Density control in RFX-mod A. Canton , S. Dal Bello Consorzio RFX, Padova, Italy

1. Density control in RFX-mod A. Canton, S. Dal Bello Consorzio RFX, Padova, Italy 13th IEA/RFP Workshop

2. Outline Introduction about RFX-mod Density behaviour in RFX-mod DESO: a diagnostic for the wall Dependence of plasma density on wall condition How to obtain a discharge with a desired plasma density on RFX-mod? Conclusions 13th IEA/RFP Workshop

3. RFX-mod first wall is entirely made of graphite Gas fuelling methods available at RFX-mod: Pre-pulse gas filling During pulse gas puffing Pellet injection Wall preloading by means of H2 GDC Wall treatment procedures: He GDC (performed on daily base or more) Baking + GDC cycles (3-5 per year) Boronization (1-3 per year) Introduction By a system of 8+8 piezoelectric valves installed at 4 toroidal positions Up to 8 pellet per shot with masses in the range 1.5-5x1020atoms 13th IEA/RFP Workshop

4. Density behaviour in RFX-mod - 1 Vacuum vessel is filled with gas before the shot. A minimum pressure of about 0.3x10-3mbar allows breakdown. Before reversal particles are not confined and are lost to the wall. Time of reversal 13th IEA/RFP Workshop

5. Density behaviour in RFX-mod - 2 RFX-mod first wall is an almost infinite reservoir of particles -> density at flat top (FT) is entirely sustained by particles fluxes from the wall -> it does not depend on the fuelled particles With a graphite wall density control requires to be able to control the status of the wall 13th IEA/RFP Workshop

6. We need to carefully control the particles both injected and extracted during the operations with plasma A new diagnostic tool, named DESO, has been implemented on RFX-mod. Aims: -> Improves the reproducibilty of the gas fuelling system a series of dummy openings of the valves (performed some minutes before the shot) stabilize their flow -> Computes the particles that fuel the discharge, both as filling and as puffing before the shot it executes the same opening of the valves programmed for the discharge and evaluates the number of particles entering the vessel by the measure of pressure -> Computes the neutral particles outgassed from the wall and extracted from the chamber after the shot DESO diagnostic system 13th IEA/RFP Workshop

7. One parameter adopted to describe the status of the wall is the Desorption factor Des% Extracted particles: -> evaluated by DESO Total fuelled particles: - By gas filling/puffing -> evaluated by DESO - By pellet injection -> pellet masses measured by the injector diagnostic system Extracted particles after the end of the pulse Des% = __________________________________________________* 100 Total fuelled particles Desorption factor Des% Des% expresses, shot by shot, the capability of the wall of absorbing the fuelled particles 13th IEA/RFP Workshop

8. Other two parameters: Particles left in the wall shot by shot: DeltaPart Particles stored in the wall at each shot: Total fuelled particles (filling, puffing, pellet injection,H2GDC) – Outgassed particles Particles stored in the wall by H2 GDC are computed following experimental results found in literature, see Winter,J. Nucl. Mater.161 (1989), p.265) Particles removed from the wall by He GDC (computed by mass spectrometer measurements) Total number of particles stored in the wall since last Baking or Boronization procedure: TotPart Particles inventory in the wall DeltaPart = Particles stored in the wall (– Particles removed from the wall by GDC) TotPart = Σ DeltaPart 13th IEA/RFP Workshop

9. Baking+GDC Behaviour of TotPart and Des% Number of particles to saturate the wall, assuming an implantation depth of the order of tens of nm [Tramontin et al., J. Nucl. Mater. 266-269 (1999), p.709] We are planning new experiments of samples exposition to the plasma in order to allow a better computation of the wall volume where particles are stored • Daily He GDCs (20-60 min) remove 1-2x1021 H atoms and allow operation, but they are not effective in preventing particle accumulation in the wall • Des% alone is not an absolute indicator of saturation state for the wall. It depends on plasma parameters (e.g. current), plasma-wall interaction, wall temperature... 13th IEA/RFP Workshop

10. Absorbing wall: wide range of density regimes Not absorbing wall: I/N forced to the value 2x10-14 Am Wall and density - 1 Analysis over a database of about 700 shots. Quantities averaged over 2ms around max plasma current. 0.3<I<1.6 MA, 1<ne<14 1019m-3 • ...but plasma itself extracts particles from the wall (PWI) • Density depends on Ohmic Power, that regulates particle influxes from the wall • Wall condition affects density... • Des% leads I/N • No correlation found between Des% and ne or I alone 13th IEA/RFP Workshop

11. Wall and density - 2 No dependence of density and/or I/N on the total number of particles stored in the wall Where are the particles stored? Implantation depth, toroidal and poloidal asymmetries can cause that stored particles are not accessible by plasma. 13th IEA/RFP Workshop

12. About particles inventory in the wall In RFX-mod we outgas a minimum part of the particles that we inject Outgassed particles depend (only?) on the temperature of the wall: limited to <100°C To avoid particles accumulation in the wall we should always fuel the discharge with the minimum gas to allow breakdown 13th IEA/RFP Workshop

13. How to obtain a discharge with a desired plasma density? - Control particle fluxes from the wall Control PWI With respect to RFX, the improved control of magnetic boundary (lower error fields and rotating modes) and a new design of the graphite tiles has allowed the operation of RFX-mod up to 1.6 MA - Control the status of the wall It means to control both particle content and their spatial distribution in the wall Optimization of He GDC with respect to the plasma to be done - Effectively fuel the discharge Gas Filling -> useless: between breakdown and reversal the particles are lost and stored in the wall Gas Puffing -> on RFX-mod proved to be inefficient due to the low flow and gas velocity of the piezo valves: neutral particles do not penetrate the plasma and accelerate particle accumulation in the wall Pellet injection -> density can be transiently raised but density control is quickly lost because of the large amount of injected particles that are left on the wall 13th IEA/RFP Workshop

14. H2 GDC as a way to obtain reproducible plasma density Wall pre-loading by means of H2 GDC is under test as a reproducible method to obtain a discharge with desired flat top density. -> Technique originally developed as a way to condition the wall after heavy treatments like boronisation. -> The same technique tentatively applied after the morning He GDC. Reproducible correspondence between the number of implanted particles and the obtained plasma density, even if there is a factor 10 between the two. Optimization in progress -> At the maximum explored plasma currents I~1.5 MA the effect of pre-load lasts for only one shot, at I<1MA it lasts for several shots (of the order of 10) The influence on the duration of the pre-load treatment of the voltage applied to the GDC anodes, that determines the energy and hence the depth of the particles implanted in the wall during the H2 GDC, and of the implant fluence that influences the depth profile, is under test. 13th IEA/RFP Workshop

15. Conclusions • In RFX-mod density control is strongly influenced by the wall • A new diagnostic tool, DESO, has been recently implemented. On the one side it is devoted to increase the reproducibility of the gas fueling system, on the other side it allows to precisely compute the particles stored in the wall and to characterise its behaviour • The relation between plasma density and wall condition has been explored over a large database of pulses. The capability of the wall to absorb particles, described by the desorption factor Des%, was found to influence the value of I/N more than the absolute number of particles stored in the wall • The strength of plasma wall interaction was found to be the other factor that determines plasma density, confirming the importance of the control of PWI in particular in view of operation at 2 MA in the next future of RFX-mod • H2 GDC is a promising way to obtain reproducible plasma density but the techinque need to be optimised 13th IEA/RFP Workshop

16. Wall and density - extras 13th IEA/RFP Workshop