TAE and EAE damping on JET

A.Fasoli, D.Testa, CRPP - EPFLC.Boswell, MIT

S.Sharapov, UKAEAand Contributors to JET-EFDA

Workprogramme and Enhancements

Burning Plasma Workshop and ITPA Meeting, Tarragona, Spain, July 2005

Toroidal and Elliptical Alfvén Eigenmodes

– cylinder: Alfvén ‘continuum’ 2(r)=k||

2(r)vA2(r)

– small scales: strong damping

– torus: Coupling of poloidal harmonics

– gaps in continuum spectrum– weakly damped, global Toroidal

Alfvén Eigenmodes (TAEs) Elliptical AEs (EAEs), ….

– Active• In-vessel antennas drive low amplitude perturbations• Resonance in plasma response (e.g. on B-probes): global mode

Active and passive AE spectroscopy on JET– Passive

• Modes observed if destabilised by fast particles (NBI, ICRH, fusion ’s)

EAEs

TAEs

ICRH = fast ion source

AEs (TAEs, EAEs, NAEs, GAEs, kinetic TAEs and EAEs) only stable global modes in Alfvén range

fmeas

dampB

B/d

t (T

/s)

+ ti

me

(s) fTAE

n=1 TAE

Ex.: single n=1 TAE tracking using saddle coils

• No ‘stable’ Alfvén Cascades seen to date

Lay-out of the talk

• Recent results from last campaign using JET saddle coils (1994-2004, now dismantled, >3000 discharges)– TAE vs EAE damping at the edge

• TAE edge damping reproduced by theory• Different scaling of EAE damping with plasma shape and edge shear?

– AE damping in core: difficult to reproduce with present models

• Outlook: the new AE antennas at JET (intermediate n’s)

TAE edge damping : experimental evidence

– Shaping of cross-section increased magnetic shear increased mode conversion strong damping

– Quantitative agreement with gyro-kinetic code PENN– Consistent with observed PNBI threshold for TAE excitation

Consequences of strong edge damping

• Damping in the plasma core can be studied with radially extended low-n AEs only for very low shaping (95<0.35, 95 <1.5), i.e. in limiter plasmas

• Strong dependence of damping on edge conditions and profiles – Example: difference in measured damping due to B-field

reversal

TAE damping: effect of B-field direction– Damping of n=1 TAE about 2-3 times larger for reverse B-field (ion

B-drift directed toward X-point, favorable for H-mode)– Comparison with ICRH-driven TAEs (n=3-10)

Calculated fast ion drive at the onset of instability

DRIVE(reverseB) > DRIVE(forwardB)

difference decreases with n

• Forward B more TAE unstable• Challenge for fluid/gyro-kinetic

models• Role of plasma edge flows

(ion B-drift direction)?

Consequences of strong edge damping

• Damping in the plasma core can be studied with radially extended low-n AEs only for very low shaping (95<0.35, 95 <1.5), i.e. in limiter plasmas

• Strong dependence of damping on edge conditions and profiles – Extreme sensitivity on details of edge– Comparison with theory partly be inconclusive (small

changes in edge profiles can be invoked), unless we look at • Scalings of measured damping• Comparison of TAE and EAE in similar conditions

TAE vs EAE damping

• Nearly identical discharges

• Ohmic, limited =1.34, <>=0.004

• Constant ne, Te, Bt, Ip

EAE

TAE

TAE

EAE

(%)

f (kH

z)

Time (s)6 7 8 9 10 11

0

1

2

3

150

200

250

300

350

400

EAE and TAE calculated gap structures

• Edge damping mechanisms for EAEs similar to TAEs?– Effect of edge magnetic shear and shaping

0 0.25 0.5 0.75 1

0.5

1

1.5

2

0

n=1 EAE dampings95 scan from ramping current and shape

3 < s95 < 4.5 scan done during a ramp in Ip and shape

Time (s)

(%)

0 5 10 150

5

1

2

3

2

4

BT (T)Ip (MA)

s95

EAE Damping Rate, #61519

Scan in s95

n=1 EAE damping rate vs s95

2 2.5 3 3.5 4 4.5

1

2

3

4

5

6

7

8

9

s95

(%)

• EAE damping small at high s95

• Similar results for elongation and triangularity

• Opposite to the n=1 TAE trend at high elongation and triangularity

• Hidden q0 dependence?

• Effect of elongation on EAE gap width?

Summary and open questions

• Low-n AE linear stability

– Edge damping

• Large , shape dependence, explained by theory for TAEs

– Extreme sensitivity on edge conditions

» Ex.: effect of B-field reversal on damping and stability

• But EAE damping seems subject to a different scaling

– Effect of q0, gap width dependence on elongation?

– Core damping

• Difficulty in reproducing measured and scalings (see following talks)

– Example of TAE damping dependence on q0

~1500 measurement points for n=1 TAE dampingq0~0.76-1.6, 1.24<95<1.55; 0<95<0.25; 1.35<ne0(1019m-3)<4.2; 1.1<Te0(keV)<5.6; 2.5<q95<4.75

– Transition for q0~1 not reproduced by continuum in CASTOR

TAE damping (in the core?): vs q0

Outlook

• Need to investigate most unstable n range for ITER: n3-15

– Identify systematic methods to compare experiments with theory in intermediate n range (many modes coexisting)

– New AE active antenna on JET

fast ion driven modes: n=3-10

saddle coils: n=0-2• JET saddle coil system limited to low n’s

+ + + +

+ + + _

+ + _ _

+ _ + _

+ _ _ +

+ + _ +

– 45mm from LCFS, 18 turns; i ~ 20A, V~1000V, 10-500kHz– Coupling of n=5 calculated to be as n=2 with saddle coils– Local value of B/B can be larger

~1m

New AE antenna spectrum in-vessel mounting

New AE antenna installed on JET Octant 8

New AE antenna installed on JET Octant 8

Overview of new AE antenna design

18-turns, inconel 718 wire, 4mm diameter,

4mm spacing

distance from LCFS ~45mm: need tiles open frame: no

loop currents

‘wings’ to attach to poloidal limiter

isolating hinges and supports, by-passed by

straps of fixed R to balance halo currents

plug&socket connector

all frame parts are Inconel 625

2 antennas on Octant 4 and Octant 8

Linear mode stability -1- AE damping mechanisms

– Direct ion, electron Landau

– Mode conversion• Directly to shear AW (‘continuum damping’) or to kinetic AW:

– large up to ~ 5-10 %

• Tunneling to shear AW or kAW: ‘radiative damping’

– Collisional damping

• el. coll/ (e/)1/2 e ~ ne / Te 3/2

TAE

=kvA (r)

=kvA(r)TAE