Alcator C-Mod Weekly Highlights April 30, 2007 FY2007 weeks of research operations: tentative target: 15 weeks Completed: 2.58 weeks Operations ---------- Plasma operations resumed at Alcator C-Mod last week, following the scheduled maintenance period. Four run days were scheduled and completed. Experiments were carried out in support of Miniproposals in the Lower Hybrid Current Drive, Diagnostic development and Operations areas, along with conditioning of the ICRF antenna systems. A total of 71 plasma discharges were obtained, with a startup reliability of about 80%; in addition, twelve beam-into-gas with toroidal and poloidal field pulses were produced for calibration of the MSE diagnostic system. Plasma operation is planned to continue this week. Operation Details ------------------ Tuesday's run was primarily devoted to conditioning of the ICRF antenna systems (MP#355). The E-port antenna was energized for the first time during this campaign; up to 1MW was successfully coupled through this antenna during Tuesday's run. Conditioning of the D- and J-port antennas was also continued. Wednesday morning's run was devoted to MP#461, "Development of zero loop voltage control algorithm", which is intended to provide feedback control of the plasma surface voltage for use in non-inductive current drive experiments in the LHCD and AT research programs. The basic concept for the algorithm being tested, which is based on use of derivative (D) control of the flux at the dominant X-point, with a constant target waveform, was validated, and discharges with surface voltage near zero and satisfactory shape control were produced. As intended for these ohmic plasmas (no external current drive and insignificant bootstrap current) the plasma current ramped down with an L/R time constant. It was found that the best results required addition of some proportional (P) gain in the feedback control, which suggests that a more robust control scheme should be developed targeting the voltage itself and employing proportional and integral gain terms, thus avoiding the need to specify an absolute value for the target flux. It was also determined that for too large values of the D gain the control loop became unstable to a low frequency oscillation (f~30Hz), associated with the response of the slowest of the poloidal field power supplies. The effect of low-pass filtering on the flux observer and modification of "feed-forward" voltages on the poloidal field coil supplies was also investigated. Wednesday afternoon was dedicated to a series of (no plasma) beam-into-gas pulses with varying toroidal and poloidal fields, in support of MP#394 (MSE Calibration). In addition to performing a standard calibration, measuring the MSE response to different field pitch angles, this experiment also investigated the effect of charge exchange on the influence of the fast ion contamination (blue feature) in beam-into-gas calibrations by varying the torus pressure by over an order of magnitude. Some data were also obtained regarding the variation of the observed polarization fraction as a function of de-tuning of the spectral filters used in the MSE measurement. The experiment on Thursday morning continued our study of combined Lower Hybrid and ICRF operation. During ICRF operation through the J-port antenna the LH coupling was determined with phasing of 45, 60, and 90 degrees (co-current drive direction). The 60 degree phasing had the best match (lowest reflection). Operation of the LH in conjuction with the E-port ICRF antenna indicated substantial reflection, though not as high as previously measured during D-port antenna operation. In both cases during the ICRF pulses, probes indicate a reduction in density at the LH coupler for the top probes, less for those at the middle of the grill. For the lower probes, E-port operation resulted in a density reduction, while J-port did not. These results are consistent with the density reduction being primarily on field lines which pass in front of the active ICRF antenna. Thursday afternoon was devoted to the first experiments using the dust injector and monitoring system (MP#462). Boron dust was injected above the outer divertor shelf and observed using laser light scattering. One of the divertor video views (DIV2) was outfitted with a narrow spectral filter at the laser line for this experiment. Initial observations indicate that the dust particles have extended paths in the radial direction. This differs from the expectation that the particles would experience a strong toroidal flow in the SOL. Two Lower Hybrid experiments were carried out on Friday. The morning continued to explore the optimization of simultaneous operation of ICRF and Lower Hybrid, concentrating on the J-port antenna. During the ICRF pulses, LH reflections ~20% were obtained, followed by increased reflectivity as the local density at the coupler dropped. During these experiments, higher density shots showed strong indications of parametric decay instability (PDI), ~20dB down from the fundamental. Symmetric sidebands at ± 78MHz from the fundamental were also observed. The experiment on Friday afternoon was in support of MP#468 "LH-driven fast electron diffusion time measurement". The purpose of this run was to perform fast square-wave modulation of the lower hybrid at various plasma conditions to measure diffusion of LH-driven fast electrons. Data were obtained at plasma currents of 500 and 800kA, with nebar~8e19/m^3, B=5.4T and 90 degree co-current drive phasing, using ~400kW of LH power. Resulting profiles of hard xray emission are being analyzed. Collaborations --------------- On Friday, Amanda Hubbard participated remotely in an experiment on NSTX based on IEA/ITPA Joint Experiment PEP-16, on comparison of small ELM regimes in C-Mod, NSTX and MAST. The other principals in this experiment were Rajesh Maingi (ORNL/NSTX) and Hendrik Meyer (UKAEA). Routine H-mode access was obtained in the low elongation/triangularity shape when drsep was set between 0 and -4 mm. However only type I ELMs and Type III ELMs were observed in the H-mode phase, even when drsep was reduced to -7mm after the L-H transition. Thus it appears that the type V ELM window on NSTX also has a restriction of a maximum X-point height or minimum kappa. These experiments will be continued at MAST next week. ICRF Systems ------------ The E-port antenna was conditioned up to 1 MW into plasma. We also confirmed the electrical location of the fast ferrite transformers (FFTs) and completed manufacturing the remaining necessary coax sections. We have worked on debugging the FFT realtime computer and associated software and plan to install the FFTs for their first plasma operation this week. We have continued to work on the gate board for FMIT#1 (D-port antenna), and have made a spare board as a replacement. We have also noted some RF interference with power supplies that was not apparent during the initial operation and will be investigating the source. Lower Hybrid System -------------------- Following a careful inspection and clean-up of the guide, the gas baffle in waveguide 23B was reassembled, and trip levels in the associated coupler protection system (CPS) were reset to provide additional protection for this guide. Power applied to this guide was restricted during last week's operation. The lower hybrid system was run in support of dedicated experiments on Thursday and Friday, and in piggyback mode as allowed on Wednesday. The control system successfully produced the modulated waveform required for the fast electron diffusion experiment on Friday. Late in the run on Friday, a tube arc occurred in klystron #10, and attempts to condition this tube back to a high power level during the run were not successful. This tube will be exchanged over the weekend and we plan to recalibrate the drive legs on Monday. Long Pulse Diagnostic Neutral Beam ----------------------------------- The diagnostic beam was operated into plasmas last week with high reliability, in support of MSE, BES and CXRS diagnostics. The beam was operating at its full voltage of 50kV with beam currents in the 5-6 amp range, for pulse lengths of 1.5 seconds. Pressure transducers installed in the beam gas feed lines have revealed unexpected pressure drops in the system during a beam pulse. Operation with mass flow control aggravates this problem due to a ratcheting effect that decreases the starting pressure on each subsequent shot, eventually causing beam faults. Further investigation into the gas supply system dynamics is required to plan improvements in the system response. Travel and Visitors -------------------- Paul Bonoli attended the Sherwood Theory Conference that was held in Annapolis, MD from April 23-25, 2007. He presented a poster titled "Comparison of Lower Hybrid Current Drive Predictions from Different Simulation Models in Reactor Relevant Regimes". This meeting was hosted by the MIT Plasma Science and Fusion Center. The Conference Organizer was Paul Bonoli and the Conference Secretary was Valerie Censabella. Manfred Bitter and Ken Hill (PPPL) visited MIT last week to work with John Rice and Alex Ince-Cushman on the newly installed high resolution soft xray spectrometer diagnostic. This diagnostic is now producing profiles of ion temperature and rotation routinely during C-Mod plasma discharges. Refinements to the analysis software are in progress. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly