Overview of the Alcator C-MOD Research Program Page: 4 of 14
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Overview of the Alcator C-MOD research programme
tokamak plasmas. Low-Z coatings reduce metallic impurity influx and diminish radiative losses leading to higher
H-mode pedestal pressure that improves global energy confinement through profile stiffness. RF sheath rectification
along flux tubes that intersect the RF antenna is found to be a major cause of localized boron erosion and impurity
generation. Initial lower hybrid current drive (LHCD) experiments (PLH < 900 kW) in preparation for future
advanced-tokamak studies have demonstrated fully non-inductive current drive at Ip ~ 1.0 MA with good efficiency,
Iarive = 0.4PLH/feoR (MA, MW, 1020 m-3,m). The potential to mitigate disruptions in ITER through massive gas-
jet impurity puffing has been extended to significantly higher plasma pressures and shorter disruption times. The
fraction of total plasma energy radiated increases with the Z of the impurity gas, reaching 90% for krypton. A
positive major-radius scaling of the error field threshold for locked modes (Bth/B ac Ro.68 o.19) is inferred from its
measured variation with BT that implies a favourable threshold value for ITER. A phase contrast imaging diagnostic
has been used to study the structure of Alfv6n cascades and turbulent density fluctuations in plasmas with an
internal transport barrier. Understanding the mechanisms responsible for regulating the H-mode pedestal height is
also crucial for projecting performance in ITER. Modelling of H-mode edge fuelling indicates high self-screening
to neutrals in the pedestal and scrape-off layer (SOL), and reproduces experimental density pedestal response to
changes in neutral source, including a weak variation of pedestal height and constant width. Pressure gradients in
the near SOL of Ohmic L-mode plasmas are observed to scale consistently as I2, and show a significant dependence
on X-point topology. Fast camera images of intermittent turbulent structures at the plasma edge show they travel
coherently through the SOL with a broad radial velocity distribution having a peak at about 1% of the ion sound
speed, in qualitative agreement with theoretical models. Fast Da diagnostics during gas puff imaging show a complex
behaviour of discrete ELMs, starting with an n ~ 10 precursor oscillation followed by a rapid primary ejection as
the pedestal crashes and then multiple, slower secondary ejections.
PACS numbers: 52.25.Vy, 52.35.Bj, 52.35.Ra, 52.40.Hf, 52.55.-s, 52.55.Fa, 52.55.Wq1. Introduction
Recent research on the high-field, high-density diverted
Alcator C-MOD tokamak has focused on the plasma physics
and plasma engineering required for ITER and for attractive
fusion reactors. C-MOD is prototypical of ITER in several
key respects including toroidal magnetic field, equilibrated
ions and electrons, plasma density, power density in the
scrape-off-layer (SOL), low momentum input, high-Z plasma-
facing components (PFCs), divertor neutral opacity and long-
pulse length compared with the skin time. The paper
is organized as follows: sections 2 and 3 summarize the
major thrusts of the C-MOD 2005-6 campaigns comprising
performance studies with all-metal walls and initial lower
hybrid current drive (LHCD) experiments. Recent disruption
mitigation studies involving massive gas puffs are described
in section 4. Experiments to infer the major-radius scaling
of the error field threshold for locked modes are reported
in section 5. Sections 6 and 7 describe new results from
the phase contrast imaging (PCI) diagnostic related to the
stucture of Alfv6n Cascades and the wavenumber spectrum
of trapped electron mode (TEM) turbulence. The next
four sections cover recent progress in the physics of the
edge plasma. New studies of the scaling of pressure
gradients in the H-mode pedestal and SOL are summarized
in sections 8 and 9. Sections 10 and 11 report new results
in imaging turbulence in the edge plasma and measurements
of the dynamics of edge localized modes. Planned facility
upgrades to C-MOD over the next two years are described in
section 12.2. Performance with all-metal walls versus low-Z
coatings
Tungsten has been selected as the primary material for PFCs in
ITER and fusion reactors based on its low tritium retention, low
erosion rate, and low neutron damage rate. High-Z metallic
PFCs including molybdenum and tungsten have hydrogen
recycling and radiative properties that differ substantially from
low-Z PFCs (carbon, beryllium) and low-Z PFCs coatings
(boron, beryllium, lithium). Most of the world's divertor
and confinement database has been developed in tokamaks
with low-Z PFCs or low-Z PFCs coatings (with ASDEX [1]
currently pursuing an all-tungsten PFCs environment), but
carbon has been relegated to a small fraction of the PFCs
surfaces in ITER and none in a high-fluence demonstration
reactor due to concerns related to tritium retention and
erosion.
Until recently, the PFCs environment in C-MOD has been
molybdenum coated with a thin layer of boron, plus boron-
nitride protective tiles near the RF antennas. In C-MOD, the
boron layer is replenished periodically using a low-temperature
electron cyclotron plasma discharge fuelled with diborane gas.
This 'boronization' process is typically carried out overnight,
depositing about 200 nm of boron over a period of 10-12 h,
and the resonance location can be swept from the inner wall
to beyond the outer limiters by scanning the toroidal field.
The deposition location can also be localized radially by
imposing a small variation in toroidal field. Prior to the 2005
experimental campaign, boron was removed from all PFCs
and the boron-nitride tiles were replaced with molybdenum
tiles to allow a direct comparison of plasma performance with
and without a boron coating, yielding a surface concentrationS599
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S. Scott, A. Bader, M. Bakhtiari, N. Basse, W. Beck, T. Biewer, S. Bernabei, P. Bonoli, et al. Overview of the Alcator C-MOD Research Program, article, November 13, 2007; Princeton, New Jersey. (https://digital.library.unt.edu/ark:/67531/metadc930615/m1/4/: accessed March 28, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.