instituto nacional de pesquisas espaciais - laboratório associado de plasma 1 iaea – 1 st rcm –...

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


ETE Spherical Torus

General view of the Spherical Tokamak Experiment – October 2004.


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.


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.


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)


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


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.


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)


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)


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).


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)


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.


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

)


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)


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)


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


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.


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.


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.


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


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


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.


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.


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

instituto nacional de pesquisas espaciais - laboratório associado de plasma 1 iaea – 1 st rcm –...

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