Alcator C-Mod Weekly Highlights May 1, 2006 FY2006 weeks of research operations: Planned: 14 weeks Completed: 9.1 weeks Operations ---------- Plasma operations continued at Alcator C-Mod last week. Three run days were scheduled and completed. A total of 59 plasma discharges were produced, including 24 high-field (7.8T) shots, with a reliability of 95%. Experiments were conducted in support of research in Transport Science and the Integreated Scenarios for ITER thrust, including two ITPA Joint Experiments. Overnight boronizations were carried out before the runs on Tuesday and Thursday. Plasma operations are planned to continue this week. Run summaries ------------- On Tuesday we attempted MP#423, "C-Mod/JET rho* scan along an ITER-relevant path". The goal was to get the low-field portion of a rho* "dimensionless" scan. These discharges would also serve as the C-Mod part of a multi-machine rho* scan, as part of the ITPA Joint Experiment CDB-8. The desired plasma parameters were Bt=2.6 T, Ip=0.54 MA, n=1.2e20, Ptotal at least 2.3 MW. The plasma shape matched an equilibrium mutually agreed upon by C-Mod, JET and DIII-D for inter-machine comparison experiments. While we did get acceptable heating, estimated at 50% absorption, via 2nd harmonic proton minority heating at 2.6 T, we were not successful in getting the dimensionless match we were looking for. Beta_N of 1.5 was achieved, but even with low ohmic target densities, the H-mode density was almost a factor of two too high, resulting in collisionality 6-7 times greater than the desired value. The lowest density shots had high levels of radiation, despite the fresh boronization. Impurities may have been generated by runaway electrons or by fast ions from the 2nd harmonic minority heating, scraping off on the outer limiter. Improved control of the density in the H-mode phase, as anticipated using the cryopump which will be installed following this campaign, may be required to achieve the specified paramters for this experiment. Part of Tuesday's run was also devoted to MP#353, on analysis of striations in Li pellet ablation clouds. This MP constitutes part of the doctoral research of an MIT graduate student. The work on Tuesday demonstrated that the injector is now working reliably. However the alignment of the viewing optics for the PSI-4 fast camera needed adjustment in order to produce useable images. Better images were obtained during piggyback operation on Thursday, following optical realignment. The goal of Wednesday's run was to continue the extension of H-mode pedestal characterization on C-Mod by obtaining H-modes at high toroidal field, with varied plasma current and input power. We successfully coupled 80MHz ICRF to the plasma using D-He3 heating at 7.9T, and were able to diagnose a limited number of H-modes at this field. Steady 2.5--3MW of ICRF power generally was needed to access H-mode at the increased field, and limitations to the total power available impacted the ability to obtain steady H-modes. Nonetheless, pedestals with rather low collisionality (nu* as low as 0.5) and reasonably good confinement were obtained. Pedestal widths did not appear to differ significantly from those at lower fields. A scan of plasma current was begun, but more shots will be required to complete this portion of the experiment. On Thursday we began experiments in support of C-Mod MP#382 (ITPA Joint Experiment PEP-7) on C-Mod/JET/Asdex-Upgrade pedestal dimensionless identity experiments. These C-Mod experiments are carried out at 7.8T, using D(He3) ICRF minority heating. Establishment of the mutually agreed upon equilibrium shape at this field was accomplished after only a few shots. ICRF heating efficiency was good, and H-mode conditions were readily obtained. Pedestal parameters were varied at q95=4.1 by varying the target density and RF power. We also made contact with the parameters from earlier inter-machine comparison experiments for which the C-Mod field was 5.2T. In addition, we obtained one comparison point at higher q95~5.2, as proposed by the Asdex team. Additional data, especially with increased RF power, is required to complete the C-Mod dataset for this Joint Experiment. The corresponding experiments are presently scheduled to begin on Asdex Upgrade within the next month, and on JET later this year. Computers and Data Acquisition Systems --------------------------------------- Two new computer servers were installed as upgrades to two existing servers: the primary C-Mod backup/archive server used for archiving all of C-Mod experiment data as well as providing linux server backups, and the PSFC mail server which provides mail, newsgroups, PSFC mailing lists and hosts the www.mdsplus.org web site and cvs repository. Both systems are now fully operational and in production use. ICRF System ------------ The D- and E-port antenna systems operated reliably during C-Mod operation last week. Operation of the J-port system was less robust, limiting the available power for some experiments. We identified an arc in the J3 phase shifter and are in the process of replacing the tracked teflon insulator with a new piece, which has a different design. We had installed a pressure transducer on the outside of the conductor and found that we could detect a signal associated with the arcing in the phase shifter. This suggests we should be able to install a more comprehensive system to allow us to determine if an arc is in the vacuum section or on the air side transmission line in the future. We have also investigated and plan to install quarterwave transformers in the J transmission line to reduce the voltage in the phase shifters and stub tuners. We have installed and tested into vacuum the quarterwave transformers in the E-port antenna transmission line. We await plasma discharges to verify the predicted voltage reduction which should enable us to run more reliably at high power with this configuration. Diagnostic Neutral Beam System ------------------------------- The changeover to the new high voltage inverter drive boards is nearly complete. All of the boards have been assembled into the inverter chassis and connected. Including spare inverters, we now have ten new inverters, which have been successfully tested at low power. Eight of the inverter chassis have been installed into the DNB high voltage inverter rack and attached to the DC connections and fiber optic inputs. The existing inverter control power supply was found to be inadequate for the new control boards, and a new supply has been ordered. We expect to begin power tests early this week, after the control power supply is installed. In discussions with Steve Scott, during his visit to PPPL, Howard Yuh (Nova Photonics, Inc.) proposed a very interesting conjecture that may explain a number of puzzling features of the MSE calibration and operation. The C-Mod MSE system is unique in that it relies on exactly perpendicular injection of the diagnostic neutral beam. This creates a population of fast ions with near-zero parallel velocity, which grad-B drift only relatively slowly out of the MSE viewing footprint. These fast ions can re-charge exchange and emit another Halpha photon at a different polarization angle due to the gyro-motion of the ions. Calculations show that a significant fraction of the gyro-orbit yields a Doppler shift that will pass through the MSE optical filters (about 1/3 of the orbit, for the edge MSE channel), and the variation in polarization angle is very large - more than 100 degrees. Calculations are in progress to evaluate the relative magnitude of the nominal MSE signal versus the secondary signal arising from re-charge exchange of the fast ions. If this mechanism is shown to be the cause of the MSE calibration anomalies, it could be fixed by re-aiming the diagnostic beam 4-8 degrees off perpendicular. There are no interferences inside the F-port extension that would preclude a tilt of this magnitude; a number of other diagnostics which view the beam would also have to be modified. Travel and Visitors ------------------- Bob Pinsker from GA visited MIT last week, and discussed a number of ICRF related hardware issues with C-Mod personnel. He has strongly endorsed installing quarter-wave transformers to reduce the voltage in the phase shifters and stub tuners. Dr Pinsker presented a PSFC Seminar entitled "Absorption of Fast Waves at Moderate to High Ion Cyclotron Harmonics on DIII-D", as well as an informal presentation on "Lessons learned from GA ICRF experience". Steve Scott visited PPPL to discuss the calibration issues for the C-Mod Motional Stark Effect diagnostic with Fred Levinton, Howard Yuh, Jill Foley and Gerrit Kramer. Valerie Izzo attended the Sherwood Fusion Theory meeting (and APS April meeting) in Dallas, TX, and presented a poster on disruption mitigation modeling. Paul Bonoli also attended the Joint Spring APS and Sherwood Theory Conference and gave an invited talk at Sherwood entitled "Quasilinear Evolution of Nonthermal Ion Distributions in the Presence of High Power Ion Cyclotron Resonance Heating". He also gave a poster presentation on behalf of John Wright entitled "Full wave coupling to a 3D antenna code using Green's function formulation of wave-particle response" Amanda Hubbard participated remotely in the "Aries Design Brainstorming Workshop" held at GA April 24-25. Joe Snipes participated remotely in the ITPA meeting of the Confinement Database and Modeling Group, which took place at PPPL from 24 - 27 April. John Rice and Alex Ince-Cushman attended the meeting of the ITPA Transport Physics Group, also held at PPPL from 24-27 April. Rice presented talks entitled "Intinsic Rotation in Alcator C-Mod" and "Summary of Spontaneous Rotation in Tokamaks". Ince-Cushman presented talks on "Poloidal Rotation in Alcator C-Mod" and "Multi-machine Scaling of Spontaneous Rotation". Bill Burke attended a conference on Real-time and Embedded systems in Framingham, MA on Thursday. A C-Mod Quarterly Review was held by videoconference on Friday, April 28. Ron Parker, Steve Wolfe, and Jim Irby presented talks on the C-Mod Lower Hybrid Experiments, Campaign Status and Plans, and Facility Status. Also attending at MIT were Earl Marmar, Miklos Porkolab, and Brian LaBombard. Adam Rosenberg, Rostom Dagazian, Steve Eckstrand, John Sauter and Chris Yetter participated from OFES in Germantown, MD. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly