Results from Helical Axis Stellarators

Boyd Blackwell, H-1 National FacilityAustralian National University

Thanks to:Enrique Ascasibar and TJ-II GroupProf. Obiki and Heliotron-J GroupDavid Anderson and HSX Crewand the H-1 Team

Outline

Acknowledgements: TJ-II, Heliotron-J, HSX and H-1 groups for their contributions and access to their data, in particular C. Alejaldre, E. Ascasibar, C. Hidalgo, T. Obiki, K. Nagasaki, D.T. Anderson, J.H.Harris, M.G. Shats, J. Howard, Nyima Gyaltsin, S. M. Collis and D.L. Rudakov.

Brief history

Comparative parametersmagnetic surfaces

plasma formation and heating

diagnostic issues

Transport

Stabilityfluctuations

Spitzer 1951Spitzer 1951 - figure-8 stellarator “spatial axis” which produces rotational transform

magnetic hill unstable to interchange

Koenig 1955Koenig 1955 - helical winding/axis: = 1 one pair of helices

Spitzer 1956Spitzer 1956 possibility of shear stabilization for higher order windings = 2,3demonstrated theoretically (resistivity 0) Johnson et al 1958Johnson et al 1958

Furth, Killeen, Rosenbluth 1963Furth, Killeen, Rosenbluth 1963 found resistive interchange instability possible even at low resistivity for small scale lengths

1964-5 several configurations proposed with magnetic well (average minimum B) found including heliac (straight).

Exploitation of avg. min B regions of bad curvature possible ballooning instability

Development of Helical Axis Stellarators

-I

+I = 1

Nagao 1977Nagao 1977 Asperator NP: toroidal helical axis stellarator (+extra helical windings)

Yoshikawa... 1982-4Yoshikawa... 1982-4 - toroidal heliac HX-1 proposal

Blackwell, Hamberger... 1984Blackwell, Hamberger... 1984 - SHEILA prototype heliac (0.2M, 0.2T, 1019m3)

Harris.. 1985 Harris.. 1985 flexible heliac: = 1 winding varies iota, well over large range

19851985 - Tohoku, H-1 and TJ-II and heliacs proposed - and Ribe’sRibe’s linear heliac UW - Operation in 1987 (Tohoku, Sendai) 1992 (H-1) and 1996(TJ-II, Spain)

1988 Nuhrenberg and Zille - 1988 Nuhrenberg and Zille - quasi-helical symmetry - restore outstanding features of straight heliac. [transport, beta limit(Monticello et. al 1983)]

1996-91996-9 Heliotron-J - combine heliotron/torsatron with advances in transport (optimise bumpy cpt, quasi-isodynamic)

1999 1999 HHelicallyelically S Symmetricymmetric EEXXperimentperiment first quasi-symmetric experiment exploit high iota, N-m scaling

Development of Helical Axis Stellarators II

Canberra, Australia external vacuum vessel

CIEMAT, Madrid internal vessel, upgrade to NBI

IAE Kyoto “inverted heliac” bumpy field cpt

TSL, Madison controlled “spoiling” of symmetry

.

Device Type Aspect Iota

H-1 Heliac 3 period heliac, toroidal>helical 5 .15

TJ-II Heliac 4 period heliac, helical>toroidal 7 0.9-2.2

Heliotron J helical axis heliotron (TFC + =1) 7-11 0.2-0.8

HSX modular coils, helical symmetry 8 1.05-1.2

Helical Axis Stellarators 2000bn,m

Canberra, Australia external vacuum vessel

CIEMAT, Madrid internal vessel, upgrade to NBI

IAE Kyoto “inverted heliac” bumpy field cpt

TSL, Madison controlled “spoiling” of symmetry

.

Device Type Aspect Iota

H-1 Heliac 3 period heliac, toroidal>helical 5 .15

TJ-II Heliac 4 period heliac, helical>toroidal 7 0.9-2.2

Heliotron J helical axis heliotron (TFC + =1) 7-11 0.2-0.8

HSX modular coils, helical symmetry 8 1.05-1.2

Helical Axis Stellarators 2000bn,m

Brief history

Comparative parameters

magnetic surfacesHeliotron J and HSX

plasma formation and heating

diagnostic issues

Transport

Stabilityfluctuations

Device Parameters of Heliotron J

Coil SystemL=1/M=4 helical coil 0.96MATToroidal coil A 0.6MATToroidal coil B 0.218MATMain vertical coil 0.84MATInner vertical coil 0.48MAT

Major radius 1.2mMinor radius of helical coil 0.28mVacuum chamber 2.1m3

Aspect ratio 7Port 65Magnetic Field 1.5TPulse length 0.5secPitch modulation of helical coil

Inner Vertical Coil

Toroidal Coil A

Outer Vertical Coil

Toroidal Coil BHelical Coil

Plasma

Vacuum Chamber

sin( )

0.4

M M

L L

The Heliotron J Device

Main VFC

TFC-A

TFC-B

Aux.VFC

HFC

Magnetic Surface Mapping

Fig.3 The magnetic surfaces at = 67.5 in the standard configuration. (a) The experimental results (corrected) and (b) The calculated magnetic surfaces.

