Alcator C-Mod Weekly Highlights October 1, 2012 FY2012 weeks of research operations Target: 18 weeks Completed: 19.0 weeks Plasma Shots: 2629 Research ___________ On Tuesday, MIT Physics Department graduate student, Yunxing Ma, successfully defended his PhD thesis, entitled "Study of H-mode access conditions on the Alcator C-Mod tokamak". His work focuses on the influence of local and global plasma parameters, and well as effects of topology, on the H-mode threshold, and provides experimental tests of several theories/models for the transition physics. Operations ----------- Plasma operations continued at Alcator C-Mod last week. Four run days were scheduled and two were completed. A total of 58 plasma discharges were produced with a reliability of 95%. The runs supported research in the Transport Physics, Pedestal Physics, ICRF Physics topical areas, as well as diagnostic calibration. A fresh boronization was carried out over Tuesday night. Scheduled runs on Friday and Saturday were canceled in order to modify the configuration of the matching network for the field-aligned ICRF antenna for 50MHz operation. Plasma operations are planned to continue this week. Data System ------------ Last Monday, some spectroscopy, magnetics, and plasma control setup data from the runs between Sept. 18 and Sept. 21 week were lost due to a problem with the backup server and the data archiving scripts. Redundant information from the plasma control system was used to replace some of the magnetics data. The HIREX-SR, IR cameras and fast camera data were restored from the saved raw data files. The archiving scripts have been updated to prevent this problem from recurring. Operation Details ----------------- The experiment on Wednesday morning supported MP#719, "Shoelace antenna operation in EDA H-mode plasmas", which comprises part of the thesis research of an MIT graduate student. The primary goal was to achieve a steady EDA H-mode, and exercise the phase lock system, including a new phase delay board, in order to drive the shoelace antenna synchronously with the Quasi-Coherent Mode (QCM). Upper single null ohmic H-mode plasmas were employed for these experiments. H-modes were achieved throughout the run, but none had strong enough QCM activity on which to phase lock. Effects of the Shoelace frequency modulation waveform appeared regularly on density fluctuations as measured by both PCI and Polarimetry diagnostics. This observation reproduces results from an earlier experiment in normal field, lower null H-mode plasmas. The previous half-day run in reversed field, which did not benefit from boronization and which did not have long-lived H-modes, did not exhibit density fluctuations driven by the antenna. Two hours on Wednesday were devoted to MP#702, "Cold pulse modulation experiments for the study of non-local heat transport and correlation with rotation", which supports the thesis research of an MIT graduate student. The goal of this run was to reproduce data from the previous week's experiment, some of which had been lost due to the data archive malfunction on Monday. Data were obtained in the Saturated Ohmic Confinement (SOC) regime at 800kA, and in the Linear Ohmic Confinement (LOC) regime at 1.1MA. Also on Wednesday, we performed a series of beam-into-gas with fields calibration experiments in support of the MSE diagnostic, MP#647, "Evaluation of MSE issues". These data will support the thesis research of an MIT graduate student. The goal was to judge how the MSE signal statistics scale with beam pulse length and signal level. Previous analysis of beam into plasma data showed a larger than expected variability in the measured polarization level, and a beam pulse to beam pulse dependence. Despite difficulties with the DNB performance, we were able to obtain relevant data to address these issues. Thursday's run was dedicated to MP#721, "Study of ICRF mode conversion flow drive and NTMs in high performance I-mode plasmas". This experiment was carried out with the J-port (field-aligned) antenna operating at 50MHz in order to access the mode conversion heating and mode conversion flow drive (MCFD) scenarios in D+He^3 plasmas. The D- and E-port dipole antennas, operating at f~80MHz, provided additional heating using the standard hydrogen minority heating scenario. It was determined that the J-port antenna power was limited to < 1.6MW, and the phase control of the four straps was not performing as expected. It was subsequently determined that these limitations were at least partly due to a mis-configuration of the matching network. The run focused on study of NTM's in I-mode discharges with total RF power up to ~4MW. We were able to extend our observations of NTM occurrence, and their dependence on beta, collisionality, and sawtooth size and frequency. These data are being analyzed. ICRF Systems ------------ The main effort last week was to operate the Field Aligned antenna at 50 MHz for the first time. The FMIT#3 and #4 transmitters were re-tuned and operated to ~2 MW each into dummy loads. After initial operation into plasma, it was determined that the coaxial transmission line network was incorrectly configured. This issue caused an error in the relative phasin of the antenna straps, resulting in degraded antenna performance (reduced heating and high voltages for given input power). Upon inspection, arcing was found in the feed lines. The lines were cleaned, reassembled, and checked for proper phasing, and will be ready for operation this week. In support of the construction of the ARRA-funded second FA antenna, the boron carbide washers and Faraday screen rods have been delivered and inspected. The boron carbide washers will go next to be copper coated (PVD) to ensure low contact resistance and the FS rods will be marked and polished in preparation for threading. Manufacturing bids for the large back plate and antenna box components have been received and a purchase order has been prepared. Lower Hybrid System ------------------- The Collector Overtemperature Sensor (COTS) fast thermocouple transmitter boxes and cables were completed this week. The fast thermocouple sensors were epoxied into the electrical isolation sleeves. These sensors will be installed on all klystron coolant outlets after the end of the run campaign. Diagnostic Neutral Beam Systems ------------------------------- The DNB performance continued to degrade during operation last week. Four (of eight) high voltage inverter circuits are non-functional during beam operation. All eight inverter modules function properly, along with their output filter modules, when tested independently. This result suggests that the fault is related to secondary transformer circuitry inside the HV transformer tank. Since it is not feasible to fix these circuits in the short time remaining in the present operating period, we are continuing to operate the beam using four inverters at reduced power. Diagnostics ----------- A presentation was given at the weekly C-Mod science meeting describing generalization of the GENIE spectroscopy viewer interface to support arbitrary spectrometers. This update adds support for the chromex spectrometer to the viewer. In addition, this viewer and the other GENIE codes have been deployed and configured for use at DIII-D and NSTX. First data from a set of new filters for the LLNL camera has imaged continuum emission spectrally close to line emission from eroded neutral molybdenum. These images confirm that most of the light observed during the limited phase of startup and at the outer strike point during diverted phases is Mo I line emission, and that localized hot spots occur on limiter surfaces during ICRH operation. The new single channel MSE real-time background subtraction polychromator has been operated with all four detectors for the first time. Studies were done in L-, H- and I-modes at a range of densities and powers and with various impurities. Two spectral regions which correlate very well with the MSE background polarization were found. Preliminary analysis indicates the system will perform well in a wide range of plasmas using polarized wavelength interpolation. Last week the AGNOSTIC team identified and repaired a problem in their RF system which had resulted in reduction of the RFQ beam energy to only ~150keV (1/6 of the design energy). After resolving this issue, the beam was operated at ~750keV, which is sufficient to enable the diagnostic measurements. An extensive set of measurements was then undertaken to ensure beam focusing and alignment, resulting in a highly Gaussian beam spot ~6mm in diameter and 0.5 micro-amp average current being injected into the C-Mod vacuum vessel. During the last week, several successful AGNOSTIC runs were conducted at night, resulting in strong detection of RFQ beam-induced gammas and neutrons from inner wall PFC tiles. The TF coils were used to steer the deuteron beam to three locations on the inner wall: near the midplane, just above the skirt, and just above the inner divertor nose. Changes in detector counts rates indicate substantial changes in PFC surface composition at these three location. Overall, ~25 GB of data were collected last week, and further data analysis (quantified count rates, spectra production, surface composition estimates, comparison to synthetic diagnostic) is currently underway. Travel and Visitors -------------------- Matt Reinke, an ORISE post-doctoral fellow, spent the last two weeks in Europe presenting Alcator C-Mod results to a variety of groups. At JET, he met with a number of people to discuss low-Z impurity seeding operations and high-Z impurity transport, and gave two seminars to the E1/E2 Task Force meeting on September 20th. One discussed the results of C-Mod's experience with impurity seeding in H-modes while the other discussed the poloidal variation of high-Z impurities. Both topics received great interest since JET's ITER-like wall campaign features a tungsten divertor. Dr. Reinke presented an overview of C-Mod work on impurity spectroscopy and transport to the ADAS Workshop in Cadarache on September 24th and discussed the possibility of integrating C-Mod's X-ray imaging spectroscopy tools into ADAS. Matt stayed on at Cadarache to meet with the Tore Supra and ITER diagnostics groups, where he presented the development of the HIREX-SR analysis code. C-Mod's results on high-Z impurity asymmetries were also presented to the Tore Supra group which is currently developing the WEST project as an upgrade to Tore Supra; WEST plans to feature a full tungsten divertor. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly