Alcator C-Mod Weekly Highlights March 10, 2008 FY2008 weeks of research operations: Target: 15 weeks Completed: 7.5 weeks Operations ---------- Research operations continued at Alcator C-Mod last week, with three run days scheduled and completed. A total of 87 plasma discharges were produced with a reliability of 90%. The runs supported experiments by the Lower Hybrid Physics, H-mode Integrated Scenarios, and Boundary Physics groups. Overnight boronization was performed prior to the plasma runs on Tuesday and Thursday. The cryopump was successfully employed for density and particle control. All tokamak subsystems performed nominally last week, with minimal unscheduled delays or interruptions in the experimental schedule. Plasma operations are planned to continue this week. Operation Details ----------------- Tuesday's experiment continued MP#488a, "Comparison of small ELM regimes on C-Mod, MAST and NSTX", which supports the C-Mod portion of ITPA Joint Experiment PEP-16. Rajesh Maingi (NSTX) and Hendrik Meyer (MAST) were at MIT to participate as co-Session Leaders for this run. Previous experiments in January had suggested that the accessibility to small ELMs in C-Mod was extremely sensitive to shaping and plasma current. The first part of this run consisted of systematic scans of lower X-point position and Ip, with the aim of determining the ELM operational window. We achieved a fine scan of X-point positions, decreasing RXL in steps of 2 to 4mm for a total of -1.2cm shift. Contrary to our previous impression, ELMs proved accessible across this range (0.54 < delta_l < 0.61). Variation in the characteristics of the ELMs as a function of triangularity over this range is being analyzed. Based on initial impressions, the most reliable figure of merit for small ELM access in this range of triangularity is simply the elevation of the pedestal Te above ~600eV. A current scan from 700kA to 850kA revealed a fairly narrow window in Ip (q95~4.7 +/- 0.3) for obtaining these ELMs. Large ELMs were also revisited in this run, by running an increased lower triangularity of 0.75 at low elongation. A broad dataset of small ELMs in near-DN shape was obtained, including useful comparative discharges with varied shaping and collisionality, which will help advance our H-mode Baseline Integrated Scenario studies. Wednesday's run was devoted to MP#531 "Scaling of edge flow-shear profiles with plasma current and magnetic field", which has the goal of investigating the sensitivity of edge plasma flows and pressure gradients to changes in plasma current and magnetic field in ohmic L-mode plasmas. In particular, the experiment is designed to investigate the magnitude of the flow velocities associated with the ExB shear layer near the last closed flux surface and to determine whether it varies with q95. Good data were obtained for the full parameter range specified in the experimental plan: (0.4 MA, 3 tesla), (0.4 MA, 6 tesla), (0.8 MA, 3 tesla), and (0.8 MA, 6 tesla) with at least three different plasma densities for each combination. The primary focus was on obtaining high-quality velocity profile data from our three scanning probes (ASP, FSP, WASP) up to the separatrix. A principal point of interest was to see if the jump in poloidal velocity across the shear layer changed with q95 as we changed q95 from ~3 to 9.5. No substantial change in the velocities was evident, based on initial analysis of the data. However, the width of the shear layer does seem to track the width of the pressure profile, becoming narrower with increasing plasma current. This trend seems to reproduce the one noted earlier for ohmic H-modes. Thursday's run supported the H-mode portion of MP#490 "Assessing LHCD in ICRF-heated L and H-modes". The aim was to establish whether, and under what conditions, measurable LHCD can be obtained in H-mode plasmas on C-Mod. The experiment was successful in demonstrating for the first time in C-Mod a clear and positive effect of LHCD into H-mod plasmas. In addition, an unexpected benefit was a clear reduction in pedestal and core density, and increase in temperatures, during LHCD. Most of the experiment was carried out at 600kA with Bt=5.4 T and LH phasing at 90 degrees. The previously developed procedure for obtaining low density H-modes, starting with LSN and transitioning to USN (SSEP~ +5mm) shortly after the L-H transition, was employed to reduce the H-mode density to nebar<1.7e20/m^3. With application of LH at net coupled power levels between 0.7 and 1.1MW we reproducibly observed further reductions in core density (to nebar~1.4e20/m^3) and in pedestal density, a reduction of radiated power, increases in both core and pedestal temperature, increased stored energy, reduction in loop voltage and in internal inductance (li), and increases in hard X-ray emissivity and non-thermal EC emission. These signatures all increased with increasing LH power. Careful analysis will be required to determine how much of the Vloop and li response was due to the density and temperature changes and how much to current drive. Either way, the waves are clearly penetrating the H-mode pedestal and being deposited in the core plasma. ICRF Systems ------------- The Final Power Amplifier (FPA) tube on the FMIT#2 transmitter (E-port) experienced a screen to grid short during operation on Thursday. Further investigation indicated the grid is also shorted to the filament and the tube vacuum is compromised. The FPA tube is being removed and will be replaced with a spare in preparation for operation this week. Long-pulse Diagnostic Neutral Beam System ----------------------------------------- The DNB is being conditioned following completion of repairs to a number of power supply components and associated fiber optic communication links to the 50 kV "Hot Deck". Conditioning pulses have reached 5 Amps at 50V. Diagnostics ------------ The high frequency RF unit used for the inner 16 channels of the FRECE electron cyclotron emission diagnostic has been shipped to University of Texas following repair and test at the vendor. The repairs consisted of rebuilding the Gunn oscillator and retuning the frequency doubler. After burn-in and reassembly, the noise figure was tested and found to be at the original specification of 22db at 3.5Ghz and 20db at 11.5GHz. Following evaluation at Texas, the unit will be reinstalled for operation on C-Mod. Travel and Visitors -------------------- During the last week of February, Robert Granetz attended an ITPA MHD meeting in Naka. The meeting was combined with the IEA Large Tokamak Workshop (concentrating on resistive wall mode physics) and the US/Japan MHD workshop. Last week Stewart Zweben was at MIT to work on the gas puff imaging diagnostic and on a comparison of the GEM turbulence code with data on edge turbulence from MP#429. Rajesh Maingi (ORNL) and Hendrik Meyer (UKAEA) visited MIT on March 4 in order to participate in an ITPA joint experiment between C-Mod, NSTX and MAST. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly