Transcript Results from Helical Axis Stellarators

Results from Helical
Axis Stellarators
Thanks to:
Enrique Ascasibar and TJ-II Group
Prof. Obiki and Heliotron-J Group
David Anderson and HSX Crew
and the H-1 Team
Boyd Blackwell, H-1 National Facility
Australian National University
Outline
Brief history
Comparative parameters
magnetic surfaces
plasma formation and heating
diagnostic issues
Transport
Stability
fluctuations
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.
Development of Helical Axis Stellarators
Spitzer 1951 - figure-8 stellarator 
“spatial axis” which produces
rotational transform
magnetic hill  unstable to interchange
Koenig 1955 - helical winding/axis:  = 1  one pair of helices
-I
+I
=1
Spitzer 1956  possibility of shear stabilization for higher order windings  = 2,3
demonstrated theoretically (resistivity  0) Johnson et al 1958

Furth, 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 II
Nagao 1977 Asperator NP: toroidal helical axis stellarator (+extra helical windings)
Yoshikawa... 1982-4 - toroidal heliac HX-1 proposal
Blackwell, Hamberger... 1984 - SHEILA prototype heliac (0.2M, 0.2T, 1019m3)
Harris.. 1985  flexible heliac:  = 1 winding varies iota, well over large range
1985 - Tohoku, H-1 and TJ-II and heliacs proposed - and Ribe’s linear heliac UW
- Operation in 1987 (Tohoku, Sendai) 1992 (H-1) and 1996(TJ-II, Spain)
1988 Nuhrenberg and Zille - quasi-helical symmetry - restore outstanding features of
straight heliac. [transport, beta limit(Monticello et. al 1983)]
1996-9 Heliotron-J - combine heliotron/torsatron with advances in transport
(optimise bumpy cpt, quasi-isodynamic)
1999 Helically Symmetric EXperiment first quasi-symmetric experiment
exploit high iota, N-m scaling
Helical Axis Stellarators 2000
Device
Type
bIota
n,m
Aspect
H-1 Heliac
3 period heliac, toroidal>helical
Canberra, Australia external vacuum vessel
5
.15
TJ-II Heliac
7
0.9-2.2
helical axis heliotron (TFC + =1)
“inverted heliac” bumpy field cpt
7-11
0.2-0.8
modular coils, helical symmetry
controlled “spoiling” of symmetry
8
1.05-1.2
4 period heliac, helical>toroidal
CIEMAT, Madrid internal vessel, upgrade to NBI
Heliotron J
IAE Kyoto
HSX
TSL, Madison
-0.1
.
B / B0   bnm cos( n  m ) ,
0 0.1
bumpy
tor curv
helical
0.2
Brief history
Comparative parameters
Helical Axis Stellarators 2000
magnetic
Device
surfaces
Type
bIota
n,m
Aspect
Heliotron J and HSX
5
.15
7
0.9-2.2
helical axis heliotron (TFC + =1)
“inverted heliac” bumpy field cpt
7-11
0.2-0.8
modular coils, helical symmetry
controlled “spoiling” of symmetry
8
1.05-1.2
H-1plasma
Heliac
period
heliac, toroidal>helical
formation3and
heating
Canberra,
Australia
diagnostic
issues external vacuum vessel
Transport
TJ-II
Heliac 4 period heliac, helical>toroidal
Stability
CIEMAT,
Madrid internal vessel, upgrade to NBI
fluctuations
Heliotron J
IAE Kyoto
HSX
TSL, Madison
-0.1
.
0 0.1
bumpy
tor curv
helical
0.2
Device Parameters of Heliotron J
Coil System
L=1/M=4 helical coil 0.96MAT
Inner Vertical Coil
Toroidal coil A
0.6MAT
Toroidal coil B
0.218MAT
Main vertical coil
0.84MAT
Inner vertical coil
0.48MAT
Major radius
1.2m
Minor radius of helical coil 0.28m
Vacuum chamber
2.1m3
Aspect ratio
7
Port
65
1.5T
Vacuum Chamber Magnetic Field
Toroidal Coil A
Pulse length
0.5sec
Toroidal
Coil
B
Plasma
Pitch modulation of helical coil
Helical Coil
Outer Vertical Coil
  
