17th TM on Research Using Small Fusion Devices, 22_24 of October 2007, Lisbon, Portugal

“Interaction of a liquid gallium jet with ISTTOK edge plasmas”

R. B. Gomes, H. Fernandes, C. Silva, ISTTOK team and the Latvian association

Associação EURATOM/IST, Centro de Fusão Nuclear, Av. Rovisco Pais, 1049-001 Lisboa, Portugal

&

Association EURATOM/University of Latvia, Institute of Solid State Physics, 8 Kengara Str. , LV-1063 Riga, Latvia.

17th TM on Research Using Small Fusion Devices, 22_24 of October 2007, Lisbon, Portugal

Advantage of liquid metal as plasma facing components.

• protection capability.

• power exhaustion capability.

Metal candidates (Li, Ga, Sn…).

• Li : very good compatibility with plasmas (Z=3).

• Ga: wider temperature range: 30 to ~ 700 ºC (at 10-7 mBar).

Aim of the project:

• to verify the feasibility of ISTTOK operation with a liquid metal limiter.

• to study the influence of a Gallium jet on the main plasma parameters, incl.

impurity content, the plasma stability and confinement;

• to measure the heat deposited on the liquid metal jet.

• to study dynamic behavior of liquid metal jets in a magnetic field.

Introduction

17th TM on Research Using Small Fusion Devices, 22_24 of October 2007, Lisbon, Portugal

Experimental Setup

LML design constraints:

• UHV environment

(10-6 mBar)

• gallium corrosion

• gallium expansion

• gallium oxidation

• electrical isolation

requirements

17th TM on Research Using Small Fusion Devices, 22_24 of October 2007, Lisbon, Portugal

Experimental Setup

LML design constraints:

• UHV environment

(10-6 mBar)

• gallium corrosion

• gallium expansion

• gallium oxidation

• electrical isolation

requirements

17th TM on Research Using Small Fusion Devices, 22_24 of October 2007, Lisbon, Portugal

Operation of the LML

Initial state: lower storage tank filled with gallium

17th TM on Research Using Small Fusion Devices, 22_24 of October 2007, Lisbon, Portugal

Operation of the LML

Filling of a layer of Liquid Metal in the collector to damp gallium

droplets reflection.

17th TM on Research Using Small Fusion Devices, 22_24 of October 2007, Lisbon, Portugal

Operation of the LML

Filling of the upper reservoir from the lower one using the

MHD pump

17th TM on Research Using Small Fusion Devices, 22_24 of October 2007, Lisbon, Portugal

Operation of the LML

Filling of the upper reservoir from the lower one using the

MHD pump

17th TM on Research Using Small Fusion Devices, 22_24 of October 2007, Lisbon, Portugal

Operation of the LML

Jet is produced by hidrostatic pressure. Gallium is received in

the lower collector.

17th TM on Research Using Small Fusion Devices, 22_24 of October 2007, Lisbon, Portugal

Operation of the LML

Jet is produced by hidrostatic pressure. Gallium is received in

the lower collector.

17th TM on Research Using Small Fusion Devices, 22_24 of October 2007, Lisbon, Portugal

Operation of the LML

Upper reservoir is refiled directly from the collector in a

closed loop.

17th TM on Research Using Small Fusion Devices, 22_24 of October 2007, Lisbon, Portugal

Testing of the Liquid Metal Loop outside ISTTOK chamber

Aim of this experimental campaign:

• Assess the operating conditions of the Liquid Metal Loop.

• Assess the reliability of every components of the system.

• Detailed characterization of the produced liquid metal jets.

• Study the influence of magnetic field gradients on the jet in

conditions as close as possible to ISTTOK field.

Test conditions:

• Testing chamber: 33 mm diameter, 276 mm height Pyrex

tube.

• All the tests are performed with ~1,3 mGa pressure on the

nozzle.

• Nozzle sizes tested: (1.45, 1.80, 2.09 and 2.30 mm).

• Measured parameters: jet flow rate, BUL and time to reach

stability.

• Magnetic field: 0.25T, 60 ms pulse generated by two coils

in the Helmoltz configuration.

17th TM on Research Using Small Fusion Devices, 22_24 of October 2007, Lisbon, Portugal

Results of the testing of the LML in the experimental rig

Achieved operating conditions compatible with ISTTOK operation.

Only non crictical failures in the LML components after one year operation.

High speed jet movies used to check stability and BUL measurements.

Stable jet successfully produced.

2,3 mm nozzle most suitable for ISTTOK operation: 13 cm BUL, 2.5 m/s flow velocity.

Tested magnetic field gradients do not affect the jet.

An efficient droplet damping device has been tested.

17th TM on Research Using Small Fusion Devices, 22_24 of October 2007, Lisbon, Portugal

Plasma-Gallium jet interaction in ISTTOK: first experimental results

ISTTOK’s experiment: (R=0.46 m, a=0.085 m, BT=0.45T, IP~4-8 kA).

Instalation of the LML from the test facility to the tokamak by modules: colector, injector, pumping + storage circuit. Ga in solid state and under Argon atmosphere to minimize oxidation.

ISTTOK plasma discharges with Gallium jet interacting with plasma successfully performed.

Comparison of ISTTOK’s main parameters and radiated power with and without Gallium jet shows no evidence of strong plasma interaction

17th TM on Research Using Small Fusion Devices, 22_24 of October 2007, Lisbon, Portugal

Heat flux measurements in ISTTOK plasma

Deposited power estimations:

• q[W/m2]=eT, e= jsat =0.5encs

• Copper wire with jet dimensions

Peak power flux <3 MW/m2 , relevant as qjetq// (B ~ jet surface).

450 W (~5% Pin) for 30 ms <~13 J deposited in the probe.

Significant damage in the copper wire, although measurements were done for 9kW discharges.

Jet is not an efficient limiter : • small area (2.3 mm thick, < ¼ perimeter).

• only 14.7% of the ohmic power is collected by the jet.

• the graphite limiter still receives most of the input power.

17th TM on Research Using Small Fusion Devices, 22_24 of October 2007, Lisbon, Portugal

Estimate of the Gallium jet surface temperature increase

Calculations performed for 16 kW discharges.

Peak temperature increase in gallium jet surface is expected to be about 98 ºC.

Gallium evaporation is low since the maximum temperature is only ~170ºC (Pv~10-22 mBar).

As expected, an increase in jet flow velocity, reduces the jet surface temperature increase.

Increase of the jet surface temperature calculated from q(r) using:

t

0P

tdt

t)tq(

κπρC

1 ΔT(t)

and: r z2+0.062) z0+vjett)2+0.062)

17th TM on Research Using Small Fusion Devices, 22_24 of October 2007, Lisbon, Portugal

Plasma-liquid Gallium jet interaction in ISTTOK: Spectroscopy measurements

Based on a ½ m imaging spectrograph + CCD camera and multi-fiber optics for collections: 5 points in the plasma (1,2 cm span).

Allows the measurement of Gallium and ions distributions in the plasma.

No lines of Gallium appears without jet.

Distribution profiles shows high intensity for Ga in the vicinity of the jet and penetration of ions (Ga+ and Ga2+) into the center of the plasma.

No absolute calibration was available.

17th TM on Research Using Small Fusion Devices, 22_24 of October 2007, Lisbon, Portugal

Plasma-liquid Gallium jet in ISTTOK: Photodiode signals

Photosensor at Ø=Øjet

viewing port

Photosensor at Ø=Øjet+165º viewing port

Interaction of Gallium with plasma appears to be only local

17th TM on Research Using Small Fusion Devices, 22_24 of October 2007, Lisbon, Portugal

Conclusions

Plasma discharges with Gallium jet interaction doesn’t seem to significantly affect plasma performance peak temperature of the jet surface is “low” low evaporation rate.

Only Local effect. What would happen if Gallium would be the main limiter? (Unable to test in ISTTOK with the current setup).

After ~400 ISTTOK discharge with Gallium there is not yet any clear signal of chamber conditions deterioration

Future work:

• Absolute measurement of Gallium density profiles by LIF.

• Measurement of total power exhaustion from plasma by jet: surface temperature by IR sensor experiment underway.

• More detailed study of the jet-plasma interaction is required.

• New setup with multi-jet configuration is planned for 2008.

• Study of plasma-jet interaction in medium size superconductive FTU tokamak in Frascati (Italy).