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Investigation of the Synergy of IBW and
LHCD for Integrated High Performance
    Operation in the HT-7 Tokamak
  Baonian Wan 1), Yuejiang Shi 1), Yanping Zhao 1), Jiafang
  Shan 1), Junyu Zhao 1), Yubao Zhu 1), Yinxian Jie 1),
  Guosheng Xu 1), Mei Song 1), Jiangang Li 1), Fukun Liu 1),
  Bojiang Ding 1), Guangli Kuang 1), Haiqing Liu 1)

      1) Institute of Plasma Physics, Chinese Academy of
                      Sciences, Hefei, China
• Control of the current density profile has been realized with off-axis
  current drive by the LHW and its synergetic effect with the IBW in the
  HT-7 tokamak. The IBW is explored as a means of improving current
  drive efficiency, creating a well-localized fast electron current channel
  and extending the high performance volume in LHCD plasmas. High
  performance via formation of an ITB-like profile in electron
  temperature, which was strongly correlated with the location of the
  LHW driven fast electron current, was achieved in the IBW and LHCD
  synergetic discharges through moving the IBW resonant layer to
  maximize the plasma performance and to avoid MHD activity. A
  variety of high performance discharges, indicated by N*H89 = 1–4,
  was produced for several tens of energy confinement times. This
  operation mode utilizing the synergetic effect of IBW and LHCD
  provides a new way to obtain steady-state operation in an advanced
  tokamak scenario.
•   Steady-state operation of a tokamak plasma is one of the basic requirements
    for fusion reactors.
•   High performance, as in an advanced tokamak operation scenario, is needed
    for the economic use of fusion reactors.
•   Control of the current density and electron pressure profiles becomes a crucial
    issue in advanced tokamak operation.
•   Off-axis LHCD is a powerful tool not only for current sustaining but also for
    current density control to achieve an advanced tokamak scenario.
•   This means potentials but also difficulties to localize LHW driven current
    density actively, if plasma performance and parameters are changed.
•   Additional heating and more reliable profile control are needed to improve the
    current drive efficiency and to increase the plasma  for obtaining high
    performance discharges under steady-state condition.
                 Introduction (cont.)
•   IBW has good features for heating electrons both locally and globally via
    electron Landau damping (ELD), and for controlling the electron pressure
•   The parallel wave number n|| of IBW is an oscillating function along the wave
    trajectory. This effect on n|| cannot drive net plasma current via ELD.
•   But it can enhance the wave damping on electrons, producing a localized
    absorption of the coupled RF power around the maximum of n||.
•   IBWs can be used in conjunction with LHWs to aid the localization of the
    non-inductive current generated in the regime of LHCD.
•   A broadening of the electron distribution shape can be produced due to
    acceleration by IBW RF field, which results in a localized increase of the
    LHCD efficiency and generation of spatially localized current channels.
•   Features of off-axis IBW heating can be integrated into LHCD plasma to
    extend high performance volume, which is needed for an advanced tokamak
                   IBW System
Frequency: 24~30MHz
RF Power: 50~350kW
RF Pulse length: 0.5~2s
Matching System:
          Two Stub-tuners
n||: Peaked at 8 for 27 MHz
Faraday shielding: graphite
                                                                       LHCD System
•   Frequency: 2.45GHz
•   Launcher: 316 grill
•   Pulse length: CW
•   Power: up to 1.2 MW
•   N||peak: 1.25~3.45
                                                     0        p
                                          DF =30 , N// =2.5                                                                                  0    p
                                                     0        p
                                                                                                                                    DF=120 , N// =3.1
                                          DF =60 , N// =2.7                                                             50                   0    p
                                                                                                                                    DF=150 , N// =3.25
                                                     0        p
                                          DF =90 , N// =2.9
       Power density (a.u.)

                              50                                                                                                             0    p
                                                                                                                                    DF=180 , N// =3.45
                                                 0        p
                                          DF =0 , N// =2.35                                       Power density (a.u)   40                   0
                                                                                                                                    DF=270 , N// =1.8



                              20                                                                                        20

                              10                                                                                        10

                               -10   -8     -6           -4       -2   0     2   4   6   8   10                         0
                                                                                                                         -10   -8       -6       -4       -2   0     2   4   6   8   10
• More than 30
  diagnostics provide
  most plasma profiles
  in one shot or spatial
  scanning shot by
  shot. Te was
  measured by a SX-
  PHA shot by shot in
  LHCD plasmas
• A CdTe detector
  based HX array is
  used as main tool in
  present investigation.
• Ip was feedback controlled by the poloidal
  field system.
• Ne was feedback controlled by a pulsed gas
  injection system.
• HT-7 main parameters:
   R=1.22m, a=0.27m, Ip = 100 ~ 250 kA,
  Bt=1.6 ~ 2 T, n e (0) = 1~ 6.51019/m3,
  Te(0) =0.5 ~ 1.5 keV (OH)
In present experiment: N||peak = 2.3 (LHCD),
       LHCD+IBW (30 MHZ)
• Ip ~ 150 kA, Bt=2 T, n e (0) ~ 1.51019/m3,
  Te(0) ~ 1keV, N||peak = 2.3
                                    In LHCD+IBW

                                   DIHX(12cm) >


                                   Ne increased

                                   Ne(r) peaked

                                   Te(0) increased
     Fast electron current density was broadened
• Peak of HXR was shifted outward.
• Simulation can account partially for LHW
  power deposition.
• An increase of global HXR imply more
  driven fast electrons by LHCD.
• LHW damping enhanced at first IBW n||
  maximum is a possible mechanism.
• Increase of HXR was mostly in 0.4~0.8a.
• The first IBW n|| maximum is at ~0.55a
            Possible mechanism
• A broadening of the electron
  distribution shape can be
  produced by IBW injection,
  which results in a localized
  increase of the LHCD
  efficiency and generation of
  spatially localized current
• Asymmetric velocity
  distribution produced by
  LHCD for IBW to damp
  preferentially in one direction.
• Direct IBW driven is small.
similar behavior as in off-axis IBW heating

                                H ~ 13.5cm
                                Ne increased
                                Ne broadened
                                Te increased
                                HXR increased
    Behavior in a reference off-axis IBW (27MHz)
                    heated plasma
• Bt=2.03T, H ~ 15.5cm
• Ne increased and broadened
• Te increased and broadened
• Te (13.8) > Te(2) in IBW
• Largest pressure gradient at close to
  resonant layer
• High performance volume extended
   Strong correlation between Pe(r) and
          radial profile of HXR
• Large gradient in Te(r) and Pe(r)
  around the IBW resonant layer.
• Peak of HXR located at the
  same position
• High performance volume
           Shot 46130 Bt=1.9T

             Shot 46110 Bt=1.8T
            LHCD+IBW (24MHz)
• FP code predict LHW power deposition is more
  centered in shot 46130 (Bt=1.9T) than in shot 46130
  (Bt=1.9T) . Prediction is not in agreement with
  observation from HXR profile.
• The first IBW n|| maximum for 46130 (Bt=1.9T) is at r
  ~ 0.52a and for 46110 (Bt=1.8T) is at r ~ 0.44a .
• Approximate agreement between observation and
  positions of the IBW n|| maxima verifies localized
  synergy effect again between LHCD and IBW
       Integrated performance
• LHCD was used to sustain plasma current and
  control current density profile.
• Features of IBW heating was integrated into
  LHCD regimes.
• IBW and LHCD synergetic discharges was
  optimized through moving the IBW resonant layer
  by changing Bt and selecting plasma parameters.
• Strategy was chosen to maximize the plasma
  performance and to avoid the MHD activities.
• IBW of 27MHz was selected for extension of high
  performance plasma volume.
Integration of performance (high Te)

 Ip=215kA, ne0=2.6x1019m-3, BT=2.0T, PLHCD=250kW,
 PIBW=130kW, Te =2.9keV, Ti=1.6keV, H89=1.6, tH/tE=53
      Integration of high performance
Bt~1.8T, H89*N >3 (4) and H93>1 (1.5) for ~50tE
Integration of high performance

Ip=135kA, ne0=2.2x1019m-3, BT=1.8T, PLHCD=190kW,
PIBW=80kW, Te =1.5keV, Ti=1.1keV, tH/tE=134
• Synergetic interaction between LHW and IBW can
  produce a well-localized non-inductive current channel.
  Strong synergy effect on driven current profile and
  increased driven efficiency was observed.
• The properties of IBWs in controlling Te and Ne profiles
  can be integrated into the LHCD plasmas to change local
  electron pressure profile and to improve the plasma
• A variety of high performance discharges indicated by
  betaN*H89 =1~3 was produced for several tens energy
  confinement times.
• The operation mode utilizing synergy effect of IBW and
  LHCD provide a new way to obtain steady-state operation
  in advanced tokamak scenario.
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