ICRF Resonance Locations in CMod at B0 4'5 T

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APS-DPP Meeting, Orlando, Fl, Nov. 10-14, 2002
LPI-2
ITB Studies with Two Frequency ICRH in Optimied
Shear Plasmas with Ip Ramp M. Porkolab, P.
Bonoli, D. Ernst, K. Fiore, A. Hubbard, Y.
Lin, E. Marmar, R. Parker, J. Rice, G.
Schilling (Princeton Plasma Physics Laboratory),
S. Wukitch, MIT Plasma Science and Fusion
Center In the present work we report on
experiments in Alcator C-Mod to form and control
Internal Thermal Barriers (ITB) in plasmas with
current ramp (1) and simultaneous injection of
two-frequency ICRH, at 70 and 80 MHz, near 4.5
T in the D(H) minority regime (2). In C-Mod, the
ITB with simultaneous application of on and
off-axis ICRH during the plasma current
flat-top phase collapses back into the EDA
H-mode when central ICRH power in excess of 1 MW
is applied (2). In addition, the internal ITB is
formed on top of an EDA H-mode with relatively
high edge density pedestal, which is not optimal
for off-axis LHCD for AT operation. Therefore it
is of interest to find alternate methods to form
more favorable target plasmas for AT plasma
operation. In previous studies in C-Mod it was
found that current ramp with central ICRH formed
Reversed Shear (RS) plasmas up to 0.2 s (5-6
Tau-E) which had relatively low edge densities
and high central temperatures (5 keV), but
without evidence of significant ITBs. In the
present work the goal is to control
micro-instabilities by RS (3) so that the
two-frequency ICRH driven ITB could be
maintained and controlled without limits on the
central heating power. 1. M. Porkolab et al,
Proc. 24th EPS , Vol. 21A, pp.569, 1997. 2.
S.J. Wukitch et al., Physics of Plasmas, Vol 9,
p. 2149, 2002. . D. Ernst, presented at the
Sherwood Conference, 2002, Rochester, NY.
Work supported by the US DOE.
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Introduction and Summary

• Goal of the experiments was to see if improved
  performance, either H-mode or ITB can be formed
  during ramp-up of current (Reversed or Optimized
  Shear) in the presence of simultaneous on and
  off-axis ICRH D(H) at Te Ti, ne 1020m-3,
  BT4-6 T, before sawteeth set in
• Two scenarios were tried (a) BT5.4T, 2MW
  on-axis power at 80 MHz, and 1 MW off-axis (r/a-
  0.5) power at 70 MHz and (b) BT4.5T, 1MW on
  axis power at 70 MHz, and 2 MW off axis (r/a
  -0.5) power at 80 MHz
• Both ELM-free and ELMy H-modes were observed with
  edge pedestal Te of 1 keV during rampup however,
  no clear evidence of internal ITB was seen
• Novel C-Mod diagnostics were used to look at
  toroidal rotation (HIREX), fluctuations (PCE) and
  q-profile (MSE)
• Higher power experiments planned for the next
  campaign

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ICRF Resonance Locations in C-Mod at B0 5.4 T

• f0 80 MHz ( 2 MW)
• XCH 1.7 cm (LFS)
• f0 70 MHz (1 MW)
• XCH 11.5 cm (LFS)

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ICRF Resonance Locations in C-Mod at B0 4.5 T

• f0 80 MHz ( 2 MW)
• XCH -9.6 cm (HFS)
• f0 70 MHz ( 1 MW)
• XCH -1.5 cm (HFS)

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EXAMPLE OF ELMy H-MODE Shot 1021025025
BT 5.4T

• RF is applied at 0.07-0.01 sec sawteeth delayed
  to 0.30 sec H mode commences at 0.18 sec, well
  into the current ramp when Te 3.5 keV,
  ne1.3x1020m-3 as plasma is still limited on
  inner divertor plate corner (up to 0.22 sec) then
  diverted by 0.24 sec frequent ELMS from 0.18-23
  sec, then less frequent elms in diverted phase H
  factor enhancement relative to OHMIC value less
  than that in ELM-free regime neutron rate
  increases to 2.0x1013 just before sawteeth set in
  at 0.30sec plasma parameters are ne2x1020 m-3,
  Te4.0 keV, (Ti not analyzed yet but expect 2.8
  keV) edge pedestal parameters during H mode
  phase are Te1.0 keV, ne1.5x1020m-3
  
  
  (Question do frequent ELMS stop at current
  ramp cessation?)
• H mode saturates (Wmhd, Te, Ti) at 0.30 sec due
  to impurity accumulation and collapses at 0.32
  sec into L mode and then back to ELM-free H mode
  at 0.45 sec

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EXAMPLE OF ELM-Free H-MODEShot 1021025005 BT
5.4T

• RF is applied at 0.07-0.01 sec sawteeth delayed
  to 0.33 sec H mode commences at 0.22 sec, near
  end of current ramp when Te4keV, Ti1.8 keV,
  ne1.5x1020m-3 as plasma is pulled away from
  inner divertor plate corner (0.22 sec) to
  diverted discharge (0.24 sec) after a few ELMS,
  plasma shifts to ELM free H-mode regime with H
  factor doubling relative to OHMIC value neutron
  rate increases to 3x1013 just before sawteth set
  in plasma parameters are ne2x1020m-3, Te4.7
  keV, Ti3.0 keV Edge pedestal parameters during
  H mode phase are Te1.0 keV, ne1x1020m-3(Questi
  on do ELMS stop due to current ramp cessation?)
• H mode saturates (Wmhd, Te, Ti) at 0.35 sec due
  to impurity accumulation and collapses at 0.4 sec
  into L mode

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EXAMPLE OF ELMy H-MODEShot 1021025026 BT5.4T

• Similar to previous shot to 025 ELMy H-mode
  followed by L Mode which is followed by ELM-free
  H-mode
• Extensive PCI measurements of turbulence during
  rampup and high frequency modes during ELM event

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ICRF Resonance Locations in C-Mod at B0 4.5 T

• f0 80 MHz ( 2 MW)
• XCH -9.6 cm (HFS)
• f0 70 MHz ( 1 MW)
• XCH -1.5 cm (HFS)

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EXAMPLE OF ELMy H-MODE Shot 1021025032
BT 4.5 T

• RF is applied at 0.07-0.01 sec sawteeth delayed
  to 0.25 sec weak H mode commences at 0.15 sec
  (?), well into the current ramp when Te 1.3 keV,
  ne1.0x1020m-3 as plasma is still limited on
  inner divertor plate corner frequent ELMS from
  0.15-0.23 sec, then revert back to L mode at 0.25
  sec power starved in center (1.1 MW RF), no
  significant H factor enhancement relative to
  OHMIC value neutron rate increases to 6.0x1012
  just before sawteeth set in and radiate power
  approaches input power and collapse to L-mode
  follows plasma parameters are unimpressive (Te
  1.7 keV) edge pedestal parameters during H mode
  phase (0.21 sec) are Te0.5 keV, ne1.0x1020m-3
• Scenario is not as good as the 5.4 T case would
  need to increase central power (70 MHz) to at
  least 2.0 MW to compare will do it next campaign

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Summary and Conclusions

• Two-frequency ICRF brought significant
  improvement in performance during current ramp-up
  as compared to the single frequency experiments
  in 1996
• H-modes are obtained both with and without ELMs,
  before sawteeth set in
• High edge pedestal temperatures and densities
  (Te 1.0 keV, ne1.5x1020m-3 ) and high central
  temperatures and densities (Te 5 keV, Ti 3.0
  keV, ne2.5x1020m-3) are obtained with 3 MW RF
  power
• Impurity accumulation terminates ELM-free H
  modes, and cessation of current ramp terminates
  Elmy H modes
• Higher power ICRF experiments, as well as further
  optimizations with off-axis mode conversion
  current drive (MCCD) will follow in the Spring
  campaign
• In 2004 off-axis LHCD will be added in these
  target plasmas

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J. Davies and J. Rice
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J. Davies and J. Rice
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