Alcator C-Mod Weekly Highlights January 21, 2008 FY2008 weeks of research operations: Target: TBD Completed: 2.6 weeks Operations ---------- Plasma operations continued last week, with four run days completed. A total of 88 plasma discharges were produced, with a startup reliability of 77%. Experiments proposed by the Lower Hybrid, MHD, H-mode Integrated Scenarios, Rotation, and Diagnostics groups were carried out. The cryopump was successfully operated during three of the four plasma runs last week, as requested. Six hours of run time were lost due to power systems, RF, and computer data acquisition delays last week. Plasma operations are scheduled to continue this week. Operation Details ----------------- Tuesday's run was devoted to the continuation of our study of coupling and reflections at the lower hybrid launcher and compatibility of ICRF and LHCD operation. For these experiments the launcher was positioned 2.5mm behind the local guard limiters; similar experiments had been conducted previously with the launcher at 1.5mm. Coupling was found to be poorer at the 2.5mm location, despite similar (~4e17/m^3) density measured by the probes at the grill. It was determined that operation of the adjacent D-port ICRF antenna resulted in heating of a piece of copper finger stock protruding from the LH probe support rail, as observed on the video view of the launcher. Injections of copper into the plasma were observed to be correlated with operation of the D-port antenna at power levels of 1MW, even when the outer gap (distance between the LCFS and main limiter) was increased to 2.5cm. Operation with up to 2MW from the J-port antenna, on the other side of the machine toroidally from the launcher, did not result in such injections. Coupling of the LH waves was degraded with the ICRF on, but still within acceptable limits. Tests with the local gas puffing system at the LH launcher did not show any clear improvement in coupling at this launcher position. On Wednesday morning, an experiment (MP#459a) to create calibration plasmas for the McPherson VUV spectrometer was carried out. Strong krypton puffing into an ICRF heated, high density, pumped USN L-mode plasma resulted in highly radiative discharges dominated by krypton emission. Calibration data were collected for the Na and Mg-like lines of krypton. The experiments demonstrated the ability of the cryopump to sustain a steady state krypton puff. The upper null,Ip=1MA,nebar~1.6e20/m^3 discharges remained in L-mode with up to 4MW of ICRF power due to the high radiated power fraction. The second half of the day on Wednesday was devoted to rotation and ion temperature measurements during strongly sawtoothing L-mode discharges, in support of MP#520. The primary diagnostic for this experiment was the high resolution spatially resolving X-ray spectrometer HIREX_SR, which provides intensity and Doppler width and shift measurements of lines of H-like and He-like Argon, and Ne-like Mo. The argon signal was enhanced using steady state puffing in conjunction with cryo-pumping, essentially the same technique employed for krypton during the previous experiment. Data were obtained at the maximum rate of 200Hz in discharges with moderate sawtooth periods in the range of 15-20 msec. The sawtooth periods and amplitude were steady during the flattop portion of the discharge, which should allow phase binning of the data to enhance signal to noise. The raw data show clear sawtooth modulation of emissivity and ion temperature profiles. The inferred rotation signals appear to be contaminated by an instrumental artifact which is being investigated. MP#513, "LHCD Effects on Stable Alfven Eigenmodes", was run on Thursday, 01/17/08. The upper Active MHD antenna was excited with a pre-programmed sweeping waveform to look for possible resonances in the frequency range of Alfven cascades early in the current rise, as well as around the TAE frequency for q=1.5 later in the flattop phase. Ohmic shots were compared with shots with additional LHCD to see what effects the LHCD have on the current density profile through changes in the measured stable Alfven eigenmode resonances. Two shots had very early LHCD from t=0 and showed some fast electron driven unstable TAEs. The early LHCD changed the current density profile and led to locked modes that then led to early disruptions. Shots with the LHCD starting at 0.1s, during the current rise, showed no clear resonances. The LHCD pulse was then moved to the flattop portion of the discharge. Data were obtained over a current scan from 0.5MA to 1MA. A number of resonances were apparent in the flattop data; further analysis is required to see whether or not resonances exist in the current rise. Detailed analysis to determine changes in the resonances due to LHCD is also underway. Friday was devoted to MP#522, "Lower lp Long Pulse L-Mode and H-Mode Advanced Scenarios". Charles Kessel (PPPL) served as Session Leader using remote collaboration tools. The experiment continued development of target discharges for AT scenarios at 450kA (q95~7) with slow current ramps, using ICRF heating injected well before the start of the current flattop at 0.5sec. H-modes were obtained during the ramp using 2-3MW of ICRF. Control of the density to levels above the low-density threshold around 8e19/m^3 was necessary for entry to H-mode, with lower density cases remaining in L-mode. H-modes produced around 200ms were short-lived (dithery), with the earliest sustained examples having L-H transitions around 0.25sec. The H-modes were ELM-free, and often suffered radiative back transition to L-mode, although