The primary activities at Alcator C-Mod during the second quarter of FY00 were: completion of a maintenance period with in-vessel access; the successful start of the year-2000 run campaign; analysis of data from that campaign; presentation of the C-Mod program to its Program Advisory Committee; participation in the Transport Taskforce Workshop; preparation for the international Plasma Surface Interactions conference; and continued participation in the critical assessment of fusion science. A more detailed account of these activities follows.
 
 

Scientific Results

Core Confinement and Edge Pedestal Results

As part of our investigation of H-mode and H-mode-pedestal physics, we have performed at set of RF power ramp experiments. The RF power has been ramped up and down through the L-H threshold at rates varying from just above 0 to 4 MW/s. During the ramps the edge T_e and n_e diagnostics have monitored the pedestal parameters. Such a ramp is shown in Figure 1, where the previously observed and reported hysteresis of the L-H threshold with power is obvious.

In these studies we find that the T_e pedestal height and width increase with RF power, while the n_e height and width are independent of power. Thus, the temperature and density pedestals can scale differently, as suggested by Diamond and Carreras (e.g. EPS 1999). When the transient response of the edge parameters is examined at the L-H threshold, we find that 1) the T_e and n_e show a clear jump at the transition, 2) the T_e jump does NOT depend systematically on the power ramp rate, 3) the temperature increase begins at the same time (within 1 ms) at all pedestal locations, and 4) the maximum rate of energy increase is roughly consistent with the net power flux across the pedestal, not the incremental flux P_tot-P_threshold. These observations have been incorporated into a simple model by Ben Carreras. Taking

dM/dt = G_in-G_{out={1-DH/DL}} G_in,

where G=heat flux, D_H(L) are the spatially uniform H-mode (L-mode) diffusion coefficients, and M=the surface integral of the pressure. This simple model makes predictions about the rates of rise of pressure which compare well with the measurements. The prompt response near the top of the pedestal is predicted to show a t^0.5 dependence. This is approximately what is observed, as is seen in Figure 2, where the T_e response near the top of the pedestal at the transition is shown.
 

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We have observed the spontaneous formation of an internal transport barrier (ITB) following the so-called "back transition" from H-mode to L-mode. This has been reported previously, but has now been the subject of further study. The existence of the ITB is shown by a large increase in the neutron rate, an increase (10 to 40%) in the central ion temperature (for a constatnt input power), and a peaking of the density profile. The density profile peaking factor increases from ~1 to ~3. This is a result of density in the outer half of the plasma dropping sharply while the central density remains essentially unchanged, as shown in Figure 3.

Thus the inverse density scale length at r/a~0.5 drops significantly, while the electron temperature scale length does not change much. This means that h_e(=[grad Te/Te] / [grad ne/ne] decreases to values between 1 and 2 after the back transition and suggests that h_e(or h_i, which is similar,but not measured in detail) may be an important parameter in the ITB formation. The dependence of the neutron rate increase upon the minimum value of h_eis shown in Figure 4, where two things can be seen: there is a dependence upon the minimum h_e, but it is not the only variable in the ITB formation.

RF Research

Recently, a version of FPPRF was incorporated into the TRANSP analysis code that allows selection of the full-wave ICRF code TORIC as an option in the bounce averaged Fokker Planck module FPPRF. This new option was tested by running TRANSP and FPPRF with the following RF options:

    (i) The old SPRUCE wave code (written by D.N. Smithe)

    (ii) A version of SPRUCE modified by the Belgian RF group to account for higher order FLR effects in the dielectric tensor elements.

    (iii) The TORIC RF module, written by M. Brambilla, which solves explicitly for the RF electric field of both the fast wave and ion Bernstein wave.

Comparisons were carried out on a TFTR Test Case with D(H) minority heating. The results are summarized below:

    (i) The SPRUCE code as modified by the Belgian RF group exhibited unphysical minority absorption off-axis similar to test cases on Alcator C-Mod.

    (ii) The old SPRUCE code gave reasonable results for minority absorption with power deposition profiles peaked on-axis at the (H) resonance layer.

    (iii) The TORIC RF code gave absorption profiles quite similar to the old SPRUCE code, but were slightly narrower and more peaked on-axis.

Based on these test cases it was concluded that the FPPRF module using TORIC as an RF option is working properly. We then proceeded to set up a test case for Alcator C-Mod with the TORIC RF code selected as the RF module. This C-Mod case is now being executed remotely at PPPL.

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We have been exploring high-poloidal-mode-number simulations for D(He3) mode conversion scenarios. Previously, 15 poloidal-mode simulations found excessive minority damping of the mode-converted ion Bernstein wave. Using 127 poloidal modes, the IBW power to electrons increased from 25% to 45% and is within experimental error bars of the measured electron power deposition. The higher-poloidal-mode-number runs allow better resolution of the IBW wave, which is a short wavelength mode. A 255 mode simulation is currently underway.

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One run day during this quarter was devoted to isolating the relative contributions of Molybdenum sources from the inner wall, outer limiter/antennas, and divertor to the core Mo levels. A new feature of these experiments was the use of emissive probes to measure directly the plasma potential during RF operation. The potentials measured by the probes (one on each side of the AB limiter) were found to correlate with operation of the antennas connected to them by field lines. In the case of the A side of the limiter this means the J4 antenna, for the B-side of the limiter this means the D-antenna. Potentials in the range of 50-200 V were observed when the corresponding antenna was energized. The measured potentials verify that sheath rectification does occur, and that the potentials are substantial. The results on the comparison of antenna sources indicate that the Mo source rate and effect on the core by the J antennas are considerably reduced from last run period. However, the source rate from the J antennas and the effect on the core were typically a factor of 2-3 larger than with the D-antenna for equivalent launched power.
 
 

Edge and Divertor Physics

Results from simple 1-D modeling of the neutral behavior in the C-Mod divertor, compared with recent divertor bypass experiments, suggest that a large fraction (~ 50%) of ions that strike the divertor plates recycle directly to the midplane as neutrals via leaks in the outer divertor plate structure (or through the bypass when open). The primary leakage paths with the bypass closed are the open divertor ports. The modeling explains the experimental observation that increasing the present leakage conductance by opening the bypass doesn't change the leakage flux significantly. The resulting drop in divertor gas pressure is also explained by the model.

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Results from an optical diagnostic investigating fluctuations in the C-Mod SOL show no obvious differences in the SOL fluctuation characteristics during EDA H-mode and L-mode plasmas. Frequency spectra at radial locations r=4, 8, and 11 mm are similar, as are the autocorrelation functions of the intensity fluctuations. (r=the distance outside the separatrix at the outer midplane.) In addition, radial profiles of intensity fluctuations have been measured using this diagnostic by scanning the separatrix dynamically during ohmic, L-mode discharges. Inside of r=20 mm the fluctuation level is 10 to 20%, rising slightly at the smaller r's. Outside of r=20 mm, it increases rapidly to ~60%. Since the dependence of the line intensity has a less-than-linear dependence upon n_e, this intensity fluctuation profile implies that the normalized density fluctuation profile decreases (with increasing r) from 40-80% at r's < 10 mm, is ~20-30% for 10<r<20 mm, and then increases sharply beyond r>20 mm. These observations show an opposite trend from the results in which an "effective diffusion coefficient" is derived from determinations of the SOL particle fluxes, gradients, and source functions. The trend in D_eff is one that decreases to quite low values as r decreases from 10 mm to 0 mm (the separatrix).

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The effects of main chamber recycling on H-mode threshold and confinement were investigated in a systematic scan of the inner gap, with the outer gap held constant at a relatively large value of approximately 20 mm. The gap to the inner wall was varied from 0 (marginally limited) to over 25 mm. Plasma current, target density, and overall plasma shape were held constant. The dominant x-point was at the bottom, and the upper, secondary x-point was outside the vessel. During the scan from 26 mm to 3 mm we observed no change in divertor gas pressure, outboard midplane gas pressure, divertor radiation, or Zeff. However, the gas pressure in the upper "divertor" region changed by a factor of about 5 and the H-mode power threshold increased by ~40%; no H-mode was obtained for the marginally limited cases. Detailed analysis of these data is in progress.

Operations and Diagnostics

The Alcator C-Mod up-to-air period ended on February 17 with pumpdown of the vessel. A partial list of invessel work during this up-to-air period includes:
 

- addition of a visible TV view of the entire J-port antenna

- new flux probes on two limiters

- new emissive probes on one limiter

- improved TTCI mirror housing

- neutral pressure gauges (5 locations) near the plasma

- refurbished (cleaned) D and E antenna

- designed, fabricated, and installed boron nitride tiles for E antenna

- added 3 additional retro-reflectors for inner wall rangefinder measurements

- added MSE shutter (PPPL and Texas)

- refurbished mirror and lens supports for MSE (PPPL and Texas)

- added 2nd set of fibers for tangential edge CXRS (Texas)

- added edge poloidal view for CXRS

- refurbished mirror assembly for McPherson spectrometer

- modified J port antenna to eliminate impurity injections (with PPPL)

- added 2nd Lyman alpha view (inner wall)

- added protective cover over H-port flux loop

In addition, we performed routine maintenance on:
 

- tiles

- telescopes and periscopes

- limiters and protection tiles

- all flappers

- various gate valves

- many system pumps

- invessel thermocouples

We also
 

- cleaned and calibrated fibers for core CXRS (Texas)

- calibrated views and refurbished fiber optics for several invessel systems

- completed the neutron calibration

- calibrated the reflectometer

- calibrated Z meter array alignment

- aligned TTCI

- did complete wipe down of vessel walls to remove old boron deposits and associated compounds.

After vessel pumpdown and leak checking, the high-temperature bakeout (100-110 degrees C) commenced on Feb. 26 and lasted for nine days. After the bake, machine pressure was 1.0e-8 Torr at a wall temperature of 35 C. A two-week period of around-the-clock ECDC discharge cleaning followed.

Tokamak plasma operation resumed March 28. The initial runs were devoted to machine startup and conditioning activities in a "reverse-field" configuration, i.e. with the toroidal field in a direction such that the ion grad-B drift is away from the lower divertor. (In C-Mod the current direction reverses with the field.) This was done in order to minimize time devoted to "reverse-field" operation later in the campaign. Operationally, the machine performance was similar to previous startup periods. Breakdown was readily obtained. The main difficulty was in machine cleanup (conditioning) which requires a significant number of tokamak discharges. Full length (>1.6 sec) discharges at 5.4T and 800kA were produced. After four days of running, the walls were still not fully conditioned, as evidenced by the rate of gas evolution from the walls, the high H/D ratio, and the relatively high loop voltage.

After the first week of "reverse-field" operation, we returned to a "normal" field configuration. Again plasma operation was readily obtained, although conditioning was still incomplete. High disruptivity was experienced except at low density (n_ebar<1.e20), and numerous molybdenum injections were observed on the bolometer, spectroscopic, and TV diagnostics. During machine startup this behavior is not atypical.

The primary goals of the startup phase of the present run campaign, i.e. reasonable conditioning of the vessel walls, commissioning the ICRF heating systems up to high power, characterization of the interactions of the diagnostic neutral beam with tokamak plasmas and initial checkout of DNB diagnostics, and re-establishing operation after boronization, were all accomplished in the five weeks of initial operation. Using all three antennas, over 5 MW of ICRF power was injected. EDA H-modes lasting up to 0.5sec were obtained using the D- and E-port dipole antennas. H-mode was also obtained using the J-port antenna alone. The first official week of the physics phase of the current campaign began on May 9.

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For this campaign a total of six TV cameras view the plasma and in-vessel hardware. The images from all six cameras are digitized and stored. The views include a wide angle view inside the vessel, a view of the J port antenna, a view of the D port antenna, a view of the E port antenna, and two nearly identical, tangential views of the divertor region. Presently the E port antenna is being viewed by the LANL-supplied fast-framing, gated, visible camera. Compressed images from the cameras are available on the WEB, allowing after-shot access by operators, diagnosticians, and any other interested parties. The site for these images is

http://www.psfc.mit.edu/cmod/PlasmaVideo/imagedir.plx

which can also be reached by clicking on the "Plasma Videos from Recent Shots" line in the C-Mod "Experimental Operations" page of the PSFC WEB-site.

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A second array looking at neutral deuterium Lyman alpha radiation is now operational. This array uses a photodiode array and a very narrow filter at 1215 Angstroms. The radiation gives the local ionization rate and neutral density with information of the electron density and temperature. Initial results indicate that the Lyman alpha radiation is approximately 10 times larger near the inner wall compared to the radiation near the outer midplane. Detailed analysis of the measured profiles is under way.

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An absolute calibration of the Michelson interferometer was carried out. The measured sensitivity vs. frequency proved very stable, when compared to measurements over a year ago. All of the other ECE diagnostics (GPC, GPC2, radiometers) are cross-calibrated to this instrument during plasma operation.

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The lithium pellet injector has been reconnected to the torus, and loaded with a variety of pellets, including pure lithium, as well as Li pellets doped with potassium chloride, silver and tungsten. Pellets were successfully fired into plasma, and experiments on P-mode discharges were carried out.

The Diagnostic Neutral Beam

The DNB operated into C-Mod plasmas for the first time May 9. During the first week of its operation, a perveance scan at 39kV was completed, voltage regulation of filament supplies was tested with and without CMOD shots, and a boron CXRS line at 7619 A was checked. The BES signals were compared for low voltage (34kV & 32kV), close-to-perveance beams and high-voltage (45kV), far-from-perveance but higher current beams. The initial BES signals were found to be small. This result is believed to be due to two principal reasons: i) the deuterium beam does not provide enough Doppler shift to separate the beam emission from the background Da emission, and ii) the beam is presently off perveance and therefore not well focussed.

Collaborations and Participation in the Fusion Science Community

A quarterly review was held onsite on February 9. Rostom Dagazian from OFES discussed various aspects of the C-Mod program with Ian Hutchinson, Earl Marmar, Jim Irby, Bill Rowan, Dave Terry, Steve Wukitch, Gerd Schilling and Joel Hosea. The discussions centered on the status and near term plans for the diagnostic neutral beam and its associated diagnostics, and on the ICRF systems.

Again on May 11, a C-Mod DOE Quarterly Review was held via videoconference. Both MIT and U. Texas made presentations. Bill Rowan (U. Texas) presented a status report on the Diagnostic Neutral Beam. Jim Irby reported on other aspects of the machine and facility. Ron Parker and Montgomery Grimes presented the status of the Lower Hybrid Project. Amanda Hubbard and Catherine Fiore summarized some of the recent physics results, and Steve Wolfe presented the experimental physics plan for the current campaign. Rostom Dagazian, Warren Marton, Curt Bolton, and Erol Oktay participated from the Office of Fusion Energy Sciences.

Norton Bretz visited from PPPL to complete work on the MSE diagnostic. Gary Taylor, PPPL, visited MIT 2/15-17 to remount the GPC2 electronics in a radiofrequency-shielded enclosure to suppress RF pickup from the J-port ICRF antenna and to discuss electronics upgrades with Amanda Hubbard.

UT-FRC personnel visiting to complete diagnostic installation work February 13-18 included Keith Carter, Perry Phillips, and David Winslow.

Joel Hosea, of PPPL, was on-site 2/7-2/9 to discuss Lower Hybrid program issues and the upcoming ICRF startup. Stefano Bernabei also came from PPPL to participate in LH program discussions (as well as give a talk at our PAC meeting).

Raffi Nazikian visited from PPPL for the first week in March to discuss upgrades to the C-Mod/PPPL Reflectometer. He also gave a talk on his plans at the C-Mod Physics Meeting. Norton Bretz also spent 2/29 - 3/3 onsite, working with MIT collaborators on the MSE control and data acquisition software.

Cynthia Phillips, PPPL, visited MIT from February 29 to March 2 to work on ICRF related problems in Alcator C-Mod with Paul Bonoli.

Perry Phillips from the Fusion Research Center was at MIT during the first two weeks in April installing the high resolution ECE system.

Randy Wilson (PPPL) came to MIT for the week to help with transmitter tuning and debugging. Joel Hosea (PPPL) came 4/6-7 to contribute to the ICRF startup and for discussions with Ron Parker on the LHCD project. Norton Bretz (PPPL) was at MIT 4/3-5 for the MSE work.

Ron Bravenec and Perry Phillips (UTx) visited C-Mod to work on the BES system and the Texas ECE system respectively. Dave Winslow was onsite working on hardware for the Texas "turbulence" probe. Alignment of the probe drive, installation of the vacuum system, and installation of the PLC electronics were completed.

Domestic Travel

Miklos Porkolab attended the FESAC MEETING in Gaithersburg on Feb. 2,3 and made a brief presentation during the "public comments" period about the PSFC's efforts to establish stronger connections with Astrophysics at MIT.

In early February Rejean Boivin was in Quebec City for the first (on site) planning meeting for the APS-ICPP meeting in October 2000. Plans were made for the outreach day and space allocation.

Tom Fredian and Josh Stillerman attended an MDSplus Developers Meeting at General Atomics during the 1^st week in February. The status of the port to unix and windows, enhancement ideas and up coming installations were discussed. Participants at the meeting included GA, UCSD, PPPL, LLNL, MIT, Lausanne and Padova.

In mid-February Martin Greenwald attended a meeting of the ESnet Steering Committee. Plans for the latest network deployment (ESnet3) as well as issues involving network research and distributed computing were discussed.

Miklos Porkolab, Ian Hutchinson, and Earl Marmar visited OFES on Feb. 29. Earl Marmar and Ian Hutchinson made a presentations to OFES staff in Germantown on the results from C-Mod from the past campaign and future plans.

Amanda Hubbard was at PPPL on March 1, as a member of the National Undergraduate Fellowship selection committee. Four students have been offered summer projects at the PSFC (3 on C-Mod, 1 on VTF).

John Rice attended the Atomic Processes in Plasmas Meeting in Reno Mar. 19-23 and presented a poster on Satellites to the High-n Rydberg Series in Ar, Cl and S from Alcator C-Mod Plasmas.

Yongkyoon In attended the Sherwood Fusion theory meeting, which was held in Los Angeles from March 27 through 29.

Montgomery Grimes (MIT) traveled to CPI (the manufacturer of the Lower Hybrid high-power klystrons) to discuss the klystron operation in this application.

Miklos Porkolab, Ian Hutchinson, and Earl Marmar attended the OFES Field Work Proposal Meeting in Germantown, Md., on April 5,6.

Martin Greenwald spent two days in early May at ORNL participating in a review of the fueling and heating technology programs.

Bob Granetz, Joe Snipes, and Jesus Ramos participated in the FIRE Workshop at PPPL during the first week in May.

Rejean Boivin, Catherine Fiore, Amanda Hubbard, Dmitri Mossessian, Ian Hutchinson, Ron Bravenec (UTx), Joe Snipes, Martin Greenwald, and Bill Rowan (UTx) participated in the Transport Task Force (TTF) meeting in Burlington, VT, during the week of April 22-29.

Miklos Porkolab and Rejean Boivin attended the APS Division of Plasma Physics Executive Committee Meeting in Long Beach, Ca., on April 29th.

International Travel

Miklos Porkolab represented APS DPP at the EPS Program Committee Meeting in Budapest, Hungary, on Feb. 25,26. Four more US oral talks were selected for the EPS Meeting in Budapest, on June 12-16. This brings up the total number of US presentations to 10, out of a total of 56 talks, including 6 US invited talks and 4 oral contributed talks. This is a respectable representation at this mainly European Meeting on Plasma Physics and Fusion research. This year the goal was to keep half of the talks non-fusion oriented. Therefore, the available fusion slots were highly contested by the many nominations from Europe, the US and Japan.

Bruce Lipschultz traveled to JET to participate in US-JET collaboration discussions held March 7-9. He was part of a group representing the different US laboratories as well as particular physics areas. Bruce collected from US labs proposals for work on edge physics for JET for 2001-2002. The meeting was very helpful for understanding the JET program and their plans. The JET task force leaders expressed strong interest in a number of US proposals including those from MIT. Bruce also spent part of 3/9 and 3/10 discussing a number of physics issues with the JET edge/divertor group. These discussions centered on similarities between C-Mod and JET in the area of wall-recycling. He brought back some typical JET data for comparison with C-Mod results. He also gave 2 presentations - SOL transport and wall recycling, and Mo impurity sources and their effect on the core plasma.

Amanda Hubbard attended a Workshop on Core and Edge Transport Barriers, held at JAERI in Tokai, Japan, and presented a talk on "Conditions for EDA H-mode". She also discussed with Drs. Kamada and Hatae ideas for comparing this regime with the 'grassy ELM' regime on JT60U.

Bob Childs attended the Executive Council Meeting 85, chaired the Vacuum Science Division meeting of IUVSTA and attended the Program Committee meeting for the International Vacuum Congress 15, to be held in San Francisco October 2001. These meetings were held in Namur, Belgium from March 31 through April 2.

Near Term Plans

Our near term plans are focussed on carrying out the year-2000 experimental run campaign. The year-2000 C-Mod Run Schedule is on the Web at

http://www.psfc.mit.edu/cmod/run_schedule.html.

Daily operations information can be found on the C-Mod Operations Calendar at

http://lost.pfc.mit.edu/cgi/calendars/cmod

which is updated when information becomes available. Those with access to the online OPS bulletin board are advised to check there for the most up-to-date schedule information.
 
 

Remote participation in C-Mod staff meetings is available using video and audio conferencing equipment. The system includes high quality room audio and video images to our remote presentation capabilities. Instructions for operation of these links can be found on the WEB at

http://www.psfc.mit.edu/cmod/forum2000/forum_remote.html

Remote participation from the community is encouraged.
 
 

Live streaming video and audio from the control room is available over the WEB during C-Mod operations for off-site collaborators and other interested parties. The video shows the control room activity between shots and switches to video of the discharge itself during each pulse. The URL is

http://www.psfc.mit.edu/cmod/control_room_rv1.html .