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 1.Cryogenic pellet injector 2.Solid pellet injector Pellet behavior inside plasma 1.Diagnostics to study pellet behavior 2.Experimental results Plasma-wall interaction: 1.Images analysis 2.Comparison with a LCFS reconstruction Conclusions and future developments

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 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 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), F L = F 0 + F p they follow the pellet with velocity V p they expand at velocity V 0 F p leads to a drift velocity that stops them F 0 stops their transverse motion CIGAR SHAPED ABLATION CLOUD As long as the particles are neutral

Introduction Pellet injectors Cryogenic pellet injector Solid pellet injector i.Aims ii.Operation iii.Control code Pellet behavior inside plasma Plasma-wall interaction Conclusions and future developments

RFX-mod cryogenic pellet injector RFX VACUUM VESSELTILTING SYSTEM DIFFERENTIAL PUMPING CHAMBERS 8-SHOT UNITBARREL

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 sabot pellet

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

Introduction Pellet injectors Pellet behavior inside plasma Diagnostics to study pellet behavior i.Fast CMOS camera ii.Position Sensitive Device Experimental results i.Measurement of the q-profile Plasma-wall interaction Conclusions 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 fps Max resolution: from 1024x1024 pixels up to 1000 fps to 128x16 pixels at fps

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 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 Magnetic field profile in a RFPRelationship between pitch of the magnetic field w (r) and safety factor reversalB t =0=>vertical ablation magnetic axis B p =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 B t, B p and

Comparison measurement-theory Using two PSD instead of one the systematic error is removed.

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. actual detectors new detector

Introduction Pellet injectors Pellet behavior inside plasma Plasma-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 H α 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 DONETO 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.