Recent Results from Helicity Injection Experiments on HIST M. - - PowerPoint PPT Presentation

Recent Results from Helicity Injection Experiments

• n HIST
• M. Nagata, H. Hasegwa, K. Kawami, T. Takamiya, Y. Kagei
• N. Fukumoto and T. Uyama

Graduate school of Engineering, University of Hyogo

Contents

• Background and Objectives
• Hilights from helicity injection studies on HIST

(Comparison between Spk and ST, Formation and sustainment of flipped ST)

• Comparison with 3D MHD simulation results
• Summary

# The university name was changed from Himeji Inst. of Tech.. 10th International ST workshop 29th Sep. -1st Oct. 2004, Kyoto, Japan

Ψbias Flux Conserver Central Conductor Magnetized Coaxial Gun Bias Flux Coil Open Flux

Kinj=2ΨbiasVg

Ig

Coaxial helicity injection (CHI) technique was introduced to classical spheromaks and spherical tokamaks to start-up and sustain a plasma current. The ability of CHI to drive a current has been already examined and the related MHD relaxation have been observed in many spheromak/ST devices. Central open flux column

Closed flux Helicity Injection Spherical Torus System

Helicity Injection Experiments and the Underlying Physics

SSPX NSTX HIT-II HIST

Plasmoid ejection, Helical twist, Magnetic reconnection, Rotation

Solar flares

Common Relaxation Phenomena Observed between Laboratory and Astrophysical Plasmas

Kink behavior created by the guns

Helicity injection experiments are also useful to study similar MHD activities

• bserved in space plasmas.

Yohkoh Hsu and Bellan、PRL

HIST

Increase in MHD activity, self-organized properties and large scale fluctuations Increase in classical diffusion, stability and small scale fluctuations TF coil current

Itf

R0 /r

0.5 1 2

q

• 0.5
• 1

Spherical Tokamak

Spheromak Spherical RFP

New Prediction: (see Dr. Kanki’s presentation) Diamagnetic high beta low-q STs with two fluid effect may be generated by driving fast flow by CT injection.

Diamagnetic Low q ST FRC

Dynamics of driven spherical system

Whether do ST plasmas collapse or survive after they pass through the rational barrier ?

T F

V

g

Iinj

tim e tim e

1) Comparison study of MHD activites between SPK and ST during CHI current drive. 2) To see what happens to ST by a rapid reversal of TF.

How to Approach to Understand Generic Properties of MHD Relaxation in Helicity-driven Toroidal System

Various utilizations of TF coil current in a single machine

Spheromak High-q ST RFP

• B. Future works
• A. Present works

m=1/n=1 rational barrier

Low-q ST Two fluid Low-q ST Flipped ST

(d) Spherical RFP

λ>λc Ψ

t.e <0

(c) Flipped ST

λ<λc Ψ

t.e<0

Bt. (b) ST

λ<λc Ψ

t.e >0

Itf

Ψ

t.e

Ψ

bias>0λ≅0 Ψ t.e >0

(a) near a Vacuum Field

Magnetized plasma Gun Flux conserver

ST Flipped RFP Flipped ST λ0 =8.53 (Eigenvalue) Flipped Sph Sph RFP

λ

Itf > 0 Itf < 0 Itf = 0

Sequence of poloidal flux topologies of driven plasmas as λ is increased from zero to above the eigenvalue λc

Helicity-driven Relaxation Theory Predicts the Existence of Flipped ST States in the Regime of TF < 0

∇×B = λB

HIST and Diagnostics

Injection Current 20 kA, Injection Voltage < 600 V Bias Flux < 5 mWb TF coil current < 0.25 MA R = 0. 30 m a = 0.24 m A =1.25

Gas Puff Valves (4) Insulator Inner Electrode Vacuum Vessel Outer Electrode Central Conductor

T

• roidal Field Coil

Inner Bias Coil V ertical Field Coil Outer Bias Coil Flux Conserver

Itf

Sustainment Bank Formation Bank

Iinj

100 80 60 40 20 2 4 1 3 5 Time (ms) 2 4 6

# 12 672

(101 9 m -3) (kA) It

n e

Surface Magnetic Probe(B

t,B p)

Rogow ski andFluxProbe

z=-74m m

MagneticProbe

1 26 19 8 12 15 5 22 17 10

(Br,Bφ,B

z)

M agnetizedCoaxial PlasmaGun Central Conductor T

• roidal

FieldCoil Spherical Flux Conserver CO

2Laser Interferom

eter

z=0m m

Toroidal modeprobe (8 chs)

It < 150 kA ∆t = 4 - 8 ms ne = 2 - 8 x 1019 m-3 Te ~ Ti =20 -40 eV

ST operation

• M. Nagata, et al., Phys. of Plasmas 10, 2932 (2003).

Comparison of Magnetic Fluctuations between Spk and ST

Intermittent generation of the toroidal current at the magnetic axis was observed in both operations.

Spheromak Spherical Tokamak

n=1 kink mode and its rotation n=0 mode dominant

Toroidal current Current density

• n the magnetic axis

n = 0 n = 1

Phase of n =1

Flux amplification/current generation in the spheromak case is associated with n=1 MHD activity. In the other hand, that in the ST is associated with repetitive merging of plasmoid injected from the gun which we proposed as a model of current drive so far. Current generation on axis Ti >Te Ti ~Te Itf = 0 Itf >> 0

Kruskal Shafranov limit t=0.340 ms t=0.365 ms time

Kink mode is unstable

Evidence of Rotating Kink Behavior Driven by MCPG

1 2

2 2

>

g c t c

I R I R λ π

vZ = 30 [km/s] , vR = 18 [km/s] Exponential growth of the kinked central column with the E×B toroidal rotation Kinked central open flux Fluctuations

• f v and B

Current drive MHD dynamo

E + ˜ v × ˜ B = η j

〉 〈 〉 〈 B v ~ , ~

0.2 0.4 0.6 0.8 1 200 400 600 800 1000 1200 1400

It

10-2 10-1 100 200 400 600 800 1000 1200 1400

n=0 n=1

Wmag [a.u.] t (τA)

Dynamo Drive of Spk Demonstrated by 3D MHD Simulation

Sustainment

Toroidal mode n=0, 1

Nonlinear evolution (Growth, nonlinear saturation and the following relaxation ) of the kinked flux column produces dynamo electric field.

Resisitve decay

Closed flux surfaces are identified only as mean fields. Edynamo = <ve×B> ~ ~

# In collaboration with Y. Suzuki and Y. Kishimoto, JAERI

2 4 6 200 400 600 800 1000 1200

Einj t

[ 10-3 ] ×

0.2 0.4 0.6 0.8 1 200 400 600 800 1000 1200

It t

10

• 5

10

• 4

10-3 10-2 10

• 1

10 200 400 600 800 1000 1200

n=0 n=1 n=2 n=3 n=4

Wmag t

Poincare plot of magnetic field at the time when the magnetic energy in the n = 1 mode gets down to ~10-4

Multi-Pulse Helicity Drive is effective for suppressing the n = 1 fluctuation. Decaying phase Driven phase

Gun voltage on Gun voltage off

Improvement of confinement quality.

Multiple Pulse Operation for Improvement of Spheromak Confinement

Closed flux surfaces are produced during the decay phase. Chaotic scattering

• f field lines

Plasmoid Ejection is Key Dynamics for Formation of ST

Stabilization of kink instability by TF

Magnetic reconnection point can be clearly identified.

Reconnection layer ~Ion skin depth ~ 3.2 cm, Electron skin depth ~ 0.07 cm, Ion-gyroradius ~ 0.4 cm

In the ST case, the global relaxation like Taylor type does not seem to

• ccur and it becomes more important

to understand local features of reconnections around the X (null)- point.(Ti >Te ? at X-point, Ti ~Te in the core region) Two-fluid reconnection theory may become important.

Plasmoid ejection speed ～60km/s

# M. Nagata et al. Phys. Rev. Lett. 90, 225001 (2003)

Self-reversal process

Observation of Self-reversal of Magnetic Fields by Reversing TF ; Relaxation from the ST toward the Flipped ST State.

