JT-60U Resistive Wall Mode (RWM) Study on JT-60U Go Matsunaga 松永 剛 Japan Atomic Energy Agency, Naka, Japan JSPS-CAS Core University Program 2008 in ASIPP

1Go Matsunaga

JSPS-CAS Core University Program 2008 in ASIPP
Plasma and Nuclear Fusion
G. Matsunaga JAEA, CUP in ASIPP
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Outline
Introduction
RWM in high-b plasmas
G. Matsunaga JAEA, CUP in ASIPP
JT-60U
Introduction
Toward fusion reactors, the high-bN operation is very attractive and advantageous, because high bootstrap current (fBS) and high fusion output (Pfus) are expected.
Finite wall resistivity makes another mode,
Resistive Wall Mode (RWM)
that limits achievable bN.
However, achievable bN is limited by low-n MHD instability.
No-wall bN-limit (bN=bNno-wall ->Cb=0)
Ideal-wall bN-limit (bN=bNideal-wall ->Cb=1)
Therefore, RWM stabilization is a key issue for high-bN operation in ITER and a fusion reactor.
Device
Size
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RWM behaviors
Error field effect
JT-60U
Positive ion based NBs (PNB)
4 tangential
CO ~ 4MW
CTR ~ 4MW
7 perpendicular
2 tangential
CO ~ 4MW
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JT-60U
Current driven RWM experiments
In order to investigate wall location effect on MHD instability, plasma-wall gap scan has been performed in OH plasma.
→ since only q-profile can determine the stability, wall effect can be clearly measured.
To destabilize current driven external kink mode, surface q was decreasing by plasma current ramping up.
G. Matsunaga JAEA, CUP in ASIPP
JT-60U
m/n=3/1 Current driven RWM is observed
qeff was just below 3, m/n=3/1 instability appeared and thermal collapse occurred.
The growth time of this mode is about 10ms.
→ On JT-60U, tw is several milliseconds.
Current driven RWM
JT-60U
G. Matsunaga, PPCF, Vol. 49, p.95(2007)
Wall stabilizing of current-driven kink mode on OH plasma
G. Matsunaga JAEA, CUP in ASIPP
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RWM growth rates vs. wall location
Increasing d/a, RWM growth rate increased.
According to AEOLUS-FT with taking into account a resistive wall, m/n=3/1kink and m/n=2/1 tearing modes are unstable.
The dependence qualitatively agrees with RWM dispersion relation without plasma rotation.
G. Matsunaga et al., PPCF, Vol. 49, pp.95-103 (2007)
m/n=2/1
JT-60U
JT-60U
Identification of critical rotation for RWM stabilizing
To identify critical plasma rotation for RWM stabilization, we only changed plasma rotation.
At 5.9s : Stored energy FB was started
→ keeping bN constant
At 6.0s : Tang NBs were switched from CTR-NB to CO-NB
→ slowly reducing Plasma rotation
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High-b RWM was observed by reducing plasma rotation
Just before collapse, n=1 radial magnetic field was growing with ~10ms growth time.
→ RWM
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Plasma rotation profiles
Since bN was kept constant, deceleration of plasma rotation was thought to make the RWM unstable.
Focusing on the plasma rotation at the q=2, critical plasma rotation is less than 1kHz.
This value is corresponding to 0.3% of Alfvén velocity.
G. Matsunaga JAEA, CUP in ASIPP
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JT-60U
Dependence of critical rotation on Cb
Target value of stored energy FB was changed to get the dependence of the critical plasma rotation.
The dependence of the critical rotation on Cb is weak.
This means that we can sustain the high-βup to the ideal wall limit.
G. Matsunaga JAEA, CUP in ASIPP
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JT-60U
JT-60U
Previously, on JT-60U, the high-bN plasmas > bNno-wall were transiently obtained.
In this campaign, we have tried to sustain the high-bN plasma > bNno-wall with plasma rotation larger than Vtcri.
We have successfully obtained the high-bN plasma for several seconds.
G. Matsunaga JAEA, CUP in ASIPP
JT-60U
On the best discharge,
bN~3.0 (Cb~0.4) was sustained by plasma rotation > Vtcri.
Sustained duration is ~5s, which is ~3 time longer than tR.
Time duration is determined by the increase of bNno-wall due to gradual j(r) penetration.
According to ACCOME, fCD80% and fBS~50% were also achieved.
~5s (~3tR)
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However, the sustainment of high-bN is not straightforward.
Because almost all discharges were limited by
Resistive Wall Mode (RWM)
Neoclassical Tearing Mode (NTM)
Energetic particle driven Wall Mode (EWM)
directly induces RWM despite Vt > Vtcri
RWM Precursor
finally, induces RWM onset
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EWM can directly induce RWM
In the wall-stabilized high-bN region, Energetic particle driven Wall Mode (EWM) is newly observed.
At RWM onset, rotation was enough for stabilization.
The EWM is dangerous for RWM
n=1
n=1
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Poloidal mode number : m~3 (Kink Ballooning-like)
Radial mode structure : globally-spread
Growth time : 1~2ms
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Mode frequency is chirping down as mode amplitude is increasing.
Initial mode frequency agrees with the precession frequency of the energetic particles from the PERP-NB.
G. Matsunaga JAEA, CUP in ASIPP
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Hot pressure of PERP-NB seems to drive
EWM is stabilized by reducing PERP-NB injection power while keeping bN constant.
→ Driving source is trapped energetic particle pressure
Dbh/btotal ~ -10%
JT-60U
The EWM were observed in high-bN plasmas.
However, the EWM requires Cb>0, NOT only high-bN.
Cb>0, bN<3.0
JT-60U
EWM stability domains
If the no-wall b limit is changed by j(r), EWM is always destabilized above the no-wall b limit.
Increasing plasma rotation, EWM boundary seems to follow it.
→ EWM has a similar stability to RWM
G. Matsunaga JAEA, CUP in ASIPP
JT-60U
Summery
RWM is a key issue in an economical aspect for future fusion reactors.
On JT-60U, RWM has been well studied;
Current driven RWM → Wall location effect,
High-b RWM → Plasma rotation stabilizing,
Instability related to RWM → Coupling to energetic particles.
JT-60U has been shut down in last August. We must wait for JT-60SA for further RWM study.
Our corroborations become important!
JT-60U
and
JT-60U
Wall location effect
External coils
Feedback control
Neutral Beam
High-b RWM
JT-60U
M. S. Chu et al., Phys. Plasma, Vol. 11, p.2497(2004)
M. S. Chu et al., Phys. Plasma, Vol. 2, p.2236(1995)
Wall
Skin
Time
Kinetic
Energy
Integral
Plasma
Potential
Energy
Vacuum
Energy
with
Resistive
Wall
Dissipation
Energy
Integral
JT-60U
Plasma rotation stabilizing effect on RWM
Some models predict that the critical rotation is several % of Alfven speed at the rational surface.
→ Dissipation and rotation are required for RWM stabilization.
How much is the critical rotation for RWM stabilization?
Future devices will have low plasma rotation.
G. Matsunaga JAEA, CUP in ASIPP
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EWM is originated from energetic particles and marginally stable RWM.
m/n=1/1 Internal-kink
Kinetic contribution
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Ideal MHD analysis by MARG2D
This mode is unstable w/o wall, however, stable with ideal wall.
The mode structure is localized in the LFS
→ Kink-Ballooning mode structure
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