Stefano Munaretto

1. 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

2. 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

3. 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

4. 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

5. 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

6. 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

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

8. RFX-mod cryogenic pellet injector 8-SHOT UNIT BARREL DIFFERENTIAL PUMPING CHAMBERS TILTING SYSTEM RFX VACUUM VESSEL

9. 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

10. RFX-mod solid pellet injector pellet sabot sabot loader optical detectors bumper and recovery box pumping gate driver gas

11. 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

12. 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

13. 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.

14. 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.

15. 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.

16. 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.

17. Pellet ablation rate Ablation rate measured by PSD Hot structure Pellet trajectory

18. 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

19. Ablation cloud temporal evolution Penetration of the pellet inside the plasma looked with the fast CMOS camera

20. 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.

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

22. 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

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

24. 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

25. 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°

26. 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

27. 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.