M
M
   sin(  )
L
L
  0.4
The Heliotron J Device
TFC-A
TFC-B
HFC
Aux.VFC
Main VFC
Magnetic Surface Mapping
STD config, 0.03 Tesla, corrected for earth’s field
(a)
(b)
Fig.3 The magnetic surfaces at  = 67.5 in the standard configuration.
(a) The experimental results (corrected) and (b) The calculated magnetic surfaces.
Heliotron-J surfaces: cfg “A” - helical divertor
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
HSX Parameters
Helically Symmetric
Experiment
UW, Madison
R = 1.2
a=0.15
B0=1.3T
4 periods
iota 1.05-1.12 well ~1%
essentially 1 term in B0 spect
28GHz@200kW
ne~3e12 for [email protected]
HSX Magnetic surfaces
Good magnetic surfaces, iota ~ 1% accurate
Drift surfaces coincide well with magnetic surfaces
- low toroidal effects, high effective iota (eff = N-m)
1.2
Transform
1.15
Measured
1.1
1.05
Calculated
1
0
0.2
0.4
0.6
r/a
0.8
1
1.2
HSX Magnetic surfaces
Good magnetic surfaces, iota ~ 1% accurate
Drift surfaces coincide well with magnetic surfaces
- low toroidal effects, high effective iota (eff = N-m)
90
8
120
60
6
4
150
30
2
180
0
210
330
240
Measured
drift surfaces
mapped to
Boozer
coordinates
300
270
90
8
120
60
6
4
150
30
2
180
0
210
330
240
300
270
Expected
drift if fully
toroidal
Helical Axis Stellarators 2000
Device
Type
bIota
n,m
Aspect
H-1 Heliac
3 period heliac, toroidal>helical
Canberra, Australia external vacuum vessel
5
.15
Comparative parameters
TJ-II Heliac 4 period heliac, helical>toroidal
magnetic
surfaces
CIEMAT,
Madrid
internal vessel, upgrade to NBI
7
0.9-2.2
plasma formation and heating (H-1, HSX)
Heliotron
J issues
helical axis heliotron (TFC + =1)
7-11
diagnostic
IAE Kyoto
Transport
“inverted heliac” bumpy field cpt
HSX
modular coils, helical symmetry
controlled “spoiling” of symmetry
TSL, Madison
0.2-0.8
8
1.05-1.2
-0.1
.
0 0.1
bumpy
tor curv
helical
0.2
H-1 Heliac: Parameters
3 period heliac: 1992
Major radius
1m
Minor radius
0.1-0.2m
Vacuum 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:
n
T

achieved / expected
3e18/1e19
~100eV(Ti)/0.5-1keV(Te)
0.1/0.5%
H-1 Heliac: Parameters
Complex geometry requires
minimum 2D diagnostic
3 period heliac: 1992
Major radius
1m
Minor radius
0.1-0.2m
Vacuum 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:
n
T

achieved / expected
3e18/1e19
~100eV(Ti)/0.5-1keV(Te)
0.1/0.5%
Cross-section of the magnet structure showing a 3x11 channel tomographic diagnostic
2D electron density tomography
Helical axis  non-circular  need true 2D
H density profile evolution (0.5T rf)
Raw chordal data
coherent drift mode in
argon, 0.08T
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 nonresonant RF
• Non-resonant heating is flexible
in B0, works better at low fields.
<ne> 1018m-3
• Resonant heating is much more
successful at high fields.
helicon/frame antenna
3.5
Helicon wave (non resonant) heating
electron density
3
Ion cyclotron resonant heating:
Hydrogen, and
Minority H in He
2.5
argon
2
H:He
1.5
1
helium
0.5
H
0
0
0.1
0.2
0.3
Magnetic Field (T)
Magnetic Field (T)
0.4
0.5
w = wwChwon
axis
CH On
axis
0.6
rotation
Ion Temperature
Camera
Intensity
temperature
radius
Hollow Ti at low B0
0
10
20
time (ms)
30
Helical Axis Stellarators 2000
Device
Type
bn,m
Iota
Aspect
H-1 Heliac
3 period heliac, toroidal>helical
Canberra, Australia external vacuum vessel
5
.15
TJ-II Heliac
7
0.9-2.2
7-11
0.2-0.8
8
1.05-1.2
4 period heliac, helical>toroidal
CIEMAT, Madrid internal vessel, upgrade to NBI
Heliotron J
IAE Kyoto
helical axis heliotron (TFC + =1)
“inverted heliac” bumpy field cpt
diagnostic issues
modular coils, helical symmetry
TSL,
Madison
controlled “spoiling” of symmetry
Transport
HSX
confinement (Heliotron-J, TJ-II, H-1)
. Stability/Fluctuations
-0.1
0 0.1
bumpy
tor curv
helical
0.2
Heliotron-J: Confinement during ECH
• ECH 400kW 53GHZ 50ms
• <> ~ 0.2%, <20% radiated
• some Fe Ti C O impurities
0.4
0.8
0.6
0.7
#1595 ~ #2416
53.2GHz ECH
1
W p (kJ)
•
•
•
•
0.5
w0/w
Initial Plasma: 700J stored energy
0.6
0.4
Plans:
0.2
will upgrade to 70GHz, 500kW
0
0.80
1.00
1.20
1.40
ultimately 4MW ~20kJ?
B
(T)
impurity control
Fig. 2 Dependence of the diamagnetic stored energy on the magnetic field strength.
vs B is peaked, 700J
explore bumpiness and hel. divertors W-Diamagnetic
max
*=0
TJ-II Heliac, CIEMAT, Spain
Helical/central conductor
•
•
•
•
•
•
•
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
Thomson scattering
1.5
1
1
T e (keV)
1.5
ne (10
19
m -3 )
• 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
0.5
0
-15
0.5
-10
-5
0
5
reff (cm)
10
15
0
-15
-10
-5
0
5
reff (cm)
10
15
Configuration Scan (iota)
• iota ~ 1.28 – 2.24, up to 1.2 x 1019 m-3 and 2.0 keV
2.0
P
inj
P
inj
P
inj
P
inj
P
inj
P
W (kJ)
1.5
1.0
inj
=300
=100
=200
=400
=500
=600
kW
kW
kW
kW
kW
kW
Hy dr ogen
Helium
0.5
0.0
1.2
1.4
1.6
1.8
Iot a(0)
2
2.2
1.2
1.4
1.6
1.8
Iot a(0)
2
2.2
Iota = 2
• When corrected for volume changes, a positive dependence on
iota is revealed in helium, (less in H) (tendency sim. to ISS95)
Confinement transitions in H-1
“Pressure” (Is) profile
evolution during transition
Parameter space map,  ~ 1.4
PRF (kW)
transition
I si (mA)
(a)
6
5
4
3
2
Modulation
inversion
B0(T)
1
•many features in common with
large machines
0
0
30
0.5
r/a
20
1
10
0
t (ms)
•associated with edge shear in Er
•easily reproduced and
investigated
Bulk Rotation Damped in Heliac
ExB and ion bulk rotation velocity in high
confinement mode: magnetic structure causes
viscous damping of rotation
V_pol
(cm/s)
2E+6
Radial force balance
(x10)
Er 
0E+0
15
20
25
r(cm)
(cm)
VExB
LCFS
(cm)
-6E+6
0
0
Vp, Vt << VExB ~ 1/(neB) dPi/dr
-2E+6
-4E+6
1
Pi  V p Bt  Vt B p
zen
Mass (ion) flow velocities
much smaller than
corresponding VExB
diagnostic issues
Transport
Stability
fluctuations
Interchange and Ballooning Modes (DTEM low )
Configuration Flexibility e.g. transform and
magnetic well (even hill!)
First Impression: No unworkable instabilties or disruptions
Issues:
Tools:
“Drift-like” instability in H-1 at low field
~ / n  T~ / T  T~ / T  30%
n
e
e
i
i
–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
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).