Robust Operation above the Ideal MHD
No-Wall Stability Limit
By
H. Reimerdes1
In collaboration with
J.W. Berkery1, A.M. Garofalo2, Y. In3, R.J. La Haye2,
M. Okabayashi4, S.A. Sabbagh1 and E.J. Strait2
1Columbia
University, New York, NY
Atomics, San Diego, CA
3FAR-Tech, Inc., San Diego, CA
4Princeton Plasma Physics Laboratory, Princeton, NJ
2General
Presented at
ReNeW Themes I and II Joint Workshop
San Diego, March 23-27, 2009
Columbia
University
Identification of the critical issue:
Resistive Wall Mode Stabilization in an AT reactor
•
Resistive wall mode stabilization is a pre-requisite for the advanced
tokamak path to a compact, steady-state fusion reactor1
– High fusion power density
➞ High βT
➞ High βN
– High bootstrap current fraction
➞ High βp
– Good bootstrap current alignment ➞ Low li
➞ Low βN,nowall
}
1. Passive (rotational) stabilization
– Most attractive RWM stabilization strategy
2. Active stabilization using magnetic feedback
– Promising backup, if passive stabilization fails or not sufficiently robust
3. Amplification of non-axisymmetric fields by stable RWM
– Has to be addressed even if RWM stabilization successful
A.D. Turnbull, et al., Phys. Rev. Lett (1995)
1
H. Reimerdes, ReNeW Workshop, March 23-27, 2009
Columbia
University
Identification of the critical issue:
1. Passive RWM stabilization
•
Observed in many devices (DIII-D, JET, JT-60U, NSTX)
– Related to rotation (also referred to as rotational stabilization)
– Kinetic effects thought to be important2,3
•
Stability boundary not well-understood
– Boundary more complex than a critical rotation4
– Recent observations5/calculations6 indicate link to fast particle
population (from neutral beam injection heating)
A. Bondeson, M.S. Chu, Phys. Plasmas (1996)
Bo Hu, R. Betti, Phys. Rev. Lett. (2004)
4
H. Reimerdes, et al., 50th DPP Meeting, Dallas 2008
5
M. Okabayashi, et al., 22nd IAEA FEC, Geneva 2008
6
J.W. Berkery, et al., 50th DPP Meeting, Dallas 2008
2
3
➜ Need to acquire an understanding of the passive RWM stabilization
mechanism, which is sufficient to extrapolate to a fusion reactor
H. Reimerdes, ReNeW Workshop, March 23-27, 2009
Columbia
University
Identification of the critical issue:
2. Active RWM stabilization using magnetic feedback
•
Transient RWM feedback control demonstrated in tokamaks
– Current driven RWM (transient instability in current ramps)7,8
– Pressure driven RWM (transient instability when passive stabilization
not sufficient, coupling to tearing at low rotation)9,10
•
Sustained RWM feedback control demonstrated in reversed field pinches
(RFPs)11
– Current driven mode remains unstable throughout discharge
– Resonant surfaces outside the plasma (no coupling to tearing modes)
C. Cates, et al., Phys. Plasmas (2000)
8
Y. In, et al., 50th DPP Meeting, Dallas 2008
9
E.J. Strait, et al., Phys. Plasmas (2004)
7
S.A. Sabbagh, et al., Phys. Rev. Lett. (2006)
P.R. Brunsell, et al., Phys. Rev. Lett. (2004)
10
11
➜ Need to
– Demonstrate sustained RWM feedback control in low rotation, high
beta plasmas
or
– Understand how RFP results are transferable to a reactor
H. Reimerdes, ReNeW Workshop, March 23-27, 2009
Columbia
University
Identification of the critical issue:
3. Amplification of non-axisymmetric fields by stable RWM
Increasing amplification of externally applied non-axisymmetric fields
(error fields and intentionally applied perturbations) at high beta leads to
a reduction of the error field tolerance12
– Plasma amplifies kink-resonant component of external field, which
can change as discharge evolves
– Self-heated plasma will not have the beneficial effect of NBI torque
decreasing the tolerance further
•
H. Reimerdes, et al., 22nd IAEA FEC, Geneva 2008
12
➜ Need to develop a reliable (and non-disruptive) error field correction
strategy
H. Reimerdes, ReNeW Workshop, March 23-27, 2009
Columbia
University
Specification of the requirements for resolving the issues:
RWM stabilization strategy for a reactor requires validated models
• Understanding of wall-stabilization can be gained by comparison of
stability limits/stability measurements with improved model predictions
– Include dependencies on parameters that will differ in a reactor
(e.g. rotation, ratio of Te and Ti, fast particle content and distribution)
• Demonstration of active FB control in high beta scenarios requires the
identification of robustly unstable scenarios
– In the absence of such a scenario the development and validation
of advanced feedback schemes can be carried out on transient
modes and RFP experiments
• Developing error field correction strategies for ITER requires improved
detection schemes
– Use interaction of non-axisymmetric fields with the plasma to detect
various components of the error field
H. Reimerdes, ReNeW Workshop, March 23-27, 2009
Columbia
University
Description of the physics thrust:
Continued research in enhanced existing machines
• Medium size tokamaks are well suited for the study of stability limits, since
they tolerate frequent disruptions (inherent to studies of stability limits)
• Extended comparison of passively stable, high beta plasmas with
improved kinetic models
– Extend parameter regime (incl. fast particle contribution, n>1 RWMs)
– Improve linear modeling (incl. finite resistivity, pol. rotation)
– Assess necessity for non-linear modeling
• Enhance diagnostics to measure quasi-static toroidal asymmetries
– Requires multiple toroidal locations
• Add electron heating to test RWM stability with reactor relevant
distribution functions
• Test advanced feedback algorithm on experiments for feedback model
validation
• Develop real-time stability measurements and/or calculations (prerequisite for reactor operation in the wall-stabilized regime)
H. Reimerdes, ReNeW Workshop, March 23-27, 2009
Columbia
University
Description of the physics thrust:
Extend the ITER AT operational regime/capabilities
No wall limit
10
Ideal wall limitg ABM p9 d9
6
Growth rate (s-1)
105
growth rate [1/s]
• Steady-state scenario (scenario 4)
with βN=2.57 uses only a small
fraction of the potential performance
gain through RWM stabilization
g PBM
VALEN: ITER with blanket (ports not passive
covered)
intc g 005PBM e+9
104
Passive growth rate
(RWM)
103
102
101
100
ITER scenario 4
10-1
2.5
3
3.5
Feedback with
6 external coils
4
4.5
ββnN
5
5.5
6
tarragona.2005
Feedback with
7 internal port coils
[J. Bialek, et al., 32nd EPS Conference, Tarragona, 2005]
• Preserve AC and n=1 capabilities of the internal and possibly the external
non-axisymmetric coils sets
H. Reimerdes, ReNeW Workshop, March 23-27, 2009
Columbia
University