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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Università degli studi di Padova, International Doctorate in Fusion Science and Engineering
(pellet H or D)
density profile control
(pellet H or D)
The pellet is a solid bullet that is injected into the plasma
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
they follow the pellet with velocity Vp
As long as the particles are neutral
they expand at velocity V0
F0 stops their transverse motion
Fp leads to a drift velocity that stops them
CIGAR SHAPED ABLATION CLOUD
DIFFERENTIAL PUMPING CHAMBERS
RFX VACUUM VESSEL
At the moment it is being installed on the experiment
bumper and recovery box
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
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 (2D-PSD)
Ablation rate measured by PSD
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
Penetration of the pellet inside the plasma looked with the fast CMOS camera
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.
Using two PSD instead of one the systematic error is removed.
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.
Fast CMOS camera can be also used to look at the Hα emission due to the plasma-wall interaction.
keys of the tiles
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°
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