any relationship between zonal flows and geodesic acoustic modes (GAMs), including the possibility of an energetic particle-induced GAM. This is to be accomplished by reproducing the unique GAM feature from a past experiment, and then determining its stability through modification of the ion cyclotron resonance heating (ICRH) tail ion distribution. Due to a power system problem, only a few useful shots were obtained on Thursday in support of this MP. Nevertheless, several interesting results were obtained. Low frequency (f < 10kHz) modes were observed prior to the L-H transition. In some cases a mode (f~3kHz, n=1) exhibited temperature fluctuations, seen on the ECE diagnostics, and extended into the core plasma. In addition a mode at around 6kHz was observed that appears to be more edge-localized and may correspond to the GAM feature observed previously. Signatures of these modes are observed on multiple diagnostics and analysis is in progress. In addition, high frequency MHD (f > 700kHz) was observed during the H-mode phase of these discharges, and these data are also being analyzed. On Friday, we carried out MP#671, "Investigation of dilution effects on turbulence and transport in low density ohmic plasmas". The purpose of this experiment is to investigate the result from modeling that diluting the main ion species of the plasma with lower Z ions results in reduced ion transport without having significant effects on electron transport. The approach entailed measuring the impurity content of unseeded plasmas with the newly calibrated impurity diagnostics, and then seeding the plasmas with Nitrogen in order to dilute the main ions by at least 15-20% (the level at which effects on transport are expected). A density scan from 0.4 to 1.4e20/m^3 (nebar) was performed in unseeded plasmas and from 0.6 to 1.3e20/m^3 in the seeded case. At the highest densities we observed marfes and poloidal detachments in both seeded and unseeded discharges. A range of results in density and Zeff were obtained for comparison with gyrokinetic modeling. ICRF System ----------- We have cleaned the FFT#2 magnets and ordered the remaining material for completing the repair. The parallel plate and ferrite sub-assembly have been reassembled. Some initial experiments were done last week puffing gas into the scrape off layer to investigate whether the SOL density profile can be modified. Data analysis is underway. Lower Hybrid System ------------------- Progress continues on the LH3 microwave control system. The AMD based server is now working with the LLC low-latency framework. A full test setup with the new control hardware and the microwave phase shifter/attenuator has been assembled. Measurements of the klystron collector coolant temperature at the klystron end of the hose connecting between the cooling system manifold and the collector have been implemented. These measurements show less time lag and a sharper heat pulse compared to previous measurements at the manifold end of the hose. Accurate measurements of the heat pulse time constant are necessary to benchmark the transmitter protection system (TPS) collector over temperature system (COTS) calculation. Several TPS nuisance fault conditions associated with coupler protection system (CPS) faults were addressed with minor changes to the control setpoints and by adjusting the CPS retry timing so that CPS and TPS retries would be better synchronized. Diagnostic Neutral Beam System ------------------------------- The DNB cathode heater, which initiates and sustains the arc source discharge, malfunctioned during the week of January 30. The cathode heater was repaired several times unsuccessfully, and two other spare cathode heater units were tried as well, but persistent problems reduced performance below that required by beam-based diagnostics. Repairs will continue during this week's maintenance day. Data Analysis ------------- We have built a streamlined workflow that allowed us to run linear gyrokinetic calculations between shots, with a turnaround time of about 12 minutes. The standard workflow requires a TRANSP run which can take several hours. This capability will be used to support decision making during future runs contributing to the 2012 JRT. Travel and Visitors -------------------- Stewart Zweben visited MIT last week along with Bill Davis from the PPPL computer division to discuss the analysis of gas puff imaging data of edge and SOL blobs with Jim Terry, Odd Eric Garcia and his student Ralph Kube from Tromso, and Olaf Grulke from IPP. Detailed comparisons were begun between Bill's blob tracking code and the one being developed by Tromso. Stewart also participated in two experiments, one of which (MP679) was to follow-up previous GPI observations of a coherent edge zonal flow in ICRH plasmas. Interesting GPI data was obtained on both the midplane and X-region fast cameras. A larger set of data on low frequency turbulence just before the L-H transition was also obtained by various other diagnostics. Professor Olaf Grulke of IPP Greifswald visited C-Mod for the week. Olaf is a long-time C-Mod collaborator in the edge physics area. For this visit he consulted with C-Mod's edge-physics group. He and Jim Terry also finalized issues relating to a manuscript on C-Mod SOL turbulence that is in preparation. A new collaboration has started at C-Mod between MIT and LLNL. Alex Tonchin-James will be working as a post-doc on impurity sources and transport research. His initial work will focus on trying to image the GH-limiter in various impurity emission lines using interference filters and an intensified camera. He is working with Vlad Soukhanovskii of LLNL (presently at PPPL) on the camera. Further work will involve becoming more involved in impurity spectroscopy here at C-Mod. Manfred Bitter and Ken Hill (PPPL) visited MIT last week to work with Luis Delgado-Aparicio. They brought and installed, temporarily, a new soft X-Ray (SXR) camera in B-port in order to estimate the SXR throughput for a future multi-energy pulse height analysis (PHA) system. The first tests have been conducted and the detector worked as expected, even in the presence of a transient poloidal field. SXR images of the plasma have been taken using lower thresholds of 4, 5, 6, and 7 keV, obtaining signals as high as several tens of thousand counts per-pixel per-second. Luis will continue taking data during the next few weeks, implementing a new rectangular collimator for imaging a poloidal cross section, as well as using integration times as low as 6-10 ms. David Pace (GA) participated by remote collaboration tools as co-Session Leader in Thursday's experiment. Melanie Preynas and Julien Hillairet (CEA), and Orso Meneghini (GA) participated remotely during Wednesday's LH experiment. Kitta MacPherson, Princeton Plasma Physics Laboratory's Director of Communications, visited the PSFC to learn more about MIT's fusion program, and to discuss ways the labs might work together to promote fusion. Her tour included a discussion with Alcator Project Head Earl Marmar, and a tour of C-Mod from graduate student Arturo Dominguez, as well as meetings with Associate Director Richard Temkin, Educator Paul Thomas (Mr. Magnet) and PSFC Director Miklos Porkolab. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly