Alcator C-Mod Weekly Highlights December 17, 2007 FY2008 weeks of research operations: Target: TBD Completed: 0.38 weeks Operations ---------- Plasma startup and conditioning operations continued at Alcator C-Mod last week. Four run days were scheduled and 3.5 completed; operations were stopped on Thursday afternoon when the MIT administration decided to close the Institute early due to a snowstorm. A total of 83 plasma discharges were produced with a startup reliability of 78%. Two Research miniproposals, addressing plasma control and vertical stability issues for ITER, were run on Tuesday. ICRF continued conditioning operations on Wednesday and Thursday, and Friday was devoted to further conditioning of the Lower Hybrid system. Plasma Operations are scheduled to continue this week. Operation Details ----------------- Two experiments on Tuesday addressed issues of vertical control at high elongation. In the first a Kalman filter was implemented in the digital plasma control system (DPCS) to reduce the control system response to noise and disturbances in the fast vertical stabilization loop. The filter was successful in reducing the amplitude of high frequency demands to the fast power supply while maintaining stable operation with plasma elongation close to the previously observed controllable limit, kappa>1.8. The experiments also identified potential improvements in the implementation which will be evaluated using off-line simulations. This work comprises part of the thesis research of an MIT graduate student. The second experiment on Tuesday, led by the same graduate student, tested a proposed metric relevant to the ITER vertical control system proposed by D. Humphries (GA). This proposal is to evaluate the "maximum delta_z", the largest uncontrolled excursion from the nominal vertical position which can be reversed by a saturated demand to the relevant actuator. Such experiments are proposed through the ITPA to be carried out by many tokamaks in order to identify the typical range of this quantity obtained in operating devices. This range would then be compared with the corresponding capability proposed for ITER to evaluate the adequacy of the ITER acutator design relative to existing practice. The C-Mod experiment was carried out using the DPCS by turning off the fast vertical control for a pre-programmed duration, while simultaneously freezing the demands to the slower equilibrium control actuators. After the pre-set interval, a saturating demand signal of the appropriate sign was applied to the power supply. The interval was varied from shot to shot to determine the maximum reversible deviation from the equilibrium position. Results were obtained on C-Mod at three elongations between 1.7 < kappa < 1.8, corresponding to different stability margins near the proposed ITER operating point. The data are being analyzed and a report will be generated, including a description of technical difficulties identified with the prescribed technique. Runs on Wednesday and Thursday were concentrated on continuing conditioning of the ICRF system. Simultaneous operation of the three antennas was achieved, with maximum total powers over 4 MW. The cryopump was used to reduce the H/D ratio during conditioning in upper null discharges, and minority heating was observed, although optimal hydrogen concentrations were not achieved. Evaluation of the fast ferrite tuner (FFT) real-time matching system on the E-port antenna continued. Results were generally satisfactory, but at the highest powers the matching solution was found to be less accurate. The reason for this effect is being investigated. Conditioning of the lower hybrid system was also carried out in piggyback operation on both days. The primary experiment on Friday was continued conditioning of the lower hybrid system. Net coupled power was increased to >700kW into moderate density (nebar~1e20/m^3) plasmas, with pulse lengths of 0.5sec. Reliable operation was demonstrated at the 700kW level, with more conditioning required at higher power. Run Planning ------------ A general C-Mod group meeting was held on Monday to discuss plans for experiments during the 2008 campaign. Topical Group and Task leaders provided summaries of the prioritization activities within their groups over the previous weeks. Lower Hybrid System ------------------- Testing of one of the klystrons under refurbishment is underway at the vendor, and tentative dates for acceptance tests, to be witnessed by MIT engineering staff, have been set for January. The lower hybrid group continued conditioning operations during the runs on Wednesday through Friday. Diagnostic Neutral Beam System ------------------------------ After installing a reconditioned cathode/heater assembly, DNB conditioning was carried out at low beam currents and voltages (2.5 A, 42 kV) for a day. This was temporary halted by a malfunctioning circuit in the control cabinet, which was repaired at the end of the week. Beam conditioning will continue next week. Travel and Visitors -------------------- Paul Bonoli attended a workshop of the ITPA Steady State Operations Group in Garching, Germany on December 10-12, 2007. He gave three talks: "Simulation of High Power ICRF Wave Heating in the ITER Burning Plasma", presented on behalf of E. F. Jaeger at ORNL; "Inter-Code Comparisons of Lower Hybrid Current Drive Models in Reactor Relevant Regimes"; and "Validation of CQL3D Against LHCD Results on Alcator C-Mod". Paul also met with Professor Marco Brambilla and Dr. Roberto Bilato of IPP to discuss physics development issues with the TORIC electromagnetic field solver. Earl Marmar and Dennis Whyte participated in the National Research Council "Committee to Review U.S. ITER Science Participation Planning Process" in Washington DC Dec. 14-15. The review is the result of the National Academies being asked by Congress, in the Energy Policy Act of 2005, to review the DOE/BPO plan for US fusion community participation in ITER. Whyte served as a member of the review panel, and Marmar gave a presentation entitled "U.S. BPO Panel: Planning for ITER". Todd Evans (General Atomics) visited MIT on Friday. In addition to presenting a PSFC Seminar on "ELM Suppression Using Edge Resonant Magnetic Perturbations in DIII-D", Todd spent much of the day in discussions of RMP experiments and non-resonant magnetic braking experiments with C-Mod scientists and students. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly