Alcator C-Mod Weekly Highlights June 18, 2007 FY2007 weeks of research operations: tentative target: 15 weeks Completed: 7.57 weeks Operations ---------- Four run days were scheduled and successfully completed last week with a plasma startup reliability of 80%. On Tuesday between shot boronization was run and an optimal location for the boronization scan was determined while using the EF4 coil to add vertical field. On Wednesday lower hybrid current drive was investigated as a function of power, and the first MSE measurements of current profile changes were obtained. On Thursday the effect of the upper divertor cryopump on H-modes in various plasma geometries was studied. On Friday both beam into gas and into plasmas was continued as part of the MSE calibration effort. On Wednesday evening we successfully performed our 3rd full boronization of this campaign, again using the EF4 magnet to provide vertical field. Results from Tuesday's run allowed us to optimize this boronization. Operation Details ------------------ On Tuesday MP 430 "Between shot boronization" was run. The goal was to determine the optimal boronization resonance location with the newly implemented vertical field. All boronizations lasted 10 minutes with the resonance at a fixed radius. As recently measured, using the Surface Science Station (S^3) diagnostic, the deposition profile is of order 10-15 cm wide. Each boronization was followed by at least two plasma discharges to ensure that the boronization had been completely depleted before the next boronization. The location found for best effect on the following discharge was at a boronization resonance position of 80 cm. Note that there are two resonances; the electron cyclotron resonance, as well as the upper hybrid resonance, which is 8 cm further out, and corresponds to a higher plasma density. The inner-most major radius of the outboard limiters is 82 cm and the mid-plane radius of the limiters is 90.5 cm. So the optimal deposition region, which extends at least from 80 - 90 cm, corresponds to boronizing the outer limiters. The width of this region is slightly less than 20 cm, which is consistent with what was measured by S^3. Note also that the S^3 results show the deposition extends up to 20 cm outward in major radius from the electron cyclotron resonance location. We will continue to refine these results, and apply them to our overnight full boronization procedures. On Wednesday MP 473 was run to investigate current drive and increase the power delivered to the plasma. Discharges with 90 deg coupler phasing showed both an ~0.5 V reduction in loop voltage and measurable current density profile change on the MSE diagnostic. Sustained power levels of up to 725 kW into the plasma were obtained. Another run to continue bringing up power levels and investigate other phasing configurations is planned for this week. Thursday's run, MP 485, was an attempt to combine H-mode operation with upper chamber cryopump operation, with the goal of documenting H-mode density control and pedestal profile response. A number of discharges exploited dynamic topology scans, which allowed us to transition to H-modes from a slightly lower single null (LSN) configuration, in which H-mode access is typically easier, to the upper single null (USN) configuration, where pumping is strongest. We achieved a number of these swept-null discharges at currents ranging from 1.0 MA to 0.6 MA, with and without cryopumping. Plasma fueling and pedestal response will require detailed analysis, but there are clear indications of pumping on the unconfined edge plasma, as upper divertor probes and the mid-plane gauge both saw the pressure drop by a factor of about two in pumped vs. un-pumped cases. We also see strong indications that ICRF performance can be improved by maintaining wall pressure below empirical neutral pressure limits experienced by the antennas. H-modes were also initiated in static USN topology, with a higher power threshold, as expected. As in our prior experience, USN H-modes were less collisional than their LSN counterparts. USN shots at 1 MA demonstrated that pumping could be used to bring down the pedestal density in H-mode. This should be pursued further at lower values of plasma current, which should provide easier access to steady EDA H-mode. Plasma performance was quite good on this day. With the combination of a fresh, well-optimized boronization and DN/USN geometries, we obtained very good confinement metrics, with high plasma temperatures, both with and without active pumping. Overall, substantial progress was made in developing attractive target discharges for low-collisionality H-mode experiments. On Friday MP#491 was run to continue calibration of the MSE diagnostic. This run is composed of two parts: (1) beam into a helium filled torus and (2) beam into plasma with Ip ramping during the discharge. The shots from Part (1) are used to calibrate the MSE diagnostic with pitch angles over the range of -10 to +5 degrees for all 10 channels. Some of these beam-into-helium gas shots are also used to study the dependences on neutral gas and the reproducibility of the measured pitch angles. The shots from Part (2) are used to calibrate the edge channel and some of the internal channels based on EFIT-calculated edge pitch angles and sawtooth-inversion radii, respectively. With two second plasma discharges, we were able to have Ip ramp from 0.8MA @ 1 sec to 0.1 MA @ 1.9 sec during the Bt flat-top, providing the pitch angle range -10 to -1 degrees for the edge channel. We also ran longer pulse plasmas to obtain a higher pitch angle range. We obtained Ip ramps from 1MA @ 1 sec to 0.4MA @ 2.3 sec, resulting in a pitch angle range from -13 to -5 degrees. In addition, we had an Ohmic-H mode phase from 1.8 to 2.1 sec in one discharge, resulting in an important data set regarding the recent observation that MSE signals can be contaminated by background partially polarized H-alpha light. This non-ICRF-driven H-mode confirms the correlation between the MSE background and the H-alpha signals seen in recent lower hybrid driven H-mode discharges. Lower Hybrid System -------------------- Work continued on design of the 2nd lower hybrid launcher, 4-way splitter design and prototype, and a new set of couplers for the 1st lower hybrid launcher. Travel and Visits ----------------- Paul Bonoli attended the PSACI Program Advisory Committee Meeting at PPPL on June 7-8, 2007. He gave a talk to the Advisory Committee on behalf of the SciDAC Center for Simulation of Wave-Plasma Interactions. Miklos Porkolab also attended the PSACI and is now serving on the PAC for the PSACI. He also attended the symposium "Recent Advances in Plasma Physics," honoring Ron Davidson at the Princeton Plasma Physics laboratory on June 11-12, and gave a presentation in the formal session, with the title: "Plasma Science and Fusion Research at the PSFC: a Historical Perspective." Tom Fredian and Josh Stillerman attended the 6th IAEA Technical Meeting (IAEA-TM) on Control, Data Acquisition, and Remote Participation for Fusion Research held in Inuyama, Japan last week. Josh was a member of the International Program Advisory Committee for the meeting and chaired one of the sessions. Tom Fredian presented a paper on Long Pulse extensions to MDSplus, and Josh Stillerman presented a paper on collaboration technology. Josh also presented a paper for Yijun Lin on "Digital real-time control for an ICRF fast ferrite tuning system on Alcator C-Mod." Tom and Josh chaired a two hour satellite MDSplus workshop at the meeting. Piero Martin, U. of Padua, visited Joe Snipes and Nick Siefert last week to discuss Alfven eigenmodes and the Fast Ion Loss Diagnostic on ASDEX-Upgrade. Leonid Dorf, LANL, worked with Jim Terry last week to install the LANL infrared spectroscopy diagnostic which views the lower divertor region. Ajai Kumar, head of laser diagnostics at the Institute for Plasma Research in Gujarat, India, visited last week. He was here to discuss applications of laser-based diagnostics with C-Mod scientists. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly