Roles of Current Drive Techniques in Tokamak

1
Roles of Current Drive Techniques in Tokamak

• Make continuous operation feasible
• Control the radial profile of the current
  density to give resilience against MHD
  instabilities
• Provide optimum current density profiles for
  better plasma confinement
• Develop high poloidal beta regimes where up to
  70 of the plasma current is carried in the form
  of the plasma generated bootstrap current

2
Non-Inductive Current Drive Techniques

• Neutral Beam Current Drive
• Lower Hybrid Current Drive
• Fast Wave Electron Current Drive
• Fast Wave Minority Ion Current Drive
• Electron Cyclotron Current Drive

3
Neutral Beam Injection
Fast ion current
Trapped electron effects
Reverse electron current in the absence of
trapped electrons

• Highest efficiency by operating at the highest
  Te, the lowest ne and by choosing the fast ion
  energy close to the critical energy Ec
• Beam driven current depends on power deposition
• 1MA with 11MW NBI on TFTR

4
Lower Hybrid (LH) Heating
For LH frequency domain of
Dielectric tensor elements,
The uncoupled slow wave dispersion relation in
the LH domain
Cutoff at the so-called perpendicular Alfven
resonance
gives propagation requirement,
5
LH Wave Accessibility Condition

• Critical value for n// from Stix-Golant
  Accessibility

• Launcher wavelength ?//,

accessibility or penetration

• low density plasmas at high magnetic field
• high parallel phase velocity

peaked profile
6
LH Heating/Current Drive

• Grill structure of wave guides with 90o phasing
  in successive toroidally adjacent wave guides
• --gt Landau damping at parallel phase velocity
• asymmetric resistivitydue to accelerated
  electrons (75 of the driven currents)

7
Experimental Low Hybrid Current Drive Efficiency

• Current drive efficiency

• Figure of merit ?,

• Full current drive of 3.6MA with 8.3MW LHCD
• at 1x1019m-3 on JT-60
• ?0.4 with LHCDFW

8
Fast Wave Current Drive

• Current drive by fast wave is similar to LHCD in
  that the waves damp at the Landau resonance by
  forces from both wave electric fields(Landau
  damping) and the gradient of wave magnetic
  fields(transit-time damping).
• Theoretical efficiency of FWCD is slightly
  greater than LHCD for the same condition since
  the fast waves damp on electrons with greater
  perpendicular velocity and reduced
  collisionality.
• FWCD has no penetration problem with the high
  plasma densities!

• Experimental achievements are relatively low
• 100kA by typically 1MW (DIII-D, Tore
  Supra)
• ?0.03 with waves of lower parallel refractive
  index
• (higher phase velocity)

9
Fast Wave Minority Ion Current Drive

• Current drive by asymmetric heating of minority
  species of ions with different charge to mass
  ratios

• Similar efficiency to NBCD

• Effect of minority current drive on the
  stability of sawtooth oscillations on JET

10
Theoretical Current Drive Efficiency
Electron Cyclotron Current Drive

• Dipolar current density profile

• Current drive efficiency

11
Prospects for Current Drive in Tokamaks
There exists a wide range of non-inductive
current drive methods possessing a variety of
different characteristics in terms of
accessibility, efficiency, plasma coupling, and
unidirectional or bipolar currents.
Experimentally the schemes are at different
stages of development with LHCD having produced
the largest current so far. The measured
efficiencies are in excellent agreement with
theoretica1 calculations which show that values
of ? (figure of merit)of the order of unity
should be possible with highly relativistic
electrons and high electron temperatures.
However, even with this level of efficiency the
provision of full current drive by an external
system would require a recirculating power of at
least 20 of the total power output. This
recirculating power is considered too large for a
reactor and so a non-inductively-driven tokamak
would rely on a large fraction of the current
being carried by the bootstrap current.