Plasma particle fuelling a general view of methods and issues David Terranova

RFX-mod program workshop, 7-9 February 2011, Padova, Italy Outline Why particle fuelling is necessary Particle fuelling techniques Issues with particle fuelling “Passive” particle fuelling: the role of the first wall Neutral particles penetration Impurities removal The divertor Summary

RFX-mod program workshop, 7-9 February 2011, Padova, Italy Plasma particle fuelling Plasma particle fuelling definition In other words it is the proportion of the deposited material that remains effectively in the discharge. The overall efficiency of a fuelling technique depends not only on the instantaneous matter deposition but also on its time of residence in the plasma and possibly induced wall pumping.

RFX-mod program workshop, 7-9 February 2011, Padova, Italy Fuelling and pumping system functions To provide and maintain the plasma density profile for the specified operation. To establish a density gradient for plasma particle flow to the edge (especially helium ash). To inject impurity gases for divertor plasma radiative cooling, wall conditioning and for plasma discharge termination on demand. To induce certain phenomena in the plasma (PEP, ELM pacing, …) To replace the deuterium-tritium (D-T) ions consumed in the fusion reaction. To exhaust He ash (and other impurities).

RFX-mod program workshop, 7-9 February 2011, Padova, Italy Gas puffing –Simple hardware. –High reliability and flexibility. –Poor deposition: 5% to 20%. Pellet injection Pellet injection (pneumatic, centrifuge) –Complex hardware. –Variable reliability, fixed pellet size. –High deposition up to almost 100%. Supersonic gas injection –Good effect mainly due to the cooling of plasma edge. –Lower first wall interaction with respect to gas puffing. –Deposition: 40% to 60%. Particle fuelling techniques

RFX-mod program workshop, 7-9 February 2011, Padova, Italy Issues in particle fuelling Particle deposition is never 100%.  B and B-curvature drift effects: –Can help HFS injected pellets. –Not useful for gas puffing. Consider pellet induced instabilities: –NTM –ELMS Study main gas particle transport. Exploiting the drift of the ablated material, High-Field-Side injection allows for a larger deposition in the plasma core at lower injection velocities with respect to Low-Field-Side injection. Issues raise on the technical possibility to inject from the HFS.

RFX-mod program workshop, 7-9 February 2011, Padova, Italy Pellet injection: HFS vs LFS Exploiting the drift of the ablated material, High-Field-Side injection allows for a larger deposition in the plasma core at lower injection velocities with respect to Low-Field-Side injection. Issues raise on the technical possibility to inject from the HFS.

RFX-mod program workshop, 7-9 February 2011, Padova, Italy Particle fuelling efficiency From a practical point of view, the optimization of ε f requires that the radius of maximum pellet deposition - tangency point between the pellet path and the flux surfaces in the off-axis injection configuration - is the closest possible to the magnetic axis. This means the combination of a launching point located on the HFS and of a high velocity. Attempts were made to harden the pellets by doping the frozen hydrogen with ~1% N 2.

RFX-mod program workshop, 7-9 February 2011, Padova, Italy Tore-supra long pulses: pellet vs gas puffing B. Pègourié, Plasma Phys. Control. Fusion 49 (2007) R87–R160

RFX-mod program workshop, 7-9 February 2011, Padova, Italy RFX-mod: pellet injection vs gas puffing Can we compare pellet injection and gas puffing experiments in RFX-mod? small medium large Pellet size Both “ long ” and “ short ” puffing periods can be considered. The DESO diagnostic allows for a precise determination of puffed particles. Courtesy of A. Canton

RFX-mod program workshop, 7-9 February 2011, Padova, Italy “Passive” particle fuelling: the role of the first wall First wall recycling: it depends on the material … First wall conditioning techniques … Particle inventory and first wall response … When the overall particle recirculation is controlled by wall recycling, it is still to be verified the possibility of controlling the particle balance and reducing the wall retention by optimizing the fuelling method (experiments done on Tore-supra).

RFX-mod program workshop, 7-9 February 2011, Padova, Italy RFX-mod first wall particle inventory Time history of the wall hydrogen content (top), and wall desorbing factor (bottom). Discontinuity in wall content are due to He glow discharge cleaning procedures. A. Canton, S. Dal Bello, 13 th IEA-RFP workshop 2008

RFX-mod program workshop, 7-9 February 2011, Padova, Italy The penetration of neutral particles is an important aspect when considering plasma particle fuelling. It can determine what technique can be used and to what efficiency. It is affected by many aspects that are interlinked :  the nature of the first wall  the nature of the source  the operational space, i.e. the typical density and temperature profiles Each of these points actually affects the others. Neutral particles penetration MST RFX-mod The low operational density of MST allows for a higher penetration and peaked density profiles differently from RFX-mod where low penetration and hollow density profiles are found. Courtesy of F. Auriemma

RFX-mod program workshop, 7-9 February 2011, Padova, Italy What about impurities removal … Also removing impurities is an issue linked to plasma particle “management”. Need to keep Z eff as small as possible. High n z implies: –High radiation losses. –Lower concentration of main gas. –Impact on confinement. A divertor is necessary. Need to address also impurities transport both with modelling and experiments: –Laser-Blow-Off –Impurity pellets LBO experiments show different behaviour in MH and QSH configurations. MH QSH S. Menmuir et al., Plasma Phys. Control. Fusion 52 (2010)

RFX-mod program workshop, 7-9 February 2011, Padova, Italy The divertor The materials facing the exhaust plasma are not in any direct contact with the main plasma. Consequently tokamaks with divertor plasmas have lower levels of impurities in the core and tend to achieve much higher temperatures in the core. A neutral gas cloud develops in the divertor region, high enough gas pressures can be achieved such that pumps are able to remove the now cold plasma exhaust. This process is crucial for the functioning of a reactor as it removes the fusion “ash” Helium. Localized and volumetric losses of plasma energy in the divertor region are important to evaluate. plasma flux in the SOL neutral atoms radiation

RFX-mod program workshop, 7-9 February 2011, Padova, Italy Tokamak and stellarator divertor Compared to the poloidal field divertor in tokamaks, island divertors in stellarators have usually longer connection lengths and shorter target-to-core distances enhancing the perpendicular-to- parallel transport ratio in the island SOL. Sketch of the principle of the divertor: single and double-null poloidal-field divertors in tokamaks and the island divertor where the island number n changes from machine to machine. Y. Feng et al. Contrib. Plasma Phys. 46, 504 – 514 (2006)

RFX-mod program workshop, 7-9 February 2011, Padova, Italy Limiter-like configuration deep reversal RFX-mod boundary The plasma boundary in high current SHAx RFP plasmas could be exploited for building a divertor by locating divertor plates with appropriate pumping in the regions of strong interaction (more and more regular as the amplitude of secondary modes reduces as plasma current is increased). Such RFP “helical-divertor” would be more similar to the island divertor in stellarators than to the tokamak divertor. divertor-like configuration shallow reversal E. Martines et al. Nucl. Fusion 50 (2010)

RFX-mod program workshop, 7-9 February 2011, Padova, Italy Summary Plasma particle fuelling is an essential issue both for present day experiments and in the perspective of a fusion reactor. Various techniques can be used with different efficiency depending on many interlinked aspects such as the technique itself and the configuration of the plasma (i.e. internal profiles). Both good and deleterious effects can take place when fuelling is done: they need to be addressed adequately. The “passive” role of the plasma facing components cannot be neglected as well. The overall efficiency of a fuelling technique depends on the instantaneous matter deposition as well as on its time of residence in the plasma: how do gas puffing and pellet injection compare on long timescales? Both main gas fuelling and impurities exhaust have to be considered. The divertor proved to be an essential part of the tokamak and stellarator configuration. What about the RFP?