Recent ICRF Experiments on the GAMMA 10 tandem mirror

M. Ichimura, Y. Yamaguchi, G-10 ICRF GroupPlasma Research Center, University of Tsukuba,Workshop on Physics of Wave Heating and Current Drive 2008.1.14-15, NFRI, Korea

Contents1.Introduction2.Higher density plasma production with phased antennas3.Evaluation of parametric decay of ICRF waves4.Summary

ICRF:Plasma ProductionIon HeatingMHD Stabilization

ECH:Potential FormationElectron Heating MHD Stabilization

1. Introduction

GAMMA 10 is an axisymmetrized tandem mirror with minimum-B anchors

GAMMA 10 Device

Z

Central cellEast anchor cell

West plug/barriercell

East plug/barriercell

West anchor cell

ICRF

B (T)

3210

ECHECH

Potential

RF1 System(fast waves)9.9, 10.3 MHz RF2 System(slow waves)

6.36 MHz

プラズマ

RF1RF2RF3

300kW ×2300kW ×2

200kW

Max. power Max. duration Frequency range

500ms500ms500ms

7.5 - 154.4 - 9.636 -76

MHzMHzMHz

Plasma

Schematics near the west end of the central cellICRF antenna and gas puffing system

ICRF systemsin the central cell

Nagoya Type IIIantenna

Double Half Turn antenna

changed to4-10MHz

Photograph near west end of the central cell

DHT Antenna

Nagoya TypeIII Antenna

Gas Box

Diamagnetic Loops

0

1

2

3

0 2 4 6

Central Cell West Anchor Cell

Type-IIIAntenna

(RF1,RF3)

B-F

ield

Str

engt

h [T

]

Z-axis [m]

DHT(RF2,RF3)

anchor midplane

midplane

0

1

2

3

4

5

0 2 4 6ω

/ Ω

ciZ-axis [m]

RF1

RF3

RF2heating

productionheating

production

Three ICRF systems (RF1, RF2 and RF3) are now in operation. Principally, Type III are driven by RF1 system and are used for the plasma production in the central cell and the ion heating in the anchor cell.The frequency is adjusted to the ion cyclotron frequency at the midplane of the anchor cell.

DHT are driven by RF2 system and used for the main ion heating in the central cell. The frequency is adjusted to the ion cyclotron frequency at near the midplane of the central cell.

In near future, the frequency of RF3 will be changed to 4 – 10 MHz for plasma production and heating.

ICRF antennas, magnetic field strength and cyclotron frequency

2. Higher density plasma production with phased antennas

In the GAMMA 10 tandem mirror, the improvement of axial confinement has been attained with potential formation

E & WPlug ECH

0 100 200Time(ms) Time(ms)

ECH

0 100 200

Density clamping is sometimes observed in ICRF produced plasmas ~ 2.5 times 1018 m-3

To extend the operation region, higher density plasma production is needed.

Motivation

Mechanism for the density clamping is considered that the eigenmode formationIn the present parameter region, that is, small plasma size, low density and the frequency near ω / ωci = 1, one radial eigenmode is only excited.There is a boundary in the axial direction. Axial eigenmodes have the optimum density discretely. The density clamping will be possible at such optimum value.

0 100 200 300

Line

Den

sity

[1

013 c

m-2

]

RF power for production [kW]

5

0

We are trying:Inject of large amount of gasPellet injectionUse of high harmonic fast waves Use of phased antenna array

Experimental configuration and results

RF1 System9.9 MHz

RF2 System6.36 MHz

Nagoya Type IIIantenna

Double Half Turn antenna (DHT)

10.3 MHz

Phase shifter

RF2 RF1DHT

DHT

Phase control

Δφ = 0 deg

Δφ = 180 deg

Antennas in east side are fixed.Antennas in west side are driven with the same frequency and the phase between west DHT and TypeIII antennas is controlled.

RF current flows

The dielectric tensor is cold plasma approximation including collisionsTwo dimensional code (Axi-symmetric mirror configuration)

wave propagation analysis in the inhomogeneous plasmaby finite element method（FEM) ( developed by Prof. A. Fukuyama, Kyoto Univ.)

B-field Density Axial Profile of B Radial Profile of n

Wave propagation analysis in the cylindrical plasmaBackground

0.00

0.02

0.03

0.05

0.07

0.09

0.10

0.12

0

1 2

3

4 5

6 7

8 9

0

2000

4000

6000

8000

AMPLITUDE[arb.]

K // [arb.]

DENSITY[×10^18 m^-3]

0-2000 2000-4000 4000-6000 6000-8000

k// increases with densityAt first, a fast wave with a fundamental radial mode is excited.

n=0.2 n=2.0 n=6.4 1018 m-3

Eigenmode formation in the axial direction

0

200

400

600

800

0 1 2 3 4 5

E-DHT and W-DHTE-DHT and W-TypeIIIW-DHT and W-TypeIIIE-TypeIII and W-TypeIII

Wav

e A

mpl

itude

(Er)

[a

.u.]

