Effect of Helical Magnetic Field Ripples on Energetic Particle Confinement in LHD Plasmas

1
Effect of Helical Magnetic Field Ripples on
Energetic Particle Confinement in LHD Plasmas

• T.Saida, M.Sasao, M.Isobe1, M.Nishiura1,
  S.Murakami2, K.Matsuoka1, A.V.Krasilnikov3,
  M.Osakabe1
• and LHD experimental group
• Department of Quantum Science and Energy
  Engineering, Tohoku University, Sendai, Japan
• 1National Institute for Fusion Science, Toki,
  Japan
• 2Department of Nuclear Engineering, Kyoto
  University, Kyoto, Japan
• 3Troitsk Institute for Innovation and Fusion
  Research, Troitsk, Russia

2
Outline of talk

• Motivation
• 2. Diagnosis system
• 3. Measurement results
• 4. Numerical analyses
• 5. Summary

3
Energetic ion orbits in Tokamak Heliotron

• Passing particle
• Trapped particle

• Passing particle
• Locally trapped particle
• Helically trapped particle

• Transition particle

4
Motivation
The improved performance for confinement of
energetic trapped particles is expected to be
obtained by optimization of magnetic
configurations

in heliotron.

• Need to demonstrate the expected confinement of
  the energetic trapped particle experimentally

Compare to the energetic particle confinement
at three different magnetic axes
Rax of 3.53, 3.6 and 3.75m in LHD
How about other particle orbits?
Inject neutral beam ions tangentially Measure
ions with perpendicular pitch angle
Pitch angle scatterings
The confinement of the other particle orbits can
be investigated.
5
Magnetic structure and energetic trapped particle
orbit
Rax3.53m
Rax3.6m
Rax3.75m
Drift surface of trapped particle
Vacuum magnetic flux surfaces
r/a0.5
r/a0.5
r/a0.5
It is predicted that the magnetic configuration
at Raxof 3.53m gives
the improved confinement of energetic
trapped particles.
6
Diagnosis system fast neutral measurement
NBI1
Rtan3.75m
No significant differences in NDD line-of-sight
at R ax of 3.53, 3.6, 3.75m
Rtan3.6-3.65m

• NBI systems

R3.68m
Natural Diamond Detector (NDD)
PHA mode
7
Initial pitch angle of energetic beam ions and
pitch angle of measured ions
Pitch angle at ionization points of tangentially
ctr.-injected NB
Pitch angles of particles reaching NDD
Rtan3.75m
Slowing down
deflection
NDD measures partially slowed down, the pitch
angle scattered perpendicular ions.
Rtan3.6-3.65m
Do NB depositions have the influence to the
particle confinement?
8
CX neutral flux and spectra at three different
configurations
9
Electron density dependence of CX neutral spectra
Estimate the effective temperature as a
function of slowing down time by taken into
account of NB deposition.
10
Effective temperature Teff
Plot effective temperature Teff as a function of
slowing down time ts
by taken into account of NB deposition positions

• Saturation value of Teff at 3.75m is the smallest
  in all cases.

• In the NBI1 and 3 case, saturation value of Teff
  at 3.6m is the largest.
• No significant difference between 3.53 and 3.6m
  is observed.
• There are no significant difference on NB
  depositions.

11
Numerical approach (Lorentz orbit code)
Calculation condition

• Magnetic configurations at Rax of 3.53, 3.6 and
  3.75m with Bt of 2.5T

• Proton with energy of 75keV and pitch angles of
  90-130 deg.

• Calculate without collisions time-backwardly from
  starting points

• Classify orbit types of energetic particles from
  the topology

Regard particle crossing over last closed flux
surface (r1) as lost particle

• Estimate the confinement region

12
Orbit topology of confined particle

• Passing particle

• Transition particle

• Helically trapped particle

• Locally trapped particle

13
Orbit classification

• No significant difference between Rax of 3.53 and
  3.6m
• The confinement at Rax of 3.75m is not improved.

14
Confinement region

• Confinement region at 3.6m is the largest among
  three configurations.

• Magnetic configuration at 3.6m has the largest
  plasma volume.

The tendency is consistent with that of
saturation value of Teff
15
Summary

• Investigate energetic particle confinement among
  three configurations experimentally

Rax3.53m
Rax3.6m
Rax3.75m
Experimental results
No significant difference
Poor confinement
(Saturation value of Teff at 3.6m
is the largest)
No significant difference on NB deposition is
observed.
No significant difference
Poor confinement
The largest confinement region
(in the case of LHD)