Plasma fuelling with cryogenic pellets in the stellarator TJ-II

• Published in: Nuclear fusion 2017-04, Vol.57 (5), p.56039
• Main Authors: McCarthy, K.J., Panadero, N., Velasco, J.L., Combs, S.K., Caughman, J.B.O., Fontdecaba, J.M., Foust, C., García, R., Hernández Sánchez, J., Navarro, M., Pastor, I., Soleto, A.
• Format: Article
• Language: English
• Subjects: 70 PLASMA PHYSICS AND FUSION TECHNOLOGY; ablation; fuelling; pellet; stellarator
• ISSN: 0029-5515; 1741-4326
• DOI: 10.1088/1741-4326/aa64fd

Cryogenic pellet injection is a widely used technique for delivering fuel to the core of magnetically confined plasmas. Indeed, such systems are currently functioning on many tokamak, reversed field pinch and stellarator devices. A pipe-gun-type pellet injector is now operated on the TJ-II, a low-magnetic shear stellarator of the heliac type. Cryogenic hydrogen pellets, containing between 3  ×  1018 and 4  ×  1019 atoms, are injected at velocities between 800 and 1200 m s−1 from its low-field side into plasmas created and/or maintained in this device by electron cyclotron resonance and/or neutral beam injection heating. In this paper, the first systematic study of pellet ablation, particle deposition and fuelling efficiency is presented for TJ-II. From this, light-emission profiles from ablating pellets are found to be in reasonable agreement with simulated pellet ablation profiles (created using a neutral gas shielding-based code) for both heating scenarios. In addition, radial offsets between recorded light-emission profiles and particle deposition profiles provide evidence for rapid outward drifting of ablated material that leads to pellet particle loss from the plasma. Finally, fuelling efficiencies are documented for a range of target plasma densities (~4  ×  1018-  ~2  ×  1019 m−3). These range from ~20%-  ~85% and are determined to be sensitive to pellet penetration depth. Additional observations, such as enhanced core ablation, are discussed and planned future work is outlined.