Electron Density [1018 m-3 ]

0

5

10

15

20

25

30

0 1 2 3 4 5

West TypeIII (Irf = 1 A)

+ East TypeIII (Irf = 1 A)

Wav

e A

mpl

itude

(Er)

[a

.u.]

Electron Density [1018 m-3 ]

West Type III (Irf = 1.414 A)

Wave amplitude depend strongly on the antenna configurationMultiple antenna is better for wave excitation

East and West TypeIII with Irf = 1 AWest TypeIII with Irf = 1.41 A

DHT antenna is more effective for the fast wave excitation than TypeIII antenna

Both DHT antennas with Irf = 1 ABoth TypeIII antennas with Irf = 1 A

Experimental results

10.3 MHz Phase shifter

RF2 RF1

Phase control

could not produce plasma

Optimum phase is almost zero

-180 -90 0 90 180Phase difference [deg.]Li

ne D

ensi

ty

[1013

cm

-2 ] 6

3

0

0 100 200 300 400

Line

Den

sity

[1

013 c

m-2

]

RF power for production [kW]

10

5

0

only Type III antenna

Type III + DHT antenna

201177 0 deg.201178 90 deg.201180 -90 deg.201181 -45 deg.201173 180 deg.

Line

Den

sity

[1

013 c

m-2

]

Time [ms]

6

3

00 50 100

Fixed at optimum phase and increased production power

3. Evaluation of parametric decay of ICRF wavesMotivationSaturation of diamagnetism is sometimes observed when ICRF heating power is increased

Possible reasonsDegradation of confinement

Macro- and micro-instabilities

Parametric decay of ICRF waves into spontaneously excited waves

AIC waves and Low frequency (LF) waves

In GAMMA 10, Alfvén ion cyclotron (AIC) waves are excited due to a strong temperature anisotropy.

AIC waves excited in the central cell of GAMMA 10 have several discrete peaks. The frequency of the AIC mode is just below the ion cyclotron frequency.

4 6 8

applied ICRF

Frequency [MHz]

AIC waves

10 -3

10 0

10 3

Frequency spectrum ofmagnetic probe signal

0

2

4

6

8

0 50 100

Dia

mag

netis

m

[10

-5W

b]

RF Power [kW]

IC R Fm A ICm m LFAzimuthal mode number:IC R Fω A ICω ω LFFrequency: = +

IC R Fk A ICk k LFAxial wave number:

Conditions for the parametric decay

RF2(6.36MHz)RF2(6.36MHz)

0

1000

2000

3000

4000

5000

80 90 100 110 120 130 140 150In

tens

ity [

a.u.

]

Dia

mag

netis

m [

10-4

Wb]

time [ms]

1

0.8

0.6

0.4

0

0.2

RF2 6.36 MHz

AIC waves

Low frequency waves

Temporal evolution of LF waves

LF waves appear when diamagnetism becomes large. Sometimes, LF waves accompany small reduction of diamagnetism.

Azimuthal mode number

Freq

uenc

y [M

Hz]

6.4

6.0

5.6

AIC Waves

RF2

-4 -3 -2 -1 0 1 2 3 4

Freq

uenc

y [M

Hz]

Mode Number

1

0.8

0.6

0.4

Low Frequency Waves

Freq

uenc

y [M

Hz]

6.4

6.0

5.6

RF2

AIC Waves

-4 -3 -2 -1 0 1 2 3 4

Freq

uenc

y [M

Hz]

1

0.8

0.6

0.4

Mode Number

Low Frequency Waves

measured with azimuthal probe array near the midplaneRF2 for ion heating will excite slow (m=-1) and fast (m=+1) waves.Only fast waves (m=+1) can be detected due to resonance layer between antenna and probe positions. (Slow waves (m= -1, 0 ) are damped)AIC waves are in slow wave branch. Always detected inm=-1.LF waves are detected with m=0,+1,+2.

If decay conditions are satisfied,RF2 AIC LF

wavesm= -1 -1 +2

0 -1 +1+1 -1 0

-4 -3 -2 -1 0 1 2 3 4Azimuthal Mode Number

Dia

mag

netis

m

[a.u

.]

2

1

0-3 -2 -1 0 1 2 3

Dia

mag

netis

m

[a.u

.]Phase Difference (Axial) [rad]

2

1

0

Axial wave number

Recently, we have started to measure the axial structure of excited waves with axial probe array.Tentatively, we obtained the different axial wave numbers for different azimuthalmode numbers

Distance between two probes is 0.77 m.Phase differences of 0 and π will indicate standing wave formation.

4. Summary

Two topics in recent ICRF experiments on GAMMA 10 are reported.

1. High density plasma production with phased antenna has been done.Conventional Double Half Turn Antenna is effective for the fast wave excitation.Plasma production depends strongly on the phase between two antennas.

Phase difference of φ = 0 means both currents enhances wave excitation.(Like slot antenna in HANBIT)

2. Parametric decay of ICRF waves are evaluated.From azimuthal mode number measurement, some modes (slow and fast waves) are candidates for the parametric decay.From the axial wave number measurement, the standing wave structure will be one of the possible solutions.