Alcator C-Mod Weekly Highlights April 17, 2007 FY2007 weeks of research operations: tentative target: 15 weeks Completed: 1.95 weeks Operations ---------- Startup and conditioning plasma operations continued at C-Mod last week. Four run days were scheduled and completed. In addition to continuing the process of machine clean-up and subsystem checkout and commissioning, all the runs this week contributed to the C-Mod research program. Progress was made in lower hybrid physics, diagnostic development, and development of operational techniques in support of the lower hybrid current drive and advanced tokamak scenario programs. A total of 74 plasma discharges were produced, with a startup reliability of over 90%. In addition to tokamak operations on Tuesday through Friday, beam-into-gas calibration shots (without fields) were carried out on Monday. The goal of this experiment was to spectroscopically determine the angle between the MSE viewing chords and the DNB trajectory. This angle is needed to set the interference filter bandpass for each MSE channel to select the appropriate fine structure component for the polarization measurement. Two experiments were scheduled for Tuesday. The first half of the day was devoted to MP#461 "Development of zero loop voltage control algorithm", which is in support of planned experiments in lower hybrid current drive and fully non-inductive current sustainment. Progress on this MP was limited due to CAMAC errors which resulted in spurious trigger signals to multiple diagnostics, including the magnetics system. Additional time will be scheduled for the completion of this development. The second half of Tuesday's run was devoted to beam-into-gas shots with vertical and toroidal fields, in support of MP#460 "Test of effect of fast ions on MSE with perpendicular injection". This experiment evaluates whether the reorientation of the DNB to 7 degrees off perpendicular has effectively reduced the contribution of emission from secondary beam neutrals to the MSE signals; this effect was conjectured to interfere with the MSE measurement of current profiles when the beam injection was purely radial. High resolution spectra were obtained with a range of field angles; these are being fully analyzed for comparison to similar data obtained prior to the rotation of the beam. Lower hybrid current drive experiments were carried out on Wednesday and Friday. On Wednesday the dependence of non-thermal electron distributions, as indicated with hard xray and electron cylotron emission (ECE) diagnostics, on plasma density (0.7 to 1.6e20/m^3), field (4.3 to 7T) and current (0.5 to 1.2MA) was investigated for different antenna phasing (n_|| spectrum). Fast electrons were observed at line-averaged density up to 1.6e20/m^3 with 90 and 120 degree phasing. The observations of fast electron tails were consistent with estimates of LH wave accessibility. LH wave coupling and reflectivity under different conditions was also addressed in experiments on Wednesday and Friday. At higher plasma current the local density at the grill was observed to decrease, leading to increased reflection. Gas was puffed at the midplane NINJA capillary near the A-B port limiter, which is located on field lines in the SOL which pass in front of the upper part of the LH launcher, in order to raise the density in front of the grill and improve the coupling. As expected, the effect on the local density was strongest on the top rows of the grill. However, the concomitant increase in core density made this technique ineffective. Also on Friday, LHCD was employed during the current rise, and in the flattop phase in conjunction with ICRF-induced H-modes. Initial results with the ICRF indicates that combined operation is more difficult when the D-port ICRF antenna is used than with J-port, presumably because the D-port antenna is magnetically well-connected with the LH launcher at C-port. LH coupling was also demonstrated during ohmic H-modes at 6 tesla; coupling in these discharges was better than during the ICRF H-modes. These experiments contribute to the third quarter research milestone "Operate LHRF into L-mode and H-mode plasmas, and investigate combined LHRF and ICRF". The run on Thursday was devoted to MP#466, "Slower Ip ramp development". The purpose of this experiment is to establish discharges with slower Ip ramp rates to enable advanced scenario q and j profile control. The goal on Thursday was to develop ohmic reference discharges that will be used later in the campaign for low-current, slow ramp ICRF/LH driven discharges including ITB and hybrid scenarios. Using an early crowbar of the OH and EF1 commutation resistors and adjustment of the Ip and shaping feedback gains, we succeeded in establishing a low initial current (Ip<200kA at 0.1sec compared to ~500kA in typical startups) and early divertor operation (t < 0.2sec) even with li>2.1. Two slow ramp cases were established. The fact that the slower ramp case (to 600kA at 1.0sec) also exhibited early diversion indicates that the crowbar and feedback settings are robust. These discharges will be combined with early heating and current drive experiments planned in the Advanced Tokamak scenario development later this year. Machine conditioning is proceeding well, with some discharges last week exhibiting H/D ratios in the 10% range. Additional plasma operation, including more high power ICRF, will be needed to further improve the H/D ratio. This week will be a maintenance week at Alcator C-Mod. No plasma runs are scheduled. Lower Hybrid System ------------------- The LH system was run every day last week, two days in support of miniproposals and in piggyback mode during other experiments. A gas baffle in one waveguide (external to the machine) was damaged and had to be replaced. We are investigating whether we can eliminate these baffles, which separate two regions which currently operate with different pressures of dry nitrogen gas. We operated LHRF successfully during ICRF operation of J- and D-port antennas. LHRF Operation was reliable at power levels up to 120kW per klystron at the intermediate directional couplers. ICRF Systems ------------ In preparation for physics operation, the D and J antennas have been conditioned to 1 MW (30 kV) and 2.5 MW (~28 kV) respectively in H-mode discharges. The stands for the fast ferrite tuner (FFT) which will be installed in the E-port transmission line have been completed and installation will begin this week. Long Pulse Diagnostic Neutral Beam ---------------------------------- The DNB operated reliably last week. Beam operation was required on all five weekdays for various CXRS and MSE physics studies, including a measurement of the "blue shift" feature, and edge MSE measurements at constant q. Beam parameters all week were 5-6 amps at 50 kV for pulse lengths up to 1.4 s. Beam modulation was varied extensively during Friday's run. There were no operational problems during the week. Diagnostics ------------ The poloidal CXRS optics which will be used for measurements in the range r/a < 1 were successfully tested with the DNB operating near its best performance. The optical system is located at a top port and requires prisms to divert the optical view to the new trajectory of the DNB. The CXRS enhancement was approximately 70% of the ambient emission. This is well within the enhancement range needed for accurate Ti analysis. Perry Phillips (UT-FRC) identified and diagnosed pickup in a few channels of the FRCECE (electron cyclotron emission diagnostic). The pickup was first noticed in channel 16 and associated with LHCD. The pickup is not large except when there are fast transients in the LH system. Better shielding for the FRCECE will be investigated. The new high resolution xray spectroscopy diagnostic (HIREX III) was installed on Thursday and began making measurements during Friday's run. Profiles of plasma rotation using doppler shifts of lines of intrinsic Mo and injected Ar were obtained. Travel and Visitors ------------------- Martin Greenwald and Ian Hutchinson traveled to Orlando for a site visit at the location of the 2007 APS-DPP meeting. Following this visit, Ian and Martin attended the DPP executive meeting in Jacksonville. Eric Eikenberry of Dectris (Switzerland), and Manfred Bitter and Ken Hill from PPPL visited during the week to assist Alex Ince-Cushman and John Rice in the commissioning of the new imaging x-ray spectrometer system. The detectors (from Dectris) were installed in the mounts, the spectrometer housing was closed and purged with helium and the system was tested with an x-ray tube. On Thursday night, April 12, the spectrometer was installed onto C-Mod and first data were collected on April 13. On Monday April 9, Dennis Whyte presented a seminar to the MIT section of the American Nuclear Society "The Challenges of Plasma-Surface Interactions in Magnetic Fusion" Chuck Kessel (PPPL) visited MIT on Thursday and Friday. On Thursday he served as Session Leader for the experiment dedicated to his MP#466 on slow Ip ramps. On Friday he worked with Amanda Hubbard and Paul Bonoli on modeling of AT discharges and planning of future C-Mod experiments, and discussed LH modeling for ITER. Randy Wilson (PPPL) was at MIT on Friday, and served as Session Leader for portions of the Lower Hybrid experiment that day. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly