Alcator C-Mod Weekly Highlights Oct 20, 2003 Startup plasma operations continued at Alcator C-Mod last week, with three run days scheduled and completed. Work also continued on Lower Hybrid and ICRF Systems; diagnostic systems are being brought back on-line. Operations are scheduled to continue this week. Operations ---------- A total of 48 plasma discharges were produced last week, with a startup reliability of 52%. The main focus of the operation was tokamak clean-up and wall conditioning. Most of the plasmas produced were in standard lower null equilibria at Ip=800kA. In addition, a number of upper null discharges were produced in order to begin conditioning of the upper strike point regions in preparation for upper and near-double null experiments planned for this campaign. The RF Group made good progress in conditioning of the D- and E-port antennas. The H/D ratio is in the 10-15% range, and needs to be reduced further by about a factor of two for efficient H-minority heating. Physics ------- The latest passive MHD spectroscopy results on C-Mod show that in addition to the usual Toroidal Alfven Eigenmodes, we now observe Alfven Cascades (AC's) in the current rise with strong ICRF heating as well as high frequency modes in flattop EDA H-mode discharges centered on the gap of the Ellipticity-induced Alfven Eigenmode (EAE) frequency. Such Alfven Cascades are observed in JT-60U and JET at the onset of ITB's when they have reversed shear q profiles. Modeling with the MISHKA code through our JET collaboration indicates that the q profile at the time of the Alfven Cascades has slightly reversed or very flat shear with a minimum q value of 3 at the minimum AC frequency. By fitting the frequency evolution of the AC's, it is possible to model the evolution of the q profile and show that the q profile remains flat or slightly reversed to a minimum q value of 2.5 as the next n=2 AC appears. Then, the modes evolve into TAE's indicating that the q profile becomes monotonic. In a few relatively low density (2 x 10^20 m^-3) EDA H-modes a series of modes are observed with frequencies from 700 kHz to 1100 kHz centered on the EAE frequency gap. The surprising feature of these modes is that they rotate in the electron diamagnetic drift direction, which is opposite to what would be expected for EAE modes with core ICRF heating. If the fast ion profile were hollow, it would be possible for EAE's to rotate in the electron direction. Ron Bravenec (U. Texas) has been running the gyrokinetic turbulence codes GS2 and GYRO in an "apples-to-apples" comparison (equivalent input parameters, same flux-tube domain). This is in preparation for simulating turbulence and transport in C-Mod. ICRF System ----------- Transmitters 1 and 2 (D- and E-port antennas) operated nominally, with conditioning of the antennas continuing throughout the week. Power levels up to 2MW were coupled to the plasma. Efforts to couple 50 MHz from transmitters 3 and 4 into the J-port antenna were unsuccessful. Extensive troubleshooting of the low level RF drive chain and the instrumentation, and open loop transmitter operation, verified all the power room equipment is operating properly. The problem appears to be in the matching of the antenna to the transmission line. Review and recommendations for a solution are underway. The J-port system will be reconfigured to 78 MHZ on Monday morning. Return to 50 MHZ will occur once a solution to the matching problem is complete and the hardware is available. Lower Hybrid System ------------------- The filament of the s/n 102 rebuilt klystron was operated. The filament voltage and current values were within expected ranges indicating both a good filament and vacuum. The klystron was reinstalled and is ready for testing to take place this week. A statement of work for the final welding of the forward waveguide has been reviewed and approved by both MIT and PPPL. The welding should proceed this week. A statement of work for the brazing of the alumina windows into the titanium couplers is under final review at MIT and will be released this week. Modifications to the couplers, agreed to by both MIT and PPPL at the recent review, are underway at PPPL that will reduce stress on the windows during the brazing process. A final cleaning process of the couplers to remove any braze material or alumina residue is also underway. Diagnostics ----------- The NSTX Imaging X-ray Crystal Spectrometer was operational during C-Mod plasma operations last week, and improved performance relative to last summer's results were observed. The instrumental improvements made so far are (1) installation of a new spherically bent crystal from Inrad Corporation, (2) installation of brass apertures before and after the crystal to reduce background due to X-rays scattered from the crystal holder, etc., and (3) preliminary lead shielding around the detector. The instrument is presently still equipped with the Korean detector, so the improved resolution is primarily due to the new crystal. The Brookhaven detector, which has a factor of 10 better spatial resolution (0.1 mm FWHM vs ~1mm), and significantly improved uniformity, is expected to be installed shortly. DNB System ----------- Beam conditioning continued during C-Mod operation last week. Matt Sampsell and Marco Valisa finished taking the data for several new tests of beam performance, including studies of beam divergence, equilibrium, and component optimization. Other studies are ongoing. Collimation of the BES filters was completed and calibration of the spectrometer was begun. A broken BES detector was replaced. Work on porting and debugging the beam penetration code to the Linux platform was begun. Travel and Visitors ------------------- Manfred Bitter and Ken Hill were at MIT 10/14-17 and made progress with the NSTX imaging X-ray crystal spectrometer on C-Mod. Matt Sampsell (U. Texas) continued a 2+ week visit to C-Mod; he participated in work on the BES diagnostic, characterization of DNB parameters, and DNB operation. Collaborating with several of the co-authors, he also finished a second draft of his APS Oral presentation "A Multi-Diagnostic Study of the QC Mode on Alcator C-Mod". Bill Beck, Bill Burke, and David Terry attended THE 20th IEEE/NPSS SOFE MEETING in San Diego last week. Bill Beck presented a poster on "ICRF Antenna Modifications on Alcator C-Mod". Bill Burke presented posters on "Broadband Amplifiers for the Active MHD Diagnostic on Alcator C-Mod", and "Serial Fiber Optic Links for the Lower Hybrid Current Drive Control System on Alcator C-Mod". Dave Terry's poster was on "Lower Hybrid Low Power Microwave Active Control System Design, Installation, and Testing on Alcator C-Mod". Two other C-Mod posters, "Alcator C-Mod Non-Axisymmetric Control Coils Design and Fabrication" by R. Vieira, et al. and "C-Mod Cryopump Design Evaluation" by J. Zaks, et al., were also presented.