Alcator C-Mod Weekly Highlights Dec 19, 2011 FY2011 weeks of research operations Target: 17 weeks Completed: 1.1 weeks Plasma Shots: 209 Operations ----------- Monday through Wednesday were scheduled maintenance days at Alcator C-Mod. Acitvities included general preventive maintenance, work on the Lower Hybrid Transmitter Protection System (TPS), reconfiguration of the J-port ICRF antenna matching network, and work on several diagnostic systems. Electron cyclotron discharge cleaning (ECDC) was continued. Two days of plasma operations were scheduled and completed on Thursday and Friday. A total of 54 plasma discharges were produced, with a startup reliability of 81%. The runs supported ICRF Physics and conditioning , and diagnostic development activities. This week will be a scheduled maintenance period. No plasma operation is planned. Physics -------- On December 15th, Matt Reinke successfully defended his Doctoral Thesis, entitled "Experimental Tests of Parallel Impurity Transport Theory in Tokamak Plasmas", submitted to the MIT Department of Nuclear Science and Engineering. In Alcator C-Mod, 2D (R,Z) measurements of photon emission from high-Z impurities reveal significant variations of impurity density on a flux surface. Poloidal variations, normalized to the flux surface average, have been measured up to ~1/3, and separate cases of impurities accumulating on the inboard and outboard side of a flux surface are observed, depending on local plasmas conditions. Experiments demonstrate that these asymmetries are due to a combination of inertia, poloidal electric fields and ion-impurity friction, and measurements are compared to existing neoclassical parallel impurity transport theory. This is the first time centrifugal force has been observed to cause a substantial asymmetry in a plasma with no external momentum input and where the flow is entirely self-generated. Magnetically trapped fast ions, sustained by ion cyclotron waves, create a poloidally varying electrostatic potential which causes high-Z impurities to accumulate on the inboard side. Existing theory is extended to include this effect by incorporating a species that has an anisotropic pressure tensor. Experimental measurements in plasmas where the minority resonance layer is scanned show good qualitative and quantitative agreement with this extended theory. The sensitivity of the asymmetry to fast-ions demonstrates the opportunity for the impurity asymmetry to be used as a novel diagnostic tool. Measured up/down asymmetries in the banana regime are unable to be explained by ion/impurity friction in the trace limit, n_z Z^2/n_i << 1. The sensitivity of such asymmetries to main ion poloidal rotation, which depends on impurity density when n_z Z^2/n_i ~ 1, suggests that explaining up/down asymmetries requires a more complex model which couples the parallel force balance of main ions and multiple impurity species. Additionally, the connection between impurity poloidal flow and poloidal density variation is highlighted. Since they are the result of the same parallel transport equation, predictions for both need to match measurements in order to validate friction-based asymmetry theory. Run Planning ------------ The C-Mod Experimental Program Committee met on Monday, Dec 12,2011. We discussed priorities for the 2012 physics campaign and considered schedule options in the light of the recently identified issues with the J-port ICRF antenna. A preliminary assessment indicates that a substantial fraction, of order 50%, of the proposed high priority program elements do not depend on power from the J-port antenna. Four new Miniproposals were reviewed and approved. Operations Details ------------------ The run on Thursday was devoted to MP#662 "HiReX Sr startup MP". The goal of this experiment was to fulfill the initial calibration, focusing and alignment requirement of the spatially resolving high resolution X-ray spectrometer. In addition, we evaluated the argon injection efficiency of the B-side lower piezo-electric valve and obtained data to support an absolute argon density measurement using beam-based spectroscopic measurements. These experiments were successful. Friday's run was devoted to MP#335c "ICRF Testing and Conditioning" and MP#669 "Rotated antenna Commissioning and Initial Characterization". These experiments support the C-Mod Plain English goal: "Investigate ICRF sheaths and impurity generation with an advanced ICRF antenna". The initial discharges were used to condition the J-antenna in its 2-strap configuration to high power in L-mode, and to examine its power handling capability, voltage limit, and impact on SOL density profiles. We then compared the J-port, D-port, and E-port antennas in terms of heating effectiveness, impurity production and neutral pressure limit. The field aligned antenna performed well in the dipole configuration. The maximum power achieved in L-mode was 1.4MW, which is comparable to the levels attained with D- and E-port. We did not attempt to increase the voltage above 40kV in these experiments. The H/(H+D) ratios were in the 10-20% range, which is still too high for efficient minority heating. The neutral pressure limit was evaluated by ramping the target plasma density during the discharge. The J-port antenna successfully operated without faulting up to densities ~2.5e20/m^3 and neutral pressures ~2.3mTorr, the highest values tested so far, and also successfully restarted at these pressures following programmed notches in the power waveform. This performance is at least a factor of two better than obtained with either the D- or E-port dipole antennas. ICRF System ----------- Electrical tests of the field aligned J-port antenna last week indicated that strap #2 is a DC open. To allow testing of the new antenna into plasma to continue, We reconfigured the J-port antenna as a two strap antenna and tested it into plasma. Straps are #3and #4 (the pair closest to K-port) were powered, while straps #1 and 2 were terminated with shorts outside the vacuum vessel. We obtained 1.4MW into L-mode plasmas at voltages approaching 40kV for short pulses. Measurements of the coupled voltage in straps #1 and #2 indicate that the shorts are in a good location to prevent #1 and #2 from acting as passive radiators. We also explored the neutral pressure limit and found that the field aligned J antenna operates at pressures over 2.3 mtorr or nebar~2.5x10^20 in L-mode, which is at least a factor of two higher than with the D- and E-port antennas. This should allow heating at higher density than previously achievable. Alan Binus had a teleconference with our vendor's engineering team to discuss manufacturing scheduling and system testing of the ARRA-funded fast ferrite tuners. The tuners are scheduled to be ready late February for dynamic testing but testing of the magnetics in January is being discussed. Lower Hybrid System ------------------- PLC and RSView logic were reprogrammed for the LH transmitter protection system (TPS) upgrade. New tags were added and interface screens were modified. The LH Team continued work on the TPS upgrade. Several boards were reprogrammed to address issues with timing and MDS communication. Fixed attenuators were added in series with adjustable attenuators on klystron drive channels to keep the adjustable attenuators in the middle of their ranges. This reduced drift in the klystron drive levels from 0.06 dB to 0.02 dB. The engineer from our TPS SBIR vendor was at the PSFC on Wednesday, Friday and Saturday to work on the lastest version of the firmware used in the TPS controller boards. Diagnostic Neutral Beam Systems ------------------------------- The diagnostic neutral beam operation was interrupted for about 3 hours last Thursday morning to repair a broken cathode heater and to recondition the beam. A new cathode heater with improved design is currently ungoing testing, and should eliminate the frequent problems that occur with the present one. The beam has been running for one second long periods into plasma for the past two weeks, ever since the cryopumps were filled with liquid helium. Diagnostics ----------- Remote operation of the upgraded Thomson scattering lasers was established during tokamak operation, and the lasers were successfully triggered during the machine shot cycle. Data acquisition was also interfaced successfully, allowing for laser timing and energy measurement. We expect to introduce the lasers into the C-Mod vessel in the coming week, enabling Thomson scattering measurements shortly thereafter. The correlation ECE diagnostic (CECE) has seen "first light" and is now undergoing optical and electronic check-out. So far, results with plasma match performance from bench tests. The 50, 60, 75, and 88GHz reflectometer diagnostics were successfully brought on-line. The Quasi-coherent mode ("Shoelace") antenna is online in passive (receiver) mode. MSE was brought online last week. The PEMs, APDs and eight of the ten filter ovens are operating correctly and the new calibration coefficents were programmed into the analysis routines. The system collected data from the diagnostic neutral beam during the two experiments last week. The new Inter-Shot Calibration (ISC) system was actuated between discharges during these two run days. This data will be used as part of the ISC commisioning process. The diagnostic response drift as calculated from the ISC system will be compared with the diagnostic response drift as calculated using EFIT to determine if the ISC scheme properly characterizes the drift in the diagnostic response. Travel and Visitors -------------------- Miklos Pokolab and Ron Parker attended the Fusion Power Associates 32nd Annual Meeting and Symposium in Washington, DC. The topic of the meeting was "Fusion Energy: Expectations" . This year Ron Parker was awarded FPA's Distinguished Career Award. He also presented a talk on pathways to Magnetic Fusion Energy. On Monday, 12 December, a video conference was held between members of the C-Mod lower hybrid team and the corresponding group at KSTAR. KSTAR is planning to install a lower hybrid system to produce long pulses and the initial coupler is following the C-Mod design. The video conference focused on fabrication issues, in particular welding and brazing of the RF vacuum window. Attending for C-Mod were Bill Beck, Rui Vieira, Greg Wallace, Seung-Gyou Baek, Orso Menenghini and Ron Parker. Participants in Korea included Dr. Young-soon Bae, Dr. Hakkun Kim, Mr. Kyungmin Kim, Dr. Seung-il Park, Dr. Haejin Kim, Ms. Heejin Do. Syun'ichi Shiraiwa presented an invited paper at the ITC21 conference, held at Toki, Gifu, Japan from November 28 to December 1, 2011. He also presented a contributed paper first-authored by Orso Meneghini. During the following week he took the opportunity while in Japan to attend a Japan-Korea RF workshop where he presented the Toki paper and discussed the C-Mod - KSTAR collaboration with Young-soon Bae, who is in charge of the KSTAR heating and current drive program. On Tuesday Dec 13, Dr. Subrata Pradhan visited the PSFC. Dr. Pradhan is head of India's superconducting tokamak project, SST-1. He discussed plasma startup issues with several C-Mod scientists, and also gave a seminar on the progress and status of the SST-1 refurbishment. Stewart Zweben (PPPL) visited MIT last week to discuss edge turbulence and blob analysis with Jim Terry, Matteo Agostini from RFX, and Odd Eric Garcia from Tromso. There exists a good database of GPI camera data from 2010/2011 which will be analyzed for blob propagation and L-H transition physics. A high priority for the present run is to obtain better data for the GPI camera viewing the outer X-point region. Bruce Lipschultz traveled to Cadarache, France to participate in two ITPA meetings. For the first meeting, that of the ITPA coordinating committee, December 11 and 12, he participated as both the international chair of the DivSOL group and the representative from Alcator C-Mod. He presented a summary of the DivSOL groups work over the past year. Highlights of the report included the considerable work on tungsten - melting studies, fuzz growth; material migration measurements and modeling; SOL parallel heat flux width studies; and an evaluation of flash heating of Be and tungsten surfaces for removal of retained fuel. Reports from 5 other ITPA groups were made as well. Bruce Lipschultz's term as chair of the DivSOL group ends with this meeting. The new chair of the group will be Emmanuelle Tsitrone of CEA-Tore Supra with the deputy chair being Huoyang Guo of ASIPP-China. On December 14, Bruce Lipschultz represented both C-Mod and the DivSOL group at the IEA/ITPA meeting. All the cross-machine experimental proposals for run time were reviewed. All proposals were approved. Presentations were also made by representatives from all 7 ITER partners. There was a discussion of the upcoming synopses submissions to the San Diego IAEA meeting. The deadline for ITPA submissions to the IO is the end of January. The next IEA/ITPA meeting is being planned for December of 2012. On Friday, December 16, Rich Hawryluk, Deputy Director General, Administration Department at the ITER Organization, visited MIT and presented a PSFC Seminar on "The role of ITER in the development of fusion energy". He also had discussions with Dennis Whyte, Amanda Hubbard, Miklos Porkolab, Earl Marmar, Joe Minervini, Bruce Lipschultz, and Jerry Hughes. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly