Alcator C-Mod Weekly Highlights April 18, 2006 FY2006 weeks of research operations: Planned: 14 weeks Completed: 7.6 weeks Operations ---------- Plasma operations continued at Alcator C-Mod last week. Three run days were scheduled and completed. A total of 84 plasma discharges were produced with a reliability of over 97%. Experiments were carried out in support of research of the Divertor/Edge, Lower Hybrid, MHD, and Transport Science groups. Plasma operations are planned to continue this week. Run summaries ------------- Wednesday's run was devoted to MP#409 "SOL Phase Space", and to MP#429 "SOL turbulence in limiter plasmas. Data were obtained for the MP#409 study in 1.1MA, 5.4T upper and lower single null discharges at high density (2 < nebar < 3.5e20/m^3). In addition, we began a series of density ramp-up discharges at 0.4MA and 4T, designed to study density limit effects. For the MP#429 study, six good limiter shots were taken with GPI imaging, fast diode, and probe data. This is probably all the data needed to characterize the turbulence in this standard condition (0.8 MA, 5.4 Tesla, kappa=1.2). Comparison shots with a similar diverted plasma were also obtained. Thursday's run was dedicated to MP#418, "Lower Hybrid Coupling Studies". Following up on prior runs, the pulse length was increased to over 100ms per pulse and up to 210ms of total duration per shot; previous experiments had been limited to <20msec per pulse. Peak power levels of 80kW/klystron in a short pulse and 70kW/klystron for the longer pulses were obtained. Based on results from this run, additional modifications to the Coupler Protection System are planned in order to further extend the pulse length at high power. During Thursday's experiments, strong signals on the HXR camera and ECE diagnostic indicated the presence of fast electrons driven by the lower hybrid waves, although no conclusive evidence of current drive was observed. Increases in total stored energy consistent with electron heating were observed. Friday's run began with a series of shots in support of MP#353 "Study of striations in Li pellet ablation clouds". The objective was to perform radial scans of the pellet ablation in standard L-mode and H-mode plasmas using the PSI-4, high speed ccd camera. The timing for the exposures of the camera would be varied, to obtain the entire radial profile. Technical difficulties with the Li pellet injector and camera triggering prevented successful completion of this experiment, which will be rescheduled for a later date. The remainder (about 6 hours) of Friday's run was devoted to MP#449 "Gas jet disruption mitigation of VDE's". Vertical displacement events (VDE's) were induced in 1MA LSN discharges and massive argon puffs were used to terminate the discharge before the disrupting plasma could reach the top or bottom of the vessel. Initially the VDE's were produced by programming the plasma position to step downward and then turning off the gain on the vertical control feedback loop, resulting in reproducible downward-going disruptions. These target discharges were then terminated by firing the gas jet at pre-set times relative to the turn-off of the vertical control. The halo current, divertor temperature (from IR camera), and radiated power fraction were monitored along with the maximum vertical displacement as the timing was varied. When the plasma was terminated after an excursion of 6-8cm the halo current was reduced by about a factor of two compared to the level when the displacement exceeded 20cm. The lag between triggering of the valve and the plasma termination is about 6-7 msec. Following these pre-programmed synchronous tests, we began a series of shots in which the Digital Plasma Control System was used to fire the gas jet in response to real-time detection of the VDE, using an algorithm based on a threshold in the absolute ZCUR error signal. This technique was successful in mitigating the VDE's resulting from the programmed turn-off of the vertical control, using a detection threshold corresponding to an error of 11mm in the vertical position, which is considerably larger than the maximum error observed in well-controlled discharges. We then tested the real-time detection and mitigation technique on a series of shots in which the vertical feedback system was enabled and the plasma elongation was programmed to increase beyond the stable limit, resulting in a growing vertical oscillation ultimately terminating in a VDE. In these cases it was found that the detection threshold had to be decreased to about 8mm for successful mitigation of halo currents; higher threshold settings resulted in larger vertical displacements before termination, as the VDE speed was increased in these "naturally" unstable discharges. Diagnostics ----------- After extensive repairs at PPPL, the 130/140 GHz reflectometer channels were brought back to MIT and re-installed on C-Mod. On Thursday reflections from the plasma were observed on both channels and the system is now ready for routine measurements. The Motional Stark Effect (MSE) diagnostic was operated on Thursday during a long series of reproducible plasmas with its wire grid polarizer in place to generate linearly polarized light with a constant polarization direction. The standard deviation of the measured angle as a function of time is more than a factor of 3.4 lower for the edge MSE channel that uses a recently-installed avalanche photodiode as the photon detector compared to the other channels that use photomultiplier tubes. Diagnostic Neutral Beam System ------------------------------ Repeated faulting of neutral beam high voltage supply components has prompted the decision to proceed immediately with the planned changeover to the new high voltage inverter drive boards. The new inverter front panels and electrostatic shields have been fabricated and are ready for installation. All eight of the inverter chassis have been removed from the cell for installation of the new boards. ICRF System ----------- D-port and E-port antenna systems (FMIT#1 and 2) were employed during experiments on Friday. Injections of Cu and Ni have been observed spectroscopically from the D-port antenna during low density, high power operation. Videos of the D-port antenna indicate discharges and high temperatures along the top antenna protection tiles. We are investigating ways to limit these effects. Tracking along the surface of the push-pull rod in the J-port transmission line phase shifter did not allow operation of the J-port antenna last week. The rod has been repaired, and installation is underway. We have manufactured and pressure tested the outer conductors for the quarter wave transformers for the E-port transmission line; the center conductors should be completed and ready for installation early this week. Travel and Visitors -------------------- Paul Bonoli attended the Steady State Operations ITPA Workshop in Naka, Japan from April 10-13, 2006. He gave two talks at this meeting titled "Plans and Results thus far for LHCD Code Benchmarking for IAEA 2006" and "AORSA and TORIC ICRF Code Benchmarking Results for the ITER Scenario 2" Glen Wurden and Leonid Dorf from the LANL P-24 Plasma Physics group worked at MIT this week to install a new infrared spectrometer on the A-Top port, using the existing infrared periscope optics. The spectrometer operates in the 0.7 to 3 micron band with a 256 element Indium-Gallium-Arsenide detector, cooled to -20C, and is scanned at ~5 milliseconds per spectrum. A variety of gratings, at 150, 300, and 600 lines/mm let the experimenter select the desired wavelength coverage and resolution. A silicon wafer beam-splitter takes part of the light to the spectrometer, and leaves the rest for the existing infrared video camera. The system is controlled via fiber optic link over a PXI crate, back to a PC in the control room, using Labview software to run the Hamamatsu infrared array. The system is installed, and operational for test purposes, and alignment procedures and software integration are underway. The Alcator magnetic fields on top of the igloo did not affect the new electronics. Stewart Zweben (PPPL) was at MIT last week and led the "SOL turbulence in Limiter Plasmas" experiment (MP#429) on Wednesday. Discussions of the new lower divertor GPI and analysis of old data also continued. Perry Phillips (Univ. of Texas) began a one-week visit to MIT last Wednesday. He installed the repaired local oscillator on the FRCECE electron cyclotron emission diagnostic, along with new magnetic shielding and an isolated power supply. All 32 channels of the FRCECE are expected to be available for this week's runs. Perry also carried out a measurement of the phase variation in the IF system and found all channels to be in phase within the resolution of system. A new fast PCI card is being installed to permit up to 22 ECE channels to be sampled at 2MHz. Amanda Hubbard travelled to PPPL on Friday to participate in the NSTX phase of the NSTX/C-Mod/MAST Small ELM comparison. This experiment, also involving Hendrik Meyer of UKAEA and Rajesh Maingi of ORNL, follows up on the C-Mod expt conducted on 4/7. Last week the PSFC hosted the 10th meeting of the ITPA Pedestal and Edge Topical Group. This meeting, organized by Amanda Hubbard and Corrinne Fogg, had 25 on-site participants from the EU, Japan, US, Russia and Korea. About 10 off-site participants connected through video-conferencing. The meeting agenda and other information can be found at http://www.psfc.mit.edu/research/alcator/mtgs/itpa.php . _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly