The primary activities at Alcator C-Mod during the fourth quarter of
FY00 were: continuation of the year-2000 run campaign; analysis of data
from that campaign; participation in the IAEA conference and the APS -
Division of Plasma Physics conference; and continued participation in the
critical assessment of fusion science. A more detailed account of these
activities follows.
Scientific Results
RF Research
An important milestone has been achieved with C-Mod's 4-strap antenna,
the so-called J-port antenna. The performance of the J-port antenna has
been improved to the point where it
is equal to that of the two dipole antennas. This includes similar, good heating efficiency and similar freedom from impurity generation. The good performance was enabled by a number of modifications that were made as we gained experience with the antenna. The key modifications were a) elimination of arcing between antenna protection tiles by installation of shorting straps between tiles, b) installation of stainless steel cap-shields at the termination points of the Faraday shield bars that prevent arcing from the current straps to the antenna backplane, and c) modification of the transmission line configuration to get good phasing between the various antenna straps. Another modification, whose evaluation awaits the next manned-access to the vacuum vessel, was the grounding (for RF frequencies) of the antenna structure to the vacuum vessel wall. The combination of all of these changes has resulted in improved ICRF-heated plasma performance, as shown in figure 1 for an EDA H-mode heated by 4 MW of ICRF, where central electron temperatures in excess of 3.5 keV at line average densities greater than 2.6x1020 m-3 were achieved. The similarities of good antenna heating efficiency and of low impurity production among the three C-Mod antennas are shown in figure 2.
Core Confinement Studies - EDA H-mode
Many aspects of the attractive Enhanced D-Alpha H-mode (EDA), seen on Alcator C-Mod, have been studied and described in previous reports. Our understanding of this regime has advanced significantly and a relatively consistent picture has emerged. Our present picture of EDA H-mode was presented in an invited talk at IAEA by Earl Marmar and an invited talk at APS-DPP by Amanda Hubbard. EDA H-mode combines very good energy confinement with the absence of impurity accumulation. In contrast to type I ELMy behavior, edge pressure and density are controlled in a continuous fashion. A quasi-coherent (QC) mode, localized to the pedestal region, has been identified which appears to be responsible for the enhanced majority and impurity particle transport. The mode has relatively short poloidal wavelength (1 cm), and typical lab-frame frequency of 100 kHz. The mode has a strong magnetic component. Direct density and electric field fluctuation measurements, made in the high gradient pedestal region with electrostatic probes, show that the mode is responsible for outgoing particle flux. The relative amplitude of the QC mode, the density profile, and the estimated outgoing
particle flux in the separatrix region are shown in figure 3. Plasma
shaping, density and magnetic shear all appear to play a role in defining
the EDA/ELM-free operational boundaries. EDA H-mode is readily obtained
in ohmic-only as well as in ICRF auxiliary-heated discharges, ruling out
the possibility that EDA is caused by fast particle or other RF driven
phenomena. EDA H-mode may bear some similarities to the small or no-ELM
regimes that have been observed on other tokamaks, including LPC H-Mode
on JET [BURENS, M., et al., Nucl. Fusion 32 (1992) 539],
and type II or grassy ELM H-Modes seen on DIII-D [OZEKI,
T., et al., Nucl. Fusion 30 (1990) 1425] and JT-60U [KAMADA,
Y., et al., Plasma Phys. Control. Fusion 38 (1996) 1387], but the
exact relationships among these different regimes is presently unclear.
An operational regime, similar to EDA, that is observed on DIII-D (Quiescent
H-mode) was the subject of an invited talk by Dr. K. Burrell of General
Atomics at the October APS-DPP conference. His comparison of the edge mode
that accompanies each machine's regime is shown below. While there are
many tantalizing similarities, there are also unexplained differences.
Perhaps the most telling distinctions are the differences in poloidal wavelength
(~1cm on C-Mod, ~100 cm on DIII-D) and in the tendency for higher neutral
pressure to favor EDA on C-Mod, while strong cryo-pumping favors the Quiescent
H-mode on DIII-D.
|
seen in DIII-D Quiescent H-mode |
seen in C-Mod EDA H-mode |
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| Increases Da level in divertor |
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| Increases particle transport across separatrix |
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| Location |
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| Frequency |
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| Frequency spread Df(FWHM)/f |
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| Toroidal mode number |
variable mix n=1-10 |
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| Poloidal wavelength |
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| Oscillations seen on |
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Pedestal Profiles and MHD Limits for EDA H-modes
Detailed measurements of density, temperature and impurity profiles
have been made for the H-Mode pedestals in both ELM-free and EDA cases.
Pedestal widths are typically in the few mm range, and the measured electron
pressure gradients are very high, approaching 107 Pascal/m in
the highest ICRF power cases. Electron density and temperature profiles
from a high-resolution edge Thomson scattering system are shown for an
EDA discharge in figure 4.
Ti profiles are not yet measured in the pedestal, but assuming Ti = Te, total plasma pressure profiles are inferred and also plotted in the figure. Modeling with the BALOO MHD stability code [MILLER, R.L., et al., Phys. Plasmas 4 (1997) 1062] indicates that, in the absence of edge bootstrap currents, these profiles would generally be at, or in some cases, well above the ideal first stability limit for ballooning modes. Nevertheless, type I ELMs are not seen. While still under investigation in the modeling, it is likely that edge bootstrap current driven in the strong gradient, pedestal region opens up a path to second stability, reminiscent of the picture developed from DIII-D [FERRON, J., et al, Phys. Plasmas 7 (2000) 1976]. The profiles of Te and ne show little or no systematic differences when comparing ELM-free to EDA regimes. One profile that is clearly different is related to impurity density. High-resolution soft x-ray imaging reveals that the impurity pedestal is significantly narrower in ELM-free than in EDA. In these cases, the measured emissivity has been shown to be directly related to the product, ne x nI, where nI, the impurity density, is dominated by highly stripped fluorine, one of the trace impurities in the plasma. Because the x-ray pedestal is located about 5 mm inside the ne pedestal, ne is nearly constant across the region of interest, and the emissivity gradient is due almost entirely to the impurity density gradient. From these measured profiles it is clear that a major difference between the EDA and ELM-free regimes is the broadening of the edge impurity profile, which in turn is directly related to the degraded impurity confinement, which is one of the desirable properties of the EDA H-mode.
Edge/Divertor Physics
Magnetic divertors were originally conceived as a means for minimizing plasma-wall contact in the main chamber by redirecting the wall interaction to a chamber that is remote from the core plasma. In this ideal picture, all particle and heat fluxes which cross the magnetic separatrix result in flows along open field lines into the divertor chamber. Experiments in Alcator C-Mod clearly demonstrate that this ideal description does not apply universally. Although the C-Mod divertor usually receives most of the conducted and convected energy fluxes from the Scrape-Off Layer (SOL) and does entrain/compress recycling impurity and fuel gases, a large fraction of ions in the SOL flow to the walls rather than flow into the divertor. The reason for such high ion fluxes to the vessel walls rather than into the divertor appears to be primarily the existence of strong cross-field particle transport in the main-chamber SOL. Cross-field particle transport in the SOL increases with distance from the separatrix, carrying particles to main-chamber wall surfaces rather than into the divertor volume [Umansky, M.V., et al., Phys. Plasmas 6 (1999) 2791.].
Utilizing the particle balance equation (
)
knowledge of the source (Si) and the ne profiles, we have derived
the radial profiles of the perpendicular ion flux in the SOL (
)
and the effective cross-field diffusivity, Deff. The local midplane
ionization source (Si) is derived from the measured local Lya
emissivity [BOIVIN, R.L., et al., Phys. Plasmas 7
(2000) 1919.], ne and Te profiles. The parallel
ion flux to the divertor is varied parametrically as a fixed fraction,
a,
of the wall flux. Three values for a are assumed
? 0.0 (no flux to the divertor throat), 0.5 and 1.0 (equal flux to divertor
throat and to wall). Figure 5 shows typical resultant profiles for the
cross-field flux, ,
for an ohmic L-mode discharge. Note that the value of at
the separatrix correspondingly increases as a
is increased. However, the value of at
the wall is unaffected by the choice of a, since
it is fixed by the measurements. The resultant effective particle diffusion
coefficient,
,
is also shown in figure 5. This definition of Deff is
not meant to imply that the transport fluxes are ëdiffusiveí. We have performed
a regression analysis of Deff at the r
=2
mm location on local values of Te (eV)
and
ne(m-3).
(r is the distance outside the separatrix at
the midplane.) The result is the following scaling:
Deff ~ 0.069 (Te/50)-3.5 (ne/1020)1.7 (m2 s-1).
Such a scaling as given is suggestive of a scaling with collisionality of the SOL plasma,
Deff ~ lei-1.7, where lei is the electron-ion mean-free path. This scaling is found to apply to a region of the SOL reaching 5 mm from the separatrix.
Operations and Diagnostics
After a short maintenance/upgrade period in July, the Alcator C-Mod run campaign for the year 2000 continued during the months of August, September, and October. The machine was pumped down on July 28. Pictures documenting the invessel status at pumpdown can be found at
http://www.psfc.mit.edu/cmod/operations/EngImages/CmodImages.asp?ref=/INVESSEL/2000/Close_Up_Survey/
Power systems tests were completed and an initial plasma attempt was made on August 16. Several full length plasmas were obtained on August 17, although carbon levels and H/D were high. On August 18 there was a substantial improvement in startup reliability and reduced carbon levels, and standard 800 kA, 5.4 T diverted discharges were obtained. Since the last quarterly review (7/18/00 through 10/18/00), there were a total of 28 run days and a total number of 508 "research" plasmas. The overall startup reliability was 83% during this time. Engineering reliability was 96%.
The Diagnostic Neutral Beam
Late in the run on September 21, C-Mod experienced a sudden loss of
vacuum which was traced to a hole in a bellows in the beamline of the DNB.
The damage was a result of a portion of the beam impinging on the bellows.
The port was blanked off and the machine pumped back down successfully,
and the vessel was baked to 100oC for two days, and then ECDC
cleaned for two days. Plasma operation resumed 6 days later. The cause
of the leak appears to have been the failure of a steering magnet whose
purpose is to remove non-neutralized ions from the beam. However, in the
analysis of the possible causes of the event, we realized that under some
circumstances (specifically high density L-mode conditions), DNB operation
on C-Mod is significantly affected by high edge neutral pressure. As presently
configured, the edge neutral pressure is also the pressure in the DNB beamline.
If the beamline pressure is too high, then re-ionization of the beam's
neutrals (via neutral-neutral collisions) will occur. The
calculated fraction of the beam lost due to this
process as a function of beamline pressure is shown in figure 6. Also shown
are the typical ranges of edge neutral pressure for various discharge conditions.
The calculations imply that under some conditions, i.e. high density L-Mode,
re-ionization of beam neutrals may occur and may explain the observed degradation
with plasma density of the BES signal. However, this process does not appear
to be significant under H-mode conditions and should also not be significant
under the conditions of the proposed AT plasmas (Lower Hybrid heated/current
driven plasmas).
Collaborations and Participation in the Fusion Science Community
A quarterly review was held onsite on October 18, with videoconference
participation at U. Texas, PPPL, and OFES. The OFES participants were Rostom
Dagazian and Chuck Finfgeld. The primary subjects of the review were 1)
the status of the machine and the present run campaign, 2) the latest results
from the 4-strap antenna, 3) the status of the DNB, and 4) the status of
the Lower Hybrid project. Presentations were given by Steve Wolfe, Jim
Irby, Steve Wukitch, Ron Parker, Ned Eisner, Ron Bravenec, Norton Bretz,
and Bill Rowan.
Papers describing research on C-Mod were presented at two major conferences held this quarter, the international IAEA meeting held October 4-10 in Serrento, Italy, and the APS Division of Plasma Physics meeting, which occurred in Quebec City, Canada on October 23-27 and was in conjunction with the 10th International Congress on Plasma Physics.
There were seven presentations (three oral and four poster) from C-Mod at the IAEA meeting:
I.H. Hutchinson - Overview of Alcator C-Mod Recent Results
E.S. Marmar - Enhanced D-Alpha H-mode Studies in the Alcator C-Mod Tokamak
B. Lipschultz - Cross-Field Transport in the SOL and its Relationship to Main Chamber and Divertor Neutral Control in Alcator C-Mod
Posters
R.J. Hastie - Mercier Instabilities in the Alcator C-Mod Tokamak
P.T. Bonoli - Numerical Modelling of ICRF Physics Experiments in the Alcator C-Mod Tokamak
R.S. Grantez - Radial and Poloidal Impurity Transport in the H-mode Edge Pedestal of Alcator C-Mod
J.R. Rice - Observations of Co-Current Toroidal Rotation in Alcator
C-Mod ICRF and Ohmic H-mode Plasmas
At the American Physical Society conference there were three invited presentations, 10 contributed orals and 28 posters. A listing of the first authors and their titles follows:
Invited Talks
Fiore, C.L. - Core Internal Transport Barriers in Alcator C-Mod Plasmas
LaBombard, B. - Particle Transport in the Scrape-Off Layer of Alcator C-Mod
Hubbard, A.E. - Pedestal Profiles and Fluctuations in C-Mod Enhanced D-alpha H-modes
Contributed Oral Presentations
Boivin, R.L. - Recent Results from the Alcator C-Mod Tokamak
Rice, J.E. - Central Toroidal Rotation Reversal with ITB Formation in Alcator C-Mod Plasmas
Snipes, J.A. - Peaked Density Profiles in H-mode in Alcator C-Mod
Greenwald, M. - Studies of EDA H-mode and Its Relation to the Micro-Stability of the Pedestal
Zweben, S.J. (PPPL)- Two Dimensional Imaging of Edge Turbulence in Alcator C-Mod
Granetz, R.S. - Disruption Neutral Point Experiment on Alcator C-Mod
Schilling, G. (PPPL) - Overview of Results from the Upgraded ICRF System on Alcator C-Mod
Nelson-Melby, E. - Observations of Mode-converted Ion Bernstein Waves in Alcator C-Mod with Phase Contrast Imaging Diagnostics
Bonoli, P. - Advanced Full-Wave Simulations of Mode Conversion Electron Heating and Current Drive in Alcator C-Mod
Mikkelsen, D.R. (PPPL) - Ion Temperature Gradient Scale Length in C-Mod: Testing Nonlinear Theory
Poster Presentations
Fiore, C.L. - Core Internal Barrier Foundation in Alcator C-Mod Plasmas
Marmar, E.S. - Experimetnal Phenomenology of the Enhanced D-Alpha H-Mode in Alcator C-Mod
Hutchinson, I.H. - Why do Alcator C-Mod plasmas rotate the way they do?
Lin, Y. - Study of Enhanced D-Alpha H-modes Using the Alcator C-Mod Reflectometry
Hughes, J.W. - High Resolution Edge Thomson Scattering Measurements on Alcator C-Mod Tokamak
Mossessian, D. - H-mode Pedestal Studies in Alcator C-Mod
In, Y. - Edge localized modes (ELMs) and their inferred dimensions
Yuh, H. - Stability Analysis of Alcator C-Mod with Gyrokinetic Code GS2
Pankin, A. (Lehigh University) - Alcator C-Mod Predictive Modeling
Chatterjee, P.E. (FRC, Univ. of TX) - Sawteeth heat pulse propagation in Alcator C-Mod
Goetz, J.A. - Operation of the Alcator C-Mod 4-Strap Antenna
Porkolab, M. - ICRF Driven Internal Thermal Barriers in Alcator C-Mod
Wukitch, S.J. - ICRF Current and Poloidal Flow Drive in Alcator C-Mod
Mazurenko, A. - Fluctuations and Fast Wave Measurements by the Phase Contrast Imaging on Alcator C-Mod
Shugart, A.J. (FRC, Univ. of TX) - Computer Modeling of the C-Mod Phase Contrast Imaging System
Eisner, E.C. (FRC, Univ. of TX) - Operation of the DNB on Alcator C-Mod
Bretz, N. (PPPL) - A Motional Stark Effect Instrument to Measure q(r) on C-Mod, PPPL
Bravenec, R.V. (FRC, Univ. of TX) - Initial Results from Beam-Emission Spectroscopy on Alcator C-Mod
Pitcher, C.S. - Edge Measurements on Alcator C-Mod using the Helium Line Ratio Technique
Chung, T. - DIVIMP Modeling on Impurity Control Studies on Alcator C-Mod
Boswell, C.J. - 2-D Temperature Measurements of the Divertor Using the Line-to-continuum Ratio Method
Winslow, D.L. (FRC, Univ. of TX) - Edge Fluctuation Measurements with a Triple Probe on Alcator C-Mod
Terry, J.L. - Fluctuation Measurements in the SOL of Alcator C-Mod
Smith, D. - Evaluation of Emissive Probe Usage in Alcator C-Mod
LaBombard, B. - Particle Transport in the Scrape-Off Layer of Alcator C-Mod
Stotler, D.P. (PPPL) - Modeling of Alcator C-Mod Divertor Baffling Experiments
Elder, D. (Univ. of Toronto) - Onion-Skin Method and EIRENE Modeling of the Alcator C-Mod Region
Rivenberg, P., Censabella, V., - Education Outreach at the MIT Plasma Science and Fusion Center
The text of these abstracts can be found on the WEB at
http://www.psfc.mit.edu/cmod/sciprogram/2000_aps_abstracts.htm
Domestic Travel
During the last week in July Rejean Boivin and Martin Greenwald participated in the APS program committee meeting in Washington for the upcoming APS-DPP meeting in Quebec.
Tom Fredian visited PPPL in July to analyze the use of the MDSplus data system on the NSTX experiment and make suggestions for improved performance and functionality. The system is installed and working well. A few fine-tuning suggestions were made.
For the third week in September Martin Greenwald was in Oak Ridge to participate in a meeting of the ESnet Steering Committee. The principal item of business was the preparation of the ESnet program plan.
Jim Irby attended the first meeting of the Fusion Facilities Operations
Committee at PPPL during the last week in September. He discussed machine
design, machine operations, and safety issues with representatives from
PPPL, GA, and DoE. A list of experts from each facility is being developed
to help all sites with technical problems.
International Travel
At the end of July Rejean Boivin travelled to INRS-Energie et Materiaux, located in Varennes
Quebec, for the thesis defense of Irina Condrea, who did research on rotation measurements in L and H modes.
In mid-September Martin Greenwald was in Varenna Italy attending the EU-US Transport task-force meeting.
Ian Hutchinson, Earl Marmar, Bruce Lipschultz, John Rice, Miklos Porkolab, Jim Hastie, and Bob Granetz presented papers based on Alcator C-Mod research at the 18th IAEA Fusion Energy Conference in Sorrento, Italy. The papers have been listed above. Also attending the conference were Joe Snipes and Ron Parker.
Before the IAEA conference Earl Marmar presented a seminar on Alcator C-Mod results to the Tore Supra group in Caderache, France.
In early October Montgomery Grimes, Ron Parker, and Dave Terry visited Thomcast AG in Turgi,
Switzerland for a design review of the Lower Hybrid power supply/modulator.
For the week October 23-27, most of the C-Mod physics staff participated
in the APS Division of Plasma Physics Meeting in Quebec City.
Near Term Plans
The remainder of the year 2000 campaign will be focused on experiments in support of ICRF development, experiments utilizing the high levels of ICRF power for the study of H-mode physics, experiments looking at SOL and edge transport/fluctuation physics, and continued development of the DNB and its associated diagnostics. A key set of experiments in the next quarter will be the creation and study of long-pulse discharges, lasting ~3.5 sec. These long-pulse discharges will be fully inductively driven discharges at 5T and 800 kA.
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 is