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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. Introduction Pellet injectors Cryogenic pellet injector

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slide1

Development of experimental devices to study first wall conditioning and transport phenomenain RFX-mod experiment

Stefano Munaretto

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

outline
Outline
  • Introduction
  • Pellet injectors
        • Cryogenic pellet injector
        • Solid pellet injector
  • Pellet behavior inside plasma
        • Diagnostics to study pellet behavior
        • Experimental results
  • Plasma-wall interaction:
        • Images analysis
        • Comparison with a LCFS reconstruction
  • Conclusions and future developments
slide3

Introduction

        • Usefulness of pellet in the fusion
        • Pellet as diagnostic
        • Pellet ablation
  • Pellet injectors
  • Pellet behavior inside plasma
  • Plasma-wall interaction
  • Conclusions and future developments
the pellet
The pellet

refueling

(pellet H or D)

density profile control

(pellet H or D)

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

wall conditioning

(pellet Li)

diagnostic

pellet as diagnostic
Pellet as diagnostic
  • transport analysis
  • pellet H or D
  • impurity pellet

pellet injection breaks stationary conditions

in stationary conditions only v/D can be studied

the magnetic confinement cannot be perfect

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

  • magnetic field diagnostic
  • ablation cloud follows magnetic field pitch
pellet ablation
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)

they follow the pellet with velocity Vp

As long as the particles are neutral

they expand at velocity V0

  • when the ablated particles are hot enough to be partially ionized they experience the Lorenz force (ms time scale),
  • FL = F0 + Fp

F0 stops their transverse motion

Fp leads to a drift velocity that stops them

CIGAR SHAPED ABLATION CLOUD

slide7

Introduction

  • Pellet injectors
        • Cryogenic pellet injector
        • Solid pellet injector
          • Aims
          • Operation
          • Control code
  • Pellet behavior inside plasma
  • Plasma-wall interaction
  • Conclusions and future developments
rfx mod cryogenic pellet injector
RFX-mod cryogenic pellet injector

8-SHOT UNIT

BARREL

DIFFERENTIAL PUMPING CHAMBERS

TILTING SYSTEM

RFX VACUUM VESSEL

rfx mod solid pellet injector
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 injector1
RFX-mod solid pellet injector

pellet

sabot

sabot loader

optical detectors

bumper and recovery box

pumping gate

driver gas

control code
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

load a sabot

the composed instruction in order to:

lunch the sabot

set free the barrel

it stops the injector when it is not working properly

slide12

Introduction

  • Pellet injectors
  • Pellet behavior inside plasma
        • Diagnostics to study pellet behavior
          • Fast CMOS camera
          • Position Sensitive Device
        • Experimental results
          • Measurement of the q-profile
  • Plasma-wall interaction
  • Conclusions and future developments
fast cmos camera
Fast CMOS camera

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

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.

two dimensional position sensitive device
Two-Dimensional Position Sensitive Device

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

  • 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.
pellet trajectory
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 trajectory1
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
Pellet ablation rate

Ablation rate measured by PSD

Hot structure

Pellet trajectory

magnetic field measurements
Magnetic field measurements

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
Ablation cloud temporal evolution

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

comparison measurement theory
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.

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

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

comparison measurement theory1
Comparison measurement-theory

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

problems with the trajectory
Problems with the trajectory

Possible reasons for the systematic error

Wrong 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.

new detector

actual detectors

slide23

Introduction

  • Pellet injectors
  • Pellet behavior inside plasma
  • Plasma-wall interaction
        • Images analysis
        • Comparison with a LCFS reconstruction
  • Conclusions and future developments
plasma wall interaction
Plasma-wall interaction

Fast CMOS camera can be also used to look at the Hα emission due to the plasma-wall interaction.

keys of the tiles

interaction

ports

warping
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
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:

deep reversal parameter

(F < -0.07)

modes with n > 24 are negligible

if the reversal parameter is shallow (F > -0.07) the mode m=0 has to be negligible wrt m=1 mode

conclusions and future developments
Conclusions and future developments

DONE

TO DO

  • 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.
  • 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.