Instituto Nacional de Pesquisas Espaciais - Laboratório Associado de Plasma 1
IAEA – 1st RCM – 2004
Plasma edge studies in the ETE spherical tokamak
G.O. Ludwig, E. Del Bosco, L.A. Berni, J.G. Ferreira, R.M. Oliveira,M.C.R. Andrade, J.J. Barroso, P.J. Castro, C.S. Shibata.
Laboratório Associado de PlasmaInstituto Nacional de Pesquisas Espaciais
12227-010 São José dos Campos, SP, Brazil
CRP – Joint research using small tokamaks7-10 November, 2004 - Lisbon, Portugal
Instituto Nacional de Pesquisas Espaciais - Laboratório Associado de Plasma 2
ETE Spherical Torus
General view of the Spherical Tokamak Experiment – October 2004.
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ETE main parameters and objectives
Major radius R0 = 0.3 m
Minor radius a = 0.2 mAspect ratio A = 1.5
Elongation = 1.6 – 1.8
Triangularity = 0.3
Magnetic field B0 = 0.4 – 0.6 T (<0.8 T)
Plasma current Ip = 0.2 – 0.4 MA
Pulse duration t < 15 – 50 ms
• Explore the physics of low aspect ratio.
• Investigate plasma edge conditions.
• Undertake diagnostics development.
• Follow worldwide spherical tokamak advancements.
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Axisymmetric configurations
The spherical torus presents both the strong toroidicity effects of compact tori (notably magnetic shear) and the good stability properties provided by the external
toroidal field of conventional tokamaks.
Instituto Nacional de Pesquisas Espaciais - Laboratório Associado de Plasma 5
Plasma current evolution
0.0 3.0 6.0 9.0 12.0 15.00
10
20
30
40
50
60
#2529#1881
#0805
#0805 (28/11/2000):First plasma)
#1881 (20/11/2001):Vertical field configuration
#2529 (25/03/2002):Discharge optimization
#3036 (25/11/2002):Toroidal and ohmic banks upgrade
#3330 (27/12/2002):Glow discharge cleaning
#3036 #3330
Pla
sma
curr
ent (
kA)
Time (ms)
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Present plasma parameters
10 15 20 25 30 35 40 450.00
0.04
0.08
0.12
0.16
Pla
sma
dens
ity (1
020 m
-3)
R (cm)
Time = 4 ms
• Current 60 kA – Pulse duration 12 ms.
• Density 0.351020 m3 – Temperature 160 eV.
• Peaks at R0 26 cm with A 2.2 (design values R0 = 30 cm and A = 1.5).
10 15 20 25 30 35 40 450
40
80
120
160
Ele
ctro
n te
mpe
ratu
re (e
V)
R (cm)
Time = 4 ms
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Plasma pictures
• High-speed CCD camera (frame speed 1/500 to 1/20,000 s) – 30 to 500 FPS (upgrade to 10,000 FPS).
• Pictures, as well as (ne, Te) profiles, show plasma displaced to the inner side.
• Need position and shape control taking into account eddy current effects.
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Typical shot (#5401)
• Plasma current = 40~60 kA, Pulse duration = 8~5 ms (>12 ms with glow discharge).
• Loop voltage at Z = 0 in gap between OH solenoid and vacuum vessel.
• Evidence of runaway discharge (hard X-rays, Iplasma ≠ 0, Vloop 0).
• Current induced in vacuum chamber = 60 kA.
0 2 4 6 8 100.00.51.0
Time (ms)
H (au)
048
Loop voltage (V)
02040 Plasma current (kA)
0 2 4 6 8 100.00.51.0
Time (ms)
Hard X-rays (au)
-101 Mirnov (au)
0.00.51.0 CIII (au)
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Coils currents and power supplies
• Parameters at ~1/3 of first phase, compatible with installed capacitor banks (~1/4).
• TF coil current 50 kA B = 0.4 T.
• OH solenoid current 20 kA Ip ~ 200 kA.
• Next: increase installed energy (2x).
0 5 10 15
0204060
Vacuum vesselcurrent (kA)
Time (ms)
-1.2-0.8-0.40.0
Equilibrium coilcurrent (kA)
-6-4-20
Ohmic heating coilcurrent (kA)
06
1218
Toroidal field coilcurrent (kA)
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Baking and glow discharge cleaning
• Vacuum vessel covered by hot tapes (11 kW) and thermal insulation blanket.
• Baking up to 120C so far (limited by viton seals) but can go up to 200C.
• Glow discharge (500 V/0.5 A, refrigerated anode) with He at 2.410-3 mbar.
• Base pressure < 810-8 mbar after baking (2-3 days) and glow (~48 hours).
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Vacuum conditioning and breakdown
1E-4 1E-31.5
2.0
2.5
3.0
3.5
4.0
4.5Breakdown curve in H
2
Hot filament offHot filament on
Ele
ctric
fiel
d (V
/m)
Pressure (mbar)
• Glow discharge and biased hot filament.
• Next: Conditioning improvement needed for removal of O2 – overcome the radiation barrier.
1E-4 1E-3
200
400
600
800 Time lag for plasma formation in H2
Hot filament offHot filament on
Tim
e la
g (
s)
Pressure (mbar)
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Thomson scattering system
• 10 J, 30 ns pulse duration, Q-switched ruby laser.
• f/6.3 lens images the scattered light on 4.5 mm 1.5 mm optical fiber bundles.
• 5-channels filter polychromator with avalanche photodiodes.
• Up to 22 plasma positions (15 mm) along 50 cm of the laser beam trajectory.
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Thomson scattering system upgrade
• Multipoint diagnostic based on time-delay technique using fibers of different lengths.
• Simultaneous temperature and density profiles – ten spatial points per polychromator.
Measured attenuation of time-delayed signals in channels 1 and 8: loss = 84%,
dispersion = 73%, total = 61%.
0.0 200.0 400.0 600.0 800.0-60
-40
-20
0
-600
-400
-200
0
112m Fiber8m Fiber
Nor
mal
ized
sig
nal
Time (ns)
Laser
Sig
nal (
mV
)
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Test of the multipoint Thomson scattering system
• Radial profile of the electron temperature obtained with the four-channelsmultiplexed Thomson scattering system (4 fibers, Ø 0.8 mm, 14 m steps).
• Three curves obtained in three different shots.
• Next: upgrade to ten channels.
22 24 26 28 30 32 34
40
60
80
100
120
4 ms 5 ms 6 ms
Tem
pera
ture
(eV
)
R (cm)
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Lithium beam probe development
• 10 keV neutral lithium beam probe for measurements of the edge plasma densityand its fluctuations.
• Current density up to 1 mA/cm2 (glassy –eucryptite source) – 80% neutralization.
• Plasma density measured in He glow discharge at 2.010-3 mbar ne=21017 m-3.
-3 -2 -1 0 1 2 3 4 50.0
0.20.4
0.6
0.8
1.0
1.2
1.4
1.6
T ion source = 1200CV acceleration = 6.0kVV extraction = 3.0kV
V colimation 2.9kV V colimation 3.5kV
Sam
pled
ion
curr
ent (A
)
Radial position (cm)
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First results of the lithium beam probe
0 1 2 3-8-7-6-5-4-3-2-10
PM
out
put v
olta
ge (m
V)
Time (s)
Pre
ssur
e (x
10-5
mba
r)
0.5
1.3
2.7
4.0
0 2 4 6 8 100.000
0.004
0.008
0.012
0.016
Den
sity
(1020
m-3)
Time (ms)
R = 48 cm• Gas injection by fast puff valve and pump out.
• Radial profile of plasma density measured with Thomson scattering, fast neutral Li beam probe
and Langmuir probe.
• Fluctuation of the plasma density at the edge.
25 30 35 40 45 50 550.00
0.04
0.08
0.12
0.16
Den
sity
(1020
m-3)
Radius (cm)
Thomson scattering Lithium beam Langmuir probe
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Lithium beam probe upgrade
• Objective lens, fiber optics (Ø 1.5 mm).
