Development of experimental devices to study first wall conditioning and transport phenomena in

RFX-mod experiment

Stefano Munaretto

Università degli studi di Padova, International Doctorate in Fusion Science and Engineering

Outline

IntroductionPellet injectors

1. Cryogenic pellet injector2. Solid pellet injector

Pellet behavior inside plasma1. Diagnostics to study pellet behavior2. Experimental results

Plasma-wall interaction:1. Images analysis2. Comparison with a LCFS reconstruction

Conclusions and future developments

Introduction Usefulness of pellet in the fusion Pellet as diagnostic Pellet ablation

Pellet injectorsPellet behavior inside plasmaPlasma-wall interactionConclusions and future developments

The pellet

The pellet is a solid bullet that is injected into the plasma

refueling(pellet H or D)

density profile control(pellet H or D)

diagnosticwall conditioning(pellet Li)

pellet H or D

impurity pellet

Pellet as diagnostic

• magnetic field diagnostic

• transport analysis

ee nvnD

in stationary

conditions only v/D can be studied

pellet injection breaks

stationary conditions

the magnetic confinement cannot

be perfect

plasma wall interaction brings to the presence of impurities inside the plasma

ablation cloud follows magnetic field pitch

Pellet ablation

when the pellet enters the plasma, it begins to be eroded. The particles are arranged in an isotropic way around it (µs time scale)

when the ablated particles are hot enough to be partially

ionized they experience the Lorenz force (ms time scale),

FL = F0 + Fp

they follow the pellet with velocity Vp

they expand at velocity V0

Fp leads to a drift velocity

that stops them

F0 stops their transverse

motion

CIGAR SHAPED ABLATION CLOUD

As long as the particles are neutral

IntroductionPellet injectors

Cryogenic pellet injector Solid pellet injector

i. Aimsii. Operationiii. Control code

Pellet behavior inside plasmaPlasma-wall interactionConclusions and future developments

RFX-mod cryogenic pellet injector

RFX VACUUM VESSELTILTING SYSTEMDIFFERENTIAL

PUMPING CHAMBERS

8-SHOT UNIT BARREL

RFX-mod solid pellet injector

At the moment it is being installed on the experiment

Aims

Transport studies

First wall conditioning

Measurement of the pitch of the magnetic field lines

Features

Pellet speed: 50÷200 m/s

Pellet size: Ø 0.2÷2 mm x 0.2÷4 mm

Materials: mainly Li and C, but also everything is solid under normal conditions

RFX-mod solid pellet injector

driver gas

sabot loader

pumping gate

optical detectors

bumper and recovery box

sabotpellet

Control code

To control the solid pellet injector a dedicated software has been developed

• to move the pistons

• to interact with RFX-mod system

• to avoid dangerous situation

the basic instructions to operate with the injector

the composed instruction in order to:

load a sabot

lunch the sabot

set free the barrel

it stops the injector when it is not working properly

IntroductionPellet injectorsPellet behavior inside plasma

Diagnostics to study pellet behaviori. Fast CMOS cameraii. Position Sensitive Device

Experimental resultsi. Measurement of the q-profile

Plasma-wall interactionConclusions and future developments

Fast CMOS camera

Looking at the pellet with the fast CMOS camera from behind it is possible to have the temporal evolution of the inclination of the ablation cloud of the pellet.

Sensor: CMOS with 17μm pixel

Shutter: electronic shutter from 16.7ms to 1.5μs independent from frame rate

Frame rate: up to 109500 fps

Max resolution: from 1024x1024 pixels up to 1000 fps to 128x16 pixels at 109500 fps

Tamron25HA

12mm f/1.8

Two-Dimensional Position Sensitive Device

212

12 L

II

IIx

xx

xx

Two-Dimensional Position Sensitive Device (2D-PSD)

212

12 L

II

IIy

yy

yy

• It is a PN junction between two layers of resistors extremely homogeneous.

• The junction is photo sensitive: electrons produced by incident photons are collected at the electrodes.

• The current collected at each electrode is proportional to the distance of the light source from the electrode itself.

x

y

z

H

K

2

Fs1

Fs2

PSDl1

PSDl2

Pellet trajectory

• Pellet position is calculated considering the projected position on two PSD sensors.

• Because of errors, the projections of the two positions do not intersect.

• The assumed position of the pellet is the midpoint of the segment perpendicular to both lines.

• Only a small part of the trajectory can be reconstructed.

• Stray magnetic field at high plasma current can damage the detector amplifier.

Pellet trajectory

• From experiments it is known that the radial velocity of the pellet is constant.

• The pellet injection speed is measured with two optical detectors.

• The PSD looking at the pellet from behind, gives us the measure of the toroidal and poloidal deflection.

• Combining the two information the pellet trajectory can be reconstructed.

Pellet ablation rate

Ablation rate measured by PSD

Hot structure

Pellet trajectory

Magnetic field measurements

(r)B

(r)B

R

rq(r)

p

t

)(

1tan)( 1

rqR

rr

Magnetic field profile in a RFP Relationship between pitch of the magnetic field w(r) and safety factor q(r)

@ reversal Bt=0 => vertical ablation cloud

@ magnetic axis Bp=0 => horizontal ablation cloud

Ablation cloud temporal evolution

Penetration of the pellet inside the plasma looked with the fast CMOS camera

Comparison measurement-theory

Combining the temporal evolution of the inclination of the ablation cloud with the pellet position it is possible to have the shape of the q profile.

The shape is similar, but the radial position is different: there is a systematic error.

q-profile from external measurements of Bt, Bp and <Bt>

Comparison measurement-theory

Using two PSD instead of one the systematic error is removed.

Problems with the trajectory

Possible reasons for the systematic errorWrong assumption, the radial velocity inside the plasma is not constant.

THE RADIAL VELOCITY IS CONSTANT

The starting point of the ablation is not right.

It will be verified using an additional optical detector close to plasma edge.

actual detectors

new detector

IntroductionPellet injectorsPellet behavior inside plasmaPlasma-wall interaction

Images analysis Comparison with a LCFS reconstruction

Conclusions and future developments

Plasma-wall interaction

Fast CMOS camera can be also used to look at the Hα

emission due to the plasma-wall interaction.

ports

keys of the tiles

interaction

Warping

Using the keys of the tiles a map of the visible area

can be reconstructed.

This area can be warped with a fitting code.

The maximum position error is ± 2°

Comparison with the Last Closed Flux Surface

theoretical reconstruction of the plasma LCFS radius from

magnetic measurements

agreement with the images of the fast

camera under particular conditions:

if the reversal parameter is shallow (F > -0.07) the

mode m=0 has to be negligible wrt m=1 mode

modes with n > 24 are negligible

deep reversal parameter (F < -0.07)

Conclusions and future developments

DONE TO DO

Installation of the solid pellet injector on

RFX-mod.

Wall conditioning with lithium injection.

Impurities transport study.

Measurement of the pitch of the magnetic

field by lithium pellet injection.

Installation of a new optical detector.

Study and development of techniques to

analyze the plasma-wall interaction.

Development and preparation of

the solid pellet injector to connect

it to RFX-mod.

Measurement of the pitch of the

magnetic field by hydrogen pellet

injection.

Studies of the pellet trajectory

inside the plasma.

Validation of the techniques to

reconstruct the LCFS.