development of experimental devices to study first wall conditioning and transport phenomena in...
TRANSCRIPT
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.