• Interference filters – 1 nm.
• Multichannel (8 x 8) photomultiplier.
• Next: 8 radial positions (< 17 cm) – gain factor 8.
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Monotron for plasma heating experiments
• Transit-time microwave tube – cylindrical cavity excited by a hollow electron beam.
• Single-cavity monotron with RF electric field tail pre-modulating the electron beam – 25% conversion efficiency.
f = 6.7 GHz, V0 = 10 kV, I0 = 10 A.
Instituto Nacional de Pesquisas Espaciais - Laboratório Associado de Plasma 19
Monotron electron gun
• Optimized design collimates electron beam – uses improved filament support.
• Annular strip of nickel coated with a (Ba,Sr,Ca)O film 3 A/cm2 (800 K, 10 A).
• Next: Tests of pulsed HV power supply (20 kV, 30 A, 50 µs), gun, and monotron.
Instituto Nacional de Pesquisas Espaciais - Laboratório Associado de Plasma 20
Foucault currents in the central column
Study of the currents induced by the OH solenoid in the 12
trapezoidal bars of the TF coil central column (cross
section shown at left).
Bode plot of the OH solenoid impedance comparing theoretical results with experimental values obtained before the central stack was assembled in ETE.
Vacuum vessel
Ohmic
TF coil
Supportstructure
(2mx2.5m)
Elongation
Equilibrium
Plasma
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Foucault currents in the vacuum vessel
Distribution of eddy currents induced by the poloidal field coils on the vacuum vessel was determined experimentally and compared with theoretical results.
Equilibrium coil
Internalcompensation coil
Elongation coil
TF coil
Plasma
Externalcompensation coil
Ring
Crown Truss
Ohmic solenoid
Vacuum vessel
(1m x 1m )
1
2345
0 6
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Foucault currents and plasma start-up simulation
• Currents in capacitor banks and external coils calculated self-
consistently with eddy currents.
• Current distributions in the vacuum vessel will be used in
zero-dimensional simulations of the plasma discharge.
• Use distributions to eliminate error-fields in the magnetic
reconstruction of the plasma equilibrium.
Instituto Nacional de Pesquisas Espaciais - Laboratório Associado de Plasma 23
Further theoretical development
• Bootstrap current dependence on plasma profiles in self-consistent equilibrium calculations.
• Effects of viscosity on plasma instabilities (simple edge plasma models).
• First attempts of magnetic reconstruction using a variant of the moments method (possibility of including edge resistivity).
• Monotrons using a novel twin-cavities concept with a bisecting grid should attain 40% electronic efficiency – comparable to gyrotrons, but simpler design (no magnetic field,
large power handling capability).
• Two coupled cavities with stepped electric field profile: monotrons with up to 45% electronic efficiency.
Instituto Nacional de Pesquisas Espaciais - Laboratório Associado de Plasma 24
Budget and planning – 2005• Main scientific staff (9 researchers) – US$ 250,000.00 (Institute)
• Four part time technicians (Institute) – Researchers & engineers urgently needed
• Maintenance of the laboratory – US$ 85,000.00 (Institute)1. Materials, services, travel/transportation – US$ 25,000.00
2. Additional capacitor bank chargers – US$ 48,000.00
3. Fast CCD camera upgrade (10,000 FPS) – US$ 12,000.00 (?)
• Diagnostics – US$ 85,000.00 (Energy research fund – under analysis)1. Upgrade of the Thomson scattering (10 x) and lithium beam (8 x) diagnostics – US$ 5,000.00
2. Magnetic pickup coils (reconstruction) and electrostatic probes (natural divertor) – US$ 15,000.00
3. Bolometers, soft X-ray arrays – US$ 15,000.00
4. VME data acquisition modules – US$ 20,000.00
5. Microwave interferometer (1 mm) – US$ 30,000.00
• Pre-ionization, preheating – US$ 82,000 (Energy research fund – under analysis)1. EC heating system (9.5 GHz, 40 kW, 500 μs) – US$ 82,000.00
Fusion activities in Brazil presently under discussion at the government level