Alcator C-Mod Weekly Highlights May 23th, 2011 FY2011 weeks of research operations Target: 15 weeks Completed: 14.5 weeks Plasma Shots: 1706 Physics ------- A new paper by Amanda Hubbard et al., "Edge energy transport barrier and turbulence in the I-mode regime on Alcator C-Mod" has been published online in Physics of Plasmas (Vol.18, Issue 5): http://link.aip.org/link/?PHP/18/056115 In an expansion of her APS invited talk, the paper reports extended studies of the I-mode regime obtained in the Alcator C-Mod tokamak. This regime, usually accessed with unfavorable ion B X Grad(B) drift, features an edge thermal transport barrier without a strong particle transport barrier. Steady I-modes have now been obtained with favorable B X Grad(B) drift, by using specific plasma shapes, as well as with unfavorable drift over a wider range of shapes and plasma parameters. With favorable drift, power thresholds are close to the standard scaling for L–H transitions, while with unfavorable drift they are 1.5–3 times higher, increasing with Ip. Global energy confinement in both drift configurations is comparable to H-mode scalings, while density profiles and impurity confinement are close to those in L-mode. Transport analysis of the edge region shows a decrease in edge χeff, by typically a factor of 3, between L- and I-mode. The decrease correlates with a drop in mid-frequency fluctuations (f~50–150 kHz) observed on both density and magnetics diagnostics. Edge fluctuations at higher frequencies often increase above L-mode levels, peaking at f~250 kHz. This weakly coherent mode is clearest and has narrowest width (df/f~0.45) at low q95 and high Tped, up to 1 keV. The Er well in I-mode is intermediate between L- and H-mode and is dominated by the diamagnetic contribution in the impurity radial force balance, without the Vpol shear typical of H-modes. The paper "Comparison of small ELM characteristics and regimes in Alcator C-Mod, MAST, and NSTX" by R. Maingi (ORNL), A.E. Hubbard (MIT), H. Meyer (Culham, UK), et. al., was published in Nucl. Fusion 51 (2011) 063036. This paper documents the results of an ITPA-coordinated experiment on small ELM regimes, i.e. those with negligible drops in stored energy associated with individual ELMs. A common small ELM regime was found and discussed in all three devices, with a common operating point of pedestal beta normalized to the poloidal field being ~ 10-15%. However the upper and lower operating boundaries in beta ped_poloidal of these ELMs differed in the three devices. Based on the observed characteristics, the ELMs were classified as Type II. In NSTX, this Type II regime was differentiated from the more common Type V ELM regime. (information supplied by R. Maingi) Raytracing/Fokker-Planck calculations have been shown to fail to model Lower Hybrid RF experiments having low n//. In particular, the simulated hard x-ray (HXR) profiles tend to be very peaked on axis and significantly overestimate the photon counts in that region. However, the results of a simulation in this regime using the LHEAF fullwave finite element code show agreement between the simulated HXR profiles and the experimental measurements. For the LHEAF simulation a SOL model which maps density and temperature profiles in 2D based on the EFIT equilibrium was used. The development of a more realistic SOL model to address LHRF density limit issues is currently under development. Operations ---------- Power supply, alternator, liquid nitrogen system, Hi-Yard, and general routine maintenance continued last week. A partial list of in-vessel work included removal of limiter thermocouples for refurbishment, and PCI and GPI (gas puff imaging) calibrations. ICRF Systems ------------ Inspection of the rotated antenna assembly to ensure proper RF contacts and fit of the antenna components is underway. Four of the six internal side plates have been modified to allow a 0.5 cm radial clearance to the antenna straps. Modifications of a number of counter bores are also required as a result of the new side plate profile. The long strip line has been brazed and is being polished prior to plating. A new IR cured masking system, needed for copper plating the antenna components, has been reported to work well. The E-beam welding technique for the bellows to the flange has been qualified. Further testing on the breakdown experiment showed that the breakdown is occurring at the edge of the specimen rather than at the location of highest electric field along the ridge line of the sample. A modification to the specimen holder to reduce the electric fields at this location will be made. Lower Hybrid System ------------------- Substantial engineering efforts were dedicated to ferrite tuner and control system ("LH ModBox") tests last week. A larger magnet yoke was fabricated for the ferrite tuner, and the system was tested at low power. The larger yoke makes placement of the ferrite components inside the waveguide less critical and leads to lower overall losses. High power tests of the latest design are scheduled for next week. Tests of the LH ModBox control system were conducted at low and high power. High power tests show overall control performance of ± 0.75 kW at 20 kW output power and ± 0.1 kW at 2 kW output power across a phase range of 360 deg. These tests were conducted at 40 kV beam voltage. A scheduled inspection of LH water system was performed. All four Cla-Val pressure regulating valves required replacement parts. The system is back to normal operation. One of our vendors visited the PSFC to discuss the TPS designs underway and to review TPS system I/O and logic, particularly the watchdog circuits. Connectors were chosen and ordered for the filament power supply system. A method of guiding the power supply boxes along the mounting channels was designed and prototypes were made at the vendor's facility. Diagnostics ----------- A successful internal review of the new TCI Fast electronics was held last week. This new system will provide fluctuation data into the several MHz range in either a local oscillator mode or a differential mode taken between any two of the ten chords. Work continued on the upgrade of the C-Mod Polarimeter from one to three chords. New components are being ordered and a full 3-D model of the three chord system is being developed. Travel and Visitors ------------------- Bill Nelson, U.S. Senator from Florida and former astronaut, visited the PSFC last Monday. He toured the C-Mod facility and discussed our program with Martin Greenwald. Benjamin Downing, a state senator representing Western Massachusetts, visited on Tuesday. He was given a tour of C-Mod by Earl Marmar and Martin Greenwald. Dr. David J. Dean, Senior Advisor to the DOE Under Secretary for Science, visited MIT on Monday. He met with Earl Marmar for a tour of the C-Mod facility and discussion of our research. David Mikkelsen, PPPL, worked with other C-Mod researchers to plan sensitivity studies for GYRO simulations of impurity transport, to begin studies of momentum transport in C-Mod plasmas exhibiting rotation reversal, and to plan further installation of TRANSP utility software at the PSFC to support GYRO simulations based on C-mod experimental data. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly