The primary activities at Alcator C-Mod during the third quarter of
FY00 were: the completion of the first part of the year-2000 run campaign;
analysis of data from that campaign; a very short maintenance period with
in-vessel access to upgrade the ICRF antennas; participation in the international
Plasma Surface Interactions conference, the High Temperature Plasma Diagnostic
conference, and the European Physical Societyís conference on plasma physics
and controlled fusion; and continued participation in the critical assessment
of fusion science. A more detailed account of these activities follows.
Scientific Results
Core Confinement/Rotation/Impurity Studies and Disruption Mitigation
In order to investigate the ICRF rotation drive theory of Perkins, White and Bonoli, an experiment was run in which the position of ICRF resonant heating was scanned from outside the magnetic axis to significantly inside the axis. This was accomplished by changing the toroidal magnetic field. The theory predicted that when the resonance is outboard of the magnetic axis the rotation drive would be in the Co-Current direction, but with it inboard it would be Counter. The experiment showed that the theory was not verified in detail. Off-axis inboard heating did not yield an immediate change in the toroidal rotation direction or magnitude. However a very interesting result was observed. The plasmas heated by off-axis inboard ICRF exhibited strong density peaking, and, coincident with this peaking, the toroidal rotation slowed down to zero or reversed. These effects are shown as time histories in Figure 1. Note that the resonance locations for the two discharges were different by only 2 cm. The density peaking is indicative of a core particle transport barrier, and profile measurements show that the barrier forms at r/a=0.5. We have speculated in the past that the existence of a "natural" co-rotation in H-mode may prevent the formation of ITBís in many circumstances. Thus, if the co-rotation could be reduced or reversed, then barriers might form more easily. Assuming that the inboard resonance of the ICRF provides some counter-current drive, these speculations are consistent with the observations, although by no means confirmed. Time-dependent profiles of Ti, vf, and vq would allow calculation of the shearing rate, which is thought to stablize the turbulence.
The reduction of vj and the ITB
formation occur at approximately the same time. The rotation slows as the
density peaks and then drops sharply. This transition behavior is very
dependent upon resonance position, as shown in Figure
2. The interesting and significant features of this phenomenon are
1) an ITB is formed and the density becomes highly peaked, 2) this peaking
survives sawteeth, 3) as the ITB forms, the plasma rotation (toroidal)
slows and eventually drops to almost zero or reverses, and 4) the edge
remains in H-mode.
In collaboration with JT-60U (specifically Dr. Y. Nakamura) we have been studying disruption mitigation on C-Mod. The concept under investigation is that of a "neutral point" position for the plasma at which the up/down imbalance of attractive forces, arising from eddy currents, disappears. Thus the goal is to shift the deleterious effects of the disruption thermal-quench (eddy currents, halo currents, and J x B forces) away from the divertor and up to the midplane. The concept was shown to be viable on JT-60U, where the typical plasma elongation is 1.3. At this elongation the plasma is inherently less unstable to vertical displacement events (VDEs) than at an elongation of 1.65, which is typical on Alcator C-Mod. Thus C-mod provides a more stringent test of the neutral point concept. The concept may also be relevant to FIRE, which would be even more elongated, but would operate at its neutral point all the time (since it would be up/down symmetric).
By inducing a thermal-quench plasma disruption and measuring the vertical
stability of the plasma as a function of pre-disruption vertical position,
the "neutral point" in C-Mod was found to be at z~+2.7 cm. (The prediction
from modeling was at z~+1.0 cm). Plasma operation at this position gave
8 ms of post-thermal-quench stability, as shown in Figure
3. In principle, this might be enough time for the PF control system
to respond, if programmed appropriately. Thus "neutral point" behavior
has been shown to exist in C-Mod. The physics appears to be somewhat different
than on JT-60U, and the robustness to variations in plasma parameters is
not yet well determined. We are continuing our collaboration with JAERI
on this subject.
A long-term study whose goal was to identify the relative contributions to the core Mo levels from the Mo sources at the inner wall, the outer limiter, the antennas, and the divertor has been carried out. The source rates were estimated by measuring MoI emission at the different surfaces. The core Mo densities were measured using core x-ray emission from Mo+33. Penetration factors, defined as PF=NCORE/GSURFACE (with units of seconds, where NCORE= the total # of Mo ions in the main plasma, and GSURFACE= the total Mo influx from a surface), were determined. We find that although the source rates from the inner wall and outer divertor are relatively high, the screening of Mo generated at these surfaces is quite good. Therefore the core Mo levels do not correlate with the fluxes from these surfaces. The core Mo content does correlate with the Mo influx from the RF antennas, and we conclude that the antennas are the primary source of core Mo, even though the source rate from the antennas is significantly smaller than that from the divertor and inner wall. These results are shown in Figure 4. (Note that the source rates have different multipliers, 1016 for the antenna, 1017 for the divertor, and 1018 for the wall.) This is one of the primary reasons for replacing the Mo tiles in the antenna shield with BN tiles (see next section).
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 these probes were found to correlate with operation of the antennas connected to them by field lines. Potentials in the range of 50-200V were observed when the corresponding antenna was energized. The measured potentials verify that sheath rectification does occur, and that the potentials are substantial. These results provide a possible reason why the screening of the antenna sources is poorer that the screening from other surfaces.
We also find that the penetration factors from all observed surfaces decrease with increasing plasma density and are lowest for the divertor.
RF Research
Performance of the J-port antenna has been improved by the modifications made on it during the maintenance period in early 2000. The modifications included a) installation of shorting straps on the antenna protection tiles (in order to keep them all at the same potential) and b) installation of stainless steel cap-shields at the termination points of the Faraday shield bars. The shorting straps have eliminated arc damage on the protection tiles, while the cap-shields appear to prevent arc damage at the Faraday shield. This has enabled operation at higher target densities. Presently ~3 MW have been injected into plasma by this antenna. This has occurred with greatly reduced impurity production and without regular large impurity injections from the antenna. In addition, the operational density limit has improved. Nonetheless, the J-port antenna provides only ~50% the heating efficiency of the D and E-port antennas.
When the J-port antenna is operated as a 2-strap antenna, it has an absorption efficiency similar to the D and E-port antennas. As a 4-strap antenna, the parallel impedance has significantly different power dependence compared to the dipole antennas, and the loading is weakly dependent on plasma position. The reduced efficiency observed when the J-port antenna is operated as a 4-strap antenna could be a result of a parasitic load. Since there is an RF voltage between vacuum vessel wall and antenna structure, it is possible that power is dissipated via an electrostatic wave or conduction. In addition there is some evidence that there is an RF discharge around the antenna feeds and possibly the straps. The good efficiency of the 2-strap operation suggests that closing the antenna box can eliminate the parasitic load. This is being done during the short in-vessel access period that occurred between July 12 and July 28. The modifications to the J-port antenna made during the July access have included 1) grounding the antenna structure to the vacuum vessel wall, 2) adding stainless steel support plates to shield the radial feeds, 3) replacing Mo protection tiles with BN tiles, and 4) improving the ground connection of corner tiles. The grounding of the antenna box has been done using ceramic capacitors so that the impedance to ground is 0.02 W at RF frequencies, while the impedance to disruption currents is ~600 W, which keeps the disruption forces manageable. There are photographs of the J-port antenna on the WEB at
http://www.psfc.mit.edu/cmod/operations/EngImages/INVESSEL/2000/Close_Up_Survey/P0000899.JPG
through
http:www.psfc.mit.edu/cmod/operations/EngImages/INVESSEL/2000/Close_Up_Survey/P0000912.JPG
Operations and Diagnostics
The Alcator C-Mod run campaign for the year 2000 began March 20th. Approximately one half of the campaign was completed by June 30th, at which time we began a short maintenance/upgrade period in order to modify the ICRF antennas as described above. During the first half of the campaign the allocation of runs was as follows:
# of run days
Testing 3
Plasma Startup/Machine Conditioning 10
ICRF Conditioning/Development 7.5
DNB Commissioning 2
Physics Runs 26.5
There were a total of 49 run days and a total number of 758 "research" plasmas. The overall startup reliability was 79%.
The in-vessel access during the maintenance period began on July 12th. The machine was pumped down on July 28.
The Diagnostic Neutral Beam
H beams have been injected into D plasmas. The current operating point for the DNB was: an accelerating voltage of 40 kV, a total current of 4.5 A with a 55 % neutralization fraction, a beam diameter of ~10 cm at the plasma. There are still a number of things that can be done to improve the quality of the beam and the reliability of its operation. These include reduction of the frequency of HV faults, improvement of beam alignment, improvement in probe plate observations of magnetic field at the beamline, understanding and reduction of the interaction of C-Mod power supplies with DNB supplies, development of an internal monitor of the beam component fraction, checking the beam focus, and continued modeling of the beam in order to understand the observed modulation and to optimize its performance.
There are presently four fiber arrays capable of viewing the beam and charge-exchange recombination spectra (CXRS) from the beam-plasma interaction. CXRS spectra have been observed, specifically the hydrogen-like boron n=8 to 7 line and the hydrogen-like carbon n=9 to 8 line.
Balmer a emission from H0 beams has also been measured, using the Beam Emission Spectroscopy (BES) system. Radial profiles of the emission show that the signal levels are presently about a factor of three above the background. This results in a signal-to-noise ratio for fluctuation measurements which is << 1. Cooling the detectors and optimizing the beam will increase the S/N. Modeling of the expected signal from the beam and the optical system shows that the actual signal is about a factor of two lower than predicted. The difference could be due to uncertainties in the beam parameters, e.g. in the beam species mix, the beam diameter, the neutralization efficiency.
The motional stark effect (MSE) system uses the same collection optics as the BES system. This diagnostic has also observed signal from the beam, albeit at non-optimized levels.
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. 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/online/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 (http://www.psfc.mit.edu).
Collaborations and Participation in the Fusion Science Community
A quarterly review was held onsite on July 18, with videoconference participation at U. Texas and at PPPL. Rostom Dagazian was present from OFES. Presentations were given by Jim Irby ("Machine and Upgrade Status"), Steve Wukitch ("ICRF Results"), Bill Rowan ("DNB Status Report"), Ron Bravenec ("First Results from BES"), Howard Yuh ("MSE Progress"), Ron Parker (Lower Hybrid Status"), Ian Hutchinson ("Off-Axis ICRF, Rotation, and Transport Barriers" and "Campaign Progress and Long-term Schedule"), Bob Granetz ("Neutral Point Disruption Experiments"), Bruce Lipschultz ("Molybdenum Sources and Transport"), Stewart Zweben ("Turbulence Imaging"), Paul Bonoli ("TORIC Modeling"), and Peter Catto (PSFC Theory Activities in Support of C-Mod").
Alan Glasser (LANL) has spent this quarter at the PSFC. Part of his time here has been spent working with Steve Wolfe on the adaptation of his Ideal MHD stability code DCON to MDSplus. The goal of this work is to produce an MDSplus-aware version of DCON that will facilitate the use of this code at C-Mod, DIII-D, NSTX, and other sites that employ the MDSplus data system. The specification of the MDSplus tree for DCON was refined, and a working copy of the Model tree generated. Nodes (with descriptive commentary) for all of the user-relevant DCON inputs and outputs are now represented in the tree. In addition to working on the MDSplus adaptation, Dr. Glasser also identified and began implementing a fix for a bug in the code that was causing invalid results for a particular class of C-Mod equilibria. During his visit, Dr. Glasser also presented a PSFC seminar on the DCON code, and held discussions with a number of physicists from the PSFC Theory Group.
Domestic Travel
On May 24, Amanda Hubbard presented the PPPL colloquium on "H-mode pedestal Physics on C-Mod".
Bob Childs attended the Board of Directors Meeting for the AVS held at Jefferson Labs, Newport News, VA in early June. Following this, he participated in a three day, AVS-sponsored workshop on Extreme High Vacuum and Surface Conditioning.
Bob Childs traveled to Wall Colmonoy Corp. in Dayton, OH and spent Jun 14-17 overseeing the hydrogen-firing of new copper felt metal sheets to be used in the fabrication of TF Magnet spare components.
Earl Marmar, Rejean Boivin, Jerry Hughes, Yijun Lin, Chris Boswell, Norton Bretz (PPPL), Ned Eisner (UTx), Bill Rowan (UTx), M.B. Sampsell (UTx), and Ricky Maqueda (LANL) attended the High Temperature Plasma Diagnostics meeting in Tucson June 20-24, and presented papers on C-Mod work. The presentations, which will be published in the Reviews of Scientific Instruments, are:
R. Boivin, et al., "High resolution measurements of neutral density and ionization rate in Alcator C-Mod",
C. Boswell, et al., "Applications of Visible CCD Cameras on the Alcator C-Mod Tokamak",
N. Bretz, et al., "A Motional Stark Effect Instrument to Measure q(r) on C-Mod",
N. Eisner, et al., "CXRS on Alcator C-Mod"
J. Hughes, et al., "High Resolution Edge Thomson Scattering Measurements on the Alcator C-Mod Tokamak",
Y. Lin, et al., "Experimental and two-dimensional full-wave study of reflectometry fluctuation measurements in the Alcator C-Mod tokamak",
R. Maqueda, et al., "Digital-image capture system for the IR camera used in Alcator C-Mod",
E. Marmar, et al., "High resolution visible continuum imaging diagnostic on the Alcator C-Mod tokamak",
M.B. Sampsell, et al., "Simulations of Beam-Emission Spectroscopy on Alcator C-Mod".
Miklos Porkolab attended the Fusion Power Associates Meeting on July 17 in San Diego and made the following presentation: Alcator C-Mod Program and Recent Results. The following 2 days he attended the FESAC meeting at General Atomics where the recent draft of the Integrated Program Planning Activity document was discussed. Earl Marmar also attended the FESAC meeting, as well as the first meeting of the Next Step Options Program Advisory Committee.
International Travel
Rejean Boivin, Sanjay Gangadhara, Brian Labombard, Bruce Lipschultz, Spencer Pitcher, and David Winslow (UTx) attended the14th International Conference on Plasma Surface Interactions (PSI) in Rosenheim Germany on May 22-26. They gave three oral and three poster presentations. In addition, posters by Chris Boswell, Rob Nachtrieb, and Jim Terry were presented. The presentations, which will be published in the Journal of Nuclear Materials, are:
R. Boivin, et al., "High resolution measurements of neutral density and ionization rate in the main chamber of the Alcator C-Mod tokamak",
C. Boswell, et al., "Observations of Cold, High Density Plasma in the Private Flux Region of the Alcator C-Mod Divertor"
S. Gangadhara, et al., "Impurity Transport Experiments in the Edge Plasma of Alcator C-Mod using Gas Injection Plumes"
B. Labombard, et al., "Cross-Field Plasma Transport and Main Chamber Recycling in Diverted Plasmas on Alcator C-Mod"
B. Lipschultz, et al., "Molybdenum sources and transport in Alcator C-Mod."
R. Nachtrieb, et al., "Helium-3 Transport Experiments in the Scrape-Off Layer with the
Alcator C-Mod Omegatron Ion Mass Spectrometer"
C.S. Pitcher, et al., "The Effect of Divertor Baffling on Alcator C-Mod Discharges"
J. Terry, et al., "Visible Imaging of Edge Turbulence in the Alcator C-Mod Tokamak"
D. Winslow, et al., "Effects of Flush-Mounted Probe Bias on Local Turbulent Fluctuations"
Following the PSI conference, Bruce Lipschultz attended a two-day workshop at Garching on "Plasma Edge Issues for Next Step Devices". There, he presented two talks on the details of tokamak operation with a Mo first wall and on wall recycling. The latter is very controversial, but it was apparent that all experiments are observing some of the characteristics of the phenomenon in which ion fluxes to the wall that are comparable to or greater than the ion flux to the divertor.
In early June Rejean Boivin presented a talk to the W7-AS and ASDEX groups on "High Resolution Bolometry on Alcator C-Mod - Applications to H-mode pedestals and neutrals."
Joe Snipes and Ian Hutchinson visited JET for the week of June 20th to participate in an experiment in which JET attempted to access the EDA H-mode regime, as seen on C-Mod.
Ian Hutchinson, Dmitri Mossessian, and Miklos Porkolab attended the 27th EPS Conference on Plasma Physics and Controlled Fusion in Budapest on June 12-16. Miklos presented the Poster "Initial Results from an Upgraded ICRF System on Alcator C-Mod". Dmitriís poster was "H-mode pedestal studies in Alcator C-Mod". Ian presented the poster, "Tokamak Plasma Rotation without Momentum Input". Later Ian gave a talk on the same subject at the 3rd Europhysics Workshop on the "Role of Electric Fields in Plasma Confinement and Exhaust" and at JET.
Miklos Porkolab also represented the APS at the EPS Board Meeting and at the Program Committee Meeting. He then also attended the 3rd Europhysics Workshop on the "Role of Electric Fields in Plasma Confinement and Exhaust" on June 18.
Near Term Plans
The full campaign for the year 2000 is planned for 72 days (18 weeks @4 days/wk). This total includes ICRF and DNB Commissioning, but not startup/machine conditioning. We are therefore halfway through the planned number of runs (26.5 + 9.5 = 36 days), and our near term plans are focussed on carrying out the second half of the year-2000 experimental run campaign. The initial proposed run allocation was 12 days to each of the five themes (Core Transport, Pedestal, Divertor/Edge, RF Physics, Performance Optimization, Particle Control, and H-mode), with an additional 12 days contingency. The "RF" allocation included ICRF conditioning and development, and the "Core" allocation included DNB development. There also exists a Long-Pulse Task Force, which is responsible for extending the plasma duration to ~3.5 sec. These long-pulse discharges will be fully inductively driven discharges at 5T and 800 kA. At present, studies on the effects of long-pulse operation on the various machine systems (e.g. heating of magnets, bus, and connections, thermal loads on power supply and bus components, protection circuitry, and RF systems) are underway. We plan to dedicate run time to this in November. The Run Schedule is on the Web at
http://www.psfc.mit.edu/cmod1/run_schedule.html
We are also beginning preparations for the annual APS meeting, occurring in Quebec City, Quebec, in October and for the IAEA conference in Sorrento, Italy, also in October.
Daily operations information can be found on the C-Mod Operations Calendar at
http://lost.pfc.mit.edu/cgi/calendars/cmod
which is updated as 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 all C-Mod staff meetings is available using video and audio conferencing equipment. The system includes high quality room audio and video images. 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 ishttp://www.psfc.mit.edu/cmod/online/control_room_rv1.html
