Alcator C-Mod Weekly Highlights Feb 2, 2004 Research operations continued at Alcator C-Mod last week. Two run days, out of three scheduled, were completed, supporting experiments in the Transport, RF Physics and Burning Plasma Support programs. Progress on the ICRF and LH systems continued. This week is a scheduled maintenance week; no plasma runs are planned. Operations ---------- A total of 42 plasmas were produced in half-day runs on Tuesday and Wednesday and a full run day on Thursday. Recovery from a small air leak on Tuesday precluded scheduled operation on Tuesday afternoon and Wednesday morning. The vacuum status had been restored to near-nominal conditions by Wednesday afternoon, following overnight ECDC and bakeout. Tuesday's run continued MP#362, "Equilibrium and Control Development for ITER-shaped plasmas at q95=3 and I_N=1.4". These equilibria, with elongation of 1.85, are challenging due to susceptibility to both vertical instability and locked modes. Tuesday's experiments, which were cut short by the onset of an air leak following a high current disruption, indicated that previous speculation that the observed locked modes were primarily due to an uncompensated m=1/n=1 component of the intrinsic error-field in C-Mod was probably not correct. Rather, the newer results suggest that operation with edge q just below 3 may be responsible for the locked mode behavior. Future experiments in this line will plan to concentrate on optimizing feedback stabilization of the vertical instability for the ITER shape at higher density and with q values above 3. The half-run on Wednesday was devoted to MP#381, which covers equilibrium development for a future ITPA Joint Experiment on EDA H-mode and related phenomena with JFT2-M. Development of a C-Mod equilibrium with the triangularity and elongation of the JFT2-M target plasma, but at the standard C-Mod aspect ratio of 3.1, was successfully completed. Good progress was also made on producing a similar shape with A approaching 5, which is the normal value for JFT2-M. Thursday's run was devoted to MP#360 "Initial test of a load tolerant configuration on E-Antenna", from the RF Physics Group. This experiment supports our Level 1 (JOULE) Milestone SC6-1e, "Design advanced, high-power, load tolerant ICRF antenna for C-Mod." A network designed to provide passive load tolerance was installed at the E-port dipole antenna. The load tolerant matching network attempts to add two lines (connected to separate antenna elements) with conjugate impedances at a tee (so-called conjugate tee). The impedance of each line is adjusted by a prematching network. Using D-port as a reference antenna, we compared the plasma repsonse to D and E-port antennas for standard C-Mod discharge: 5.4 T, Ip=1 MA, and D(H) minority heating scenario. Although the load tolerant behavior was not realized, we were able to investigate the system thoroughly. Initial analysis indicates that one of the key underlying assumptions regarding the load tolerant configuration, that each antenna element acts independently of the others, is violated. The data indicate the straps were more tightly coupled in the presence of plasma than in vacuum. Parameter scans of plasma current, density, and gap suggests this coupling is fairly insensitive to plasma conditions and may allow for a load tolerant configuration to be developed if the coupling can be measured or modeled. Future measurements of the S matrix of the antennas with plasma present would provide the necessary information. Even with strong current imbalance in the E-port antenna, the plasma response was similar to that of D-port antenna in L-mode up to 1 MW of power. Computer Infrastructure ----------------------- Over the last two weeks the main C-Mod data acquisition, data archive, user file, and mail servers were replaced. Two new dual Xeon servers were installed, one to handle data acquisition and data serving tasks, the other to provide file service for user home directories. These new servers have sufficient capacity that either could operate all of our critical computer functions in the event that one of the two goes down. Mail service was separated out, and moved to the original data acquisition computer. The operating systems on all of our linux workstations have been upgraded. Physics/Collaborations ----------------------- On Saturday, 26 January, JET ran a C-Mod similarity experiment under the command of Geoff Cordey to attempt to distinguish the scaling of confinement with collisionality or density normalized to the Greenwald limit. They attempted to match as closely as possible the C-Mod shape with dimensionless parameters based on C-Mod shot 1001018013, which was a relatively high density high beta discharge. The preliminary results appear to disagree with the ITER scaling law IPB98(y,2) in that Btau scaled proportional to nu*^-0.35 but agrees with the dimensionless parameters including nu* rather than n/nG. However, care must be taken regarding the absorbed power in C-Mod so further analysis is required to clarify the dependence of the confinement on collisionality. Cordey has proposed that C-Mod also try to provide a better match to the JET results by running a similar discharge at higher density. On Jan 29, under the direction of Tim Hender, JET continued its portion of the ITPA Joint Experiment on error field identity comparison with C-Mod. Most of the required JET pulses, including density scans at q=3.2 and 4.2 and a higher BT case corresponding to 8 tesla operation in C-Mod, are now in hand. Analysis of these shots is proceeding, and the remaining required JET pulses are expected to be carried out over the next week. The remaining C-Mod experiments will be scheduled soon. Additional experiments are planned to be carried out by DIII-D. The objective is to clarify the size and Bt scalings of the locked mode threshold under conditions of relevance to ITER. Perry Phillips and Alan Lynn of UT-FRC are using ray tracing studies to investigate the degree of spatial localization that is available in the measurement of density fluctuations with ECE. These studies also give some insight into the sensitivity of the measurement to the density profile. Ron Bravenec of UT-FRC has analyzed recent beam width measurements using the BES optics. The new measurements are consistent with data taken as far back as August 2002. This indicates that the beam width has been quite consistent. The measurements were made as a function of beam current and show the expected perveance effects. Bill Rowan of UT-FRC continued his analysis of the high resolution MSE spectra taken in mid-January. These were compared with the model used in the analysis of the MSE data. Both component mix and aperture broadening were important to the comparison. With regard to the critical pi components used for the current profile measurement, the high resolution spectra are in good agreement with the model. Other portions of the spectrum are in qualitative agreement. Bill and Ron Bravenec are working with Steve Scott (PPPL) and Howard Yuh to resolve the remaining, modest discrepancies. ICRF Systems ------------ Monday and Friday were scheduled as maintenance days, with emphasis on activities related to repair and modification of the FMIT#4 transmitter. Teams also continued maintenance on this system during off-shift hours. The FMIT#4 breaker has been inspected and shown to operate properly. Additional functional tests and modification of switchgear indication circuitry need to be performed. The wiring and logic have been traced in both the breaker and crowbar. Updated drawings are being generated. The reassembly and replacement of wiring has begun in the crowbar cabinet. The repair activities are planned to continue this week. Lower Hybrid System -------------------- Work continued at MIT on the Lower Hybrid launcher assembly. This work will continue this week, along with the start of thermocouple and heater wiring installation. Stainless-steel shims have been prepared at PPPL for use in coupling the Forward Wave Guide to the Rear Wave Guide assembly. All spare three-channel titanium coupler prototype samples will be sent to MIT for ongoing brazing tests. A very successful braze test of a copper plated titanium plate to one of the metalized alumina windows was performed last week. Follow up tests using a three-window prototype are planned for this week. Progress continued on installation of the circulators. All circulators have been mounted. All of the circulator waveguide connections are made, with the exception of some dummy loads. All of the water connections have been made, and have been pressure and flow tested with no leaks. All of the circulators have been pressure tested. High power testing of the first circulator is planned for this week; a representative of the supplier is expected to be present for this test. Travel and Visitors ------------------- Joe Frangipani and Dave Miller (PPPL) spent the week at MIT working on the LH launcher assembly.