(a) (b)

STD config, 0.03 Tesla, corrected for earth’s field

Configuration “A” is designed to create a helical divertor region shown in red and yellow.

The position of the plasma is shown relative to the helical conductor and the vacuum vessel

Other configurations

• island divertor

• standard

from T. Mizuuchi, M. Nakasuga et al. Stellararor Workshop 1999

Heliotron-J surfaces: cfg “A” - helical divertor

Helically Symmetric Experiment

UW, Madison

R = 1.2 a=0.15B0=1.3T 4 periodsiota 1.05-1.12 well ~1% essentially 1 term in B0 spect

28GHz@200kWne~3e12 for 50kW@0.5T

HSX Parameters

HSX Magnetic surfacesGood magnetic surfaces, iota ~ 1% accurateDrift surfaces coincide well with magnetic surfaces

- low toroidal effects, high effective iota (eff = N-m)

HSX Magnetic surfacesGood magnetic surfaces, iota ~ 1% accurateDrift surfaces coincide well with magnetic surfaces

- low toroidal effects, high effective iota (eff = N-m)

Measured drift surfaces mapped to Boozer coordinates

Expected drift if fully toroidal

Canberra, Australia external vacuum vessel

CIEMAT, Madrid internal vessel, upgrade to NBI

IAE Kyoto “inverted heliac” bumpy field cpt

TSL, Madison controlled “spoiling” of symmetry

.

Device Type Aspect Iota

H-1 Heliac 3 period heliac, toroidal>helical 5 .15

TJ-II Heliac 4 period heliac, helical>toroidal 7 0.9-2.2

Heliotron J helical axis heliotron (TFC + =1) 7-11 0.2-0.8

HSX modular coils, helical symmetry 8 1.05-1.2

Helical Axis Stellarators 2000bn,m

Comparative parameters

magnetic surfaces

plasma formation and heating (H-1, HSX)

diagnostic issuesTransport

H-1 Heliac: Parameters3 period heliac: 1992Major radius 1mMinor radius 0.1-0.2mVacuum chamber 33m2 excellent access

Aspect ratio 5+ toroidal

Magnetic Field 1 Tesla (0.2 DC)Heating Power 0.2(0.4)MW GHz ECH

0.3MW 6-25MHz ICH

Parameters: achieved / expected n 3e18/1e19

T ~100eV(Ti)/0.5-1keV(Te)

0.1/0.5%

H-1 Heliac: Parameters3 period heliac: 1992Major radius 1mMinor radius 0.1-0.2mVacuum chamber 33m2

Aspect ratio 5+Magnetic Field 1 Tesla (0.2 DC)Heating Power 0.2(0.4)MW GHz ECH

0.3MW 6-25MHz ICH

Parameters: achieved / expected n 3e18/1e19

T ~100eV(Ti)/0.5-1keV(Te)

0.1/0.5%

Complex geometry requires minimum 2D diagnostic

Cross-section of the magnet structure showing a 3x11 channel tomographic diagnostic

2D electron density tomography

coherent drift mode in argon, 0.08TH density profile evolution (0.5T rf)

Helical axis non-circular need true 2D

Movie Clip (AVI)

Raw chordal data Tomographically inverted data

HSX ECH Plasma

Utilize 2nd harmonic ECH at 28GHz to examine confinement of deeply-trapped electrons

Plasma production and heating: resonant and non-

resonant RF

<ne> 1018m-3

• Non-resonant heating is flexible in B0, works better at low fields.

• Resonant heating is much more successful at high fields. helicon/frame antenna

= Chon axisMagnetic Field (T)

radi

us

Ion Temperature Camera

Hollow Ti at low B0

0 10 20 30 time (ms)

Inte

nsity

te

mpe

ratu

re

rot

atio

n

Canberra, Australia external vacuum vessel

CIEMAT, Madrid internal vessel, upgrade to NBI

IAE Kyoto “inverted heliac” bumpy field cpt

TSL, Madison controlled “spoiling” of symmetry

.

Device Type Aspect Iota

H-1 Heliac 3 period heliac, toroidal>helical 5 .15

TJ-II Heliac 4 period heliac, helical>toroidal 7 0.9-2.2

Heliotron J helical axis heliotron (TFC + =1) 7-11 0.2-0.8

HSX modular coils, helical symmetry 8 1.05-1.2

Helical Axis Stellarators 2000bn,m

diagnostic issues

Transport

confinement (Heliotron-J, TJ-II, H-1)Stability/Fluctuations

Heliotron-J: Confinement during ECH

• ECH 400kW 53GHZ 50ms

• <> ~ 0.2%, <20% radiated

• some Fe Ti C O impurities

Plans:

• will upgrade to 70GHz, 500kW

• ultimately 4MW ~20kJ?

• impurity control

• explore bumpiness and hel. divertorsFig. 2 Dependence of the diamagnetic stored energy on the magnetic field strength.

B*=0 (T)

Wp (kJ)

#1595 ~ #241653.2GHz ECH

0/

0.80 1.00 1.20 1.400

0.2

0.4

0.6

0.8

1

0.4 0.5 0.6 0.7

W-Diamagnetic vs B is peaked, 700J max

Initial Plasma: 700J stored energy

TJ-II Heliac, CIEMAT, Spain

• R = 1.5 m, a < 0.22 m, 4 periods

• B0 < 1.2 T

• PECRH < 600 kW from 2 ECH systems

• PNBI < 3 MW under installation

• helium and hydrogen plasma

• Te ~ 2keV, low radiated powers (<20%)

• wall desorption rate limits operation in He at P< 600 kW

Helical/central conductor

• Helium plasmas with injected power of 300 kW• Neoclassical Monte-Carlo agrees well

Inferred positive ambipolar Er, confinement time ~ 5ms ~ ISS95

yet no serious accumulation of impurities

0

0.5

1

1.5

-15 -10 -5 0 5 10 15

n e(1

019

m-3

)

reff

(cm)

Thomson scattering

0

0.5

1

1.5

-15 -10 -5 0 5 10 15

T e

(keV

)

reff

(cm)

• iota ~ 1.28 – 2.24, up to 1.2 x 1019 m-3 and 2.0 keV

0.0

0.5

1.0

1.5

2.0

1.2 1.4 1.6 1.8 2 2.2

Pinj

=300 kWP

inj=100 kW

Pinj

=200 kWP

inj=400 kW

Pinj

=500 kWP

inj=600 kW

W (

kJ)

Iota(0)

Helium

1.2 1.4 1.6 1.8 2 2.2Iota(0)

Hydrogen

Iota = 2

• When corrected for volume changes, a positive dependence on iota is revealed in helium, (less in H) (tendency sim. to ISS95)

Configuration Scan (iota)

Confinement transitions in H-1

6

5

4

3

2

1

00

0 . 5

1r / a

01 0

2 03 0

t ( m s )

I s i ( )m A( a )

M o d u l a t i o ni n v e r s i o n

“Pressure” (Is) profile evolution during transition

transition

PRF (kW)

B0(T)

•many features in common with large machines

•associated with edge shear in Er

•easily reproduced and investigated

Parameter space map, ~ 1.4

ExB and ion bulk rotation velocity in high confinement mode: magnetic structure causes

viscous damping of rotation

-6E+6

-4E+6

-2E+6

0E+0

2E+6

15 20 25

V_pol(cm/s)

(x10)

VExB

r(cm) (cm)

LCFS (cm)

pttpir BVBVPzen

E 1

0 0

Vp, Vt << VExB ~ 1/(neB) dPi/dr

Radial force balance

Mass (ion) flow velocities much smaller than corresponding VExB

Bulk Rotation Damped in Heliac

diagnostic issues

Transport

Stability fluctuations

Issues: Interchange and Ballooning Modes (DTEM low )

Tools: Configuration Flexibility e.g. transform andmagnetic well (even hill!)

First Impression: No unworkable instabilties or disruptions

“Drift-like” instability in H-1 at low field

–Triple-Mach-Triple probe

–disappears as B increases

Helical axis high iota short connection length

All devices need > 0.5-1% to test ballooning stability%30/

~/

~/~ iiee TTTTnn

TJ-II Turbulence/Fluctuation studies

• ExB sheared flows observed near edge rational surfaces (8/5, 4/2)

• Spectra mainly <200kHz, 10-40% (edge?), correlation time 10ms

• MHD (ELM-like) events (for W~1kJ) - magnetic activity - spike in the H signal.

• Fluctuations increase with magnetic hill near edge• resistive ballooning?

Summary - Future• Confinement in heliacs ~ISS95 or better (2keV, ~5ms). Ion beam

probe to elucidate role of Eradial in improved confinement

• New configurations with improved neoclassical transport initial results promising, await mature data, analysis

• HSX/H-J can compare similar configurations with vastly different neoclassical transport predictions.

• Confinement transitions possible at low power, many similarities with large devices/powers. Investigate effect of E-field imposed by localised ECH.

• No serious impurity accumulation problems yet. Real test when the ions are strongly heated

• No fatal instabilities observed yet. Several devices should have the heating capacity to test ballooning limits, at least in degraded configurations (consequence of flexibility).