Note that not only toroidal flux but also poloidal flux reverses the sign spontaneously during the relaxation process.

0.2 0.4 0.6 0.8 1

n mode (kG)

n=0 n=1 n=2 n=3

0.5 0.6 0.7 0.55 0.65 0.75

0.39 0.00

• 0.21

0.21

Time (ms)

(d)

R (m)

0.105 0.173 0.241 0.309 0.377 0.445

Jt (MA/m-2)

(c)

• 0.4
• 0.2

0.2 0.4

• 70
• 50
• 30
• 10

10 30 50 70

Bt.e , <B > (kG)

t.core

It (kA)

Edge toroidal field Core toroidal field Toroidal current (a) Shot #4586

Flipped ST Large growth of the n=1 kink mode RFP ST +Jt

• Jt

# In collaboration with S. Masamune, Kyoto Inst. of Tech. and M. Katsurai, U of Tokyo

A B C E F D

3D MHD Simulation of Self-organizing from ST to F-ST Relaxed States

Spontaneous reversal of not only toroidal but also poloidal flux.

The system relaxes to a lower energy state by rearranging current distribution. The parallel current profile λ becomes

• peaked. Kink of the central open flux is essential to the self-

reversal process.

2 4 6 8 10 12 0.2 0.4 0.6 0.8 1.0

λ r

t=0 t=20 t=80 t=485 t=820

Magnetic reconnection between the open and closed field lines. # Y. Kagei et al. PPCF, 45, L17 (2003) # In collaboration with Y. Suzuki and Y. Kishimoto, JAERI and T. Hayashi, NIFS

Fast Camera Images Display Kink instability around the Center Conductor during the Current-reversal Process

(g) (h)

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

• 20

20 40 60 Time[ms] It[kA ] 0.33 0.34 0.35 0.36 0.37 0.38

• 15
• 10
• 5

Tim e[ms] It[kA ] 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

• 40
• 30
• 20
• 10

10 Ig[kAturn]

(a) (b) (c) (d) (e) (f)

RF-ON

#10152

0.340ms 0.345ms 0.350ms 0.355ms 0.360ms 0.365ms 0.370ms 0.375ms

TF It

Unique magnetic field lines geometry: Bt: opposite direction, Bp: same direction

Key Question; Can We Sustain the Flipped ST plasmas in Spite of No Central Open Flux ?

Non-flipped region Flipped region

q ~ Itf/It >1 Iinj > 2Itf >2It.

No Magnetic Reconnection But, the F-ST is isolated from the electrodes, so can we drive it by helicity injection?

Ejection condition: Iinj > 2Itf

The F-ST configuration is consisted of only closed flux surfaces so that it may have a better confinement quality ! ? How to drive current? A key point is to cause the kink deformation

• f the non-flipped field lines.

Large injection current is required to sustain a large plasma current in the F-ST.

• It

Bt

• It

B p

Tim e [m s] 1 .5 1.6 1.7 1.8 1.9 2.0 0.3 0.0

• 0.2

0.2 0.1

• 0.1

40 20

• 20
• 40

Tim e [m s]

• 40
• 20

20 40 # 4 3 0 9 Su stainm ent p hase Quiescence phase Reversal phase ST formation ph ase 0 .5 1.0 1.5 2 .0 2.5 3 .0

Dynamo Current Drive of F-ST Plasmas by Kink behavior of Non-flipped Open Flux

Inboard Outboard Time (ms) Drive current

Non-reversed region

Toroidal current density

CT injection experiment into a small torus chamber to study two-fluid effect and the m=1 helical relaxed state of RFP plasmas. We have reviewed the MHD relaxation observed in the driven system by varying TF.

The rotational kink behavior of the open column (central or outboard open flux) is common basic feature of dynamo activities, which plays a major

role in CHI current drive in spheromak and low-q ST plasmas. 3D MHD simulation investigated the nonlinear evolution of the kink instability during sustainment and revealed its dynamo drive. Plasmoid ejection and the following magnetic reconnection process may play an important role in the formation and sustainment of the ST.

Summary

Future plans

An OH coil and a new FC with cut will be installed on the HIST machine.