some had reduced radiation and lasted for over 250 ms. The earliest H-mode attempts at 200 ms did appear to delay sawteeth and/or change their character. A cursory look at the temperature profiles showed that the longer H-modes had good pedestal temperatures and were steady, in spite of being ELM-free.More detailed examination will focus on systematic changes in internal inductance (li), loop voltage, stored energy, and other plasma parameters. Program Planning ----------------- On Tuesday, the C-Mod Experimental Program Committee met to consider miniproposals for near-term experiments. Five new or revised MP's were reviewed and approved. The next EPC meeting has been scheduled for January 28. Physics --------- The first beam was obtained on the new 1.7 MV tandem ion accelerator. The accelerator, which was obtained by Dennis Whyte from Harvard last year, was refurbished and recommissioned by Research Engineer Pete Stahle and MIT graduate student Harold Barnard. The beam facility is located in an MIT Nuclear Science and Engineering Laboratory, adjacent to Alcator, and will provide extensive capabilities for surface analysis of plasma-facing components from C-Mod, including an external beam which will enable surface analysis of large components. ICRF Systems ------------- We modified the E antenna transmission line network to accomodate the FFT voltage limit of 34 kV. This was accomplished by installing a fixed stub between the FFTs and the antenna to lower the voltage in the FFTs. As expected, the maximum voltage in the FFTs is ~33% lower than in the transmission line before the pre-matching stub. For test discharges, the voltage in the line before the pre-matching is 20 kV and the FFT voltage is ~14 kV. This change however appears to have modified the tuning feedback in an unexpected manner and requires some further testing. The transmitters and antennas were run in support of experiments three days last week. Part of one run was impacted when the FMIT#4 grid supply faulted and required servicing. Lower Hybrid System -------------------- Design work on the new four-way splitter is proceeding. Design of a windowless prototype, to be used for testing, has been completed and procurement for fabrication has begun. Testing of the new circulators continued. Low-power (network analyzer) tests were performed on five of the units. Results were close to those reported by the vendor, and are within acceptable ranges. The vendor reports that the processing of the klystron currently under refurbishment is going well. 250kW of RF output power for short pulses (2-3 msec) have been achieved at 50kV beam voltage, indicating that we are on track for the scheduled acceptance testing this week, to be witnessed by MIT engineers. The lower hybrid transmitter system was successfully used in support of experiments on Tuesday and Thursday last week. Diagnostics ----------- The Beam Emission Spectroscopy (BES) diagnostic is being prepared for DNB width measurements just inside the separatrix during plasma operation and for DNB attenuation measurements during beam operation. The data is needed to improve measurements of boron density. The BES data acquisition system was resassembled and tested by Igor Bespamyatnov and Bill Rowan (UT-FRC). The correlation between fiber location in the BES/MSE dissector and location observed in the plasma changed following modifications to the MSE optics, as expected. New BES fiber locations were selected, and the fibers moved accordingly. Travel and Visitors -------------------- MIT scientists and engineers in the RF Group met with a vendor's representative for discussions of high power tube life and performance issues. A meeting was also held with the manufacturer of our fast ferrite tuner system concerning development of the next version of the FFT to be applied at C-Mod. Bruce Lipschultz presided at the 10th ITPA SOL/divertor meeting in Avila Spain January 7-10. He also presented 4 talks at the meeting - 'Divertor characteristics puzzles', 'Enhanced sheaths during ICRF', 'Projection of ITER wall fluxes', and 'A new surface ion beam diagnostic' (the last on behalf of D. Whyte). Following the ITPA meeting he traveled to Garching and on January 11 presented a joint ASDEX-Upgrade & materials science division seminar on 'D retention in a high-Z tokamak, Alcator C-Mod'. Following the talk he spent the day in discussions with the materials science group headed by J. Roth on the C-Mod and ion-beam results. Lastly, Bruce Lipschultz traveled to the FOM Institute at Rijnhuizen to be part of their scientific advisory committee meeting January 14-15. Jim Terry was at General Atomics in San Diego last week to participate in the DIII-D Program Advisory Committee meeting. Brian Labombard attended the ITPA meeting on SOL/divertor physics and a meeting of the international program committee for the upcoming Plasma-Surface Interaction Conference (May 2008). Both meetings were held in Avila, Spain. Leonid Zakharov (PPPL) visited the PSFC on Wednesday and Thursday last week. He met with Bob Granetz, Brian LaBombard, and Steve Wolfe to discuss measurements related to his model of halo currents and kink modes during vertical disruption events, and related issues. Perry Phillips (UT-FRC) was at MIT last week to work on repair of the FRCECE diagnostic. The high frequency (core channel) module was removed and the components were shipped to UT in Austin. The local oscillator appears to be working properly. The next step in the repair, testing the RF gain using a noise source, is in progress. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly