Investigation of the Boundary Layer during the Transition from Volume to Surface Dominated H − Production at the BATMAN Test Facility Christian Wimmer, Loïc Schiesko, Ursel Fantz and the NNBI-Team Max-Planck-Institut für Plasmaphysik ICIS 2015 – 16th International Conference on Ion Sources (23 – 28 Aug 2015)

Investigation of the Boundary Layer during the Transition from Volume to Surface Dominated H − Production at the BATMAN Test Facility Max-Planck-Institut für Plasmaphysik Outline 1.Prototype H − source for ITER N-NBI (⅛ size) 2.Source diagnostics at BATMAN 3.Influence of caesiation on extr. currents, potentials and H − density 4.Conclusion

Prototype H − source for ITER N-NBI (⅛ size) Driver: plasma generation (H, H x +, e −, …) ICP: f = 1 MHz, P = kW T e = 10 eV, n e = m -3 Expansion volume with magnetic filter field: reduction of T e and n e in front of the plasma grid (≈ factor 10) → Reduce H − destruction: H − + e −  H + 2e − Caesiated Plasma grid (PG): production of H − and extraction: H, H x + + surface e −  H − caesium layer (low work function) caesium oven driver Ø = 24.5 cm ℓ = 14 cm expansion chamber 32x59x23 cm 3 Faraday screen RF coil magn. filter field Cs H - Cs 2 Volume close to PG: boundary layer

Prototype H − source for ITER N-NBI (⅛ size) 3 Co-extracted electrons: heat-load on second grid (max. several 10 MW / m 2 ) limits tolerable amount: j e /j H − < 1 positively biased Positive bias of PG: reduction of j e 1) 2) 3) S N N S S N extraction system 1)plasma grid 2)extraction grid 3)grounded grid Clean source without Cs: volume production of H − dominant H 2 (ν) + e − (T e ≈ 1 eV) → H − + H Stripping losses in accelerator: (H − + H 2 → H + H 2 + e − ) p ≤ 0.3 Pa

BATMAN test facility BATMAN (BAvarian Test MAchine for Negative ions): 4 Pulsed operation: Plasma phase: < 10 s (incl. 4 s beam extraction) 200 s vacuum phase between pulses Standard configuration of magnetic filter field (magnets in diagnostic flange) Main aim: Investigation of physical aspects!

BATMAN test facility BATMAN (BAvarian Test MAchine for Negative ions): 5 Spring 2015 campaign: Starting with a cleaned source (Cs free → volume production of H − ) Slowly evaporating Cs: transition to surface dominated H − constant operational parameters (p = 0.3 Pa, P RF = 60 kW, U ex = 5 kV) Following this process with diagnostics in boundary layer

Diagnostic setup 6 CRDS probe bottom probe top Characterization of the plasma close to PG: Cavity Ring-Down Spectroscopy (CRDS) → n H − (2.2 cm to PG) Langmuir probes (0.7 cm to PG) → Φ pl, T e, n i + Standard measures: Source performance j H −, j e Bias voltage U bias (constant bias current: I bias = 5 A)

Cs conditioning: source performance 7 1 st day of Cs conditioning: j H − : increase by a factor of 9.7 (1.7 mA/cm 2 → 14.8 mA/cm 2 ) j e : decrease by a factor of 1.8 (43.1 mA/cm 2 → 24.3 mA/cm 2 ) Thus: j e / j H − decreases by a factor of 17 Obviously: Minority species reacts more sensitive on Cs conditions (here: H − in badly conditioned source)

Cs conditioning: transition to ion-ion plasma 8 Transition electron-ion towards ion-ion plasma: H − becomes dominant negatively charged particle

Cs conditioning: potentials 9 Plasma potentials: Φ pl,top & Φ pl,bot lowered by ≈ 10 V (change of charge fluxes from & towards wall) Bias voltage: Lowered by the same value as plasma potential → Potential difference in PG sheath (U bias – Φ pl ) almost constant

Cs conditioning: negative ions 10 Dependence n H − ↔ j H − : j H − > 6 mA/cm 2 : linear correlation (regularly seen for const. operational parameters) j H − < 6 mA/cm 2 : knee towards pure volume production Possible reasons for knee: different measurement areas for n H − & j H −, possible gradient in Cs coverage on PG change of transport of H − from surface towards volume during conditioning (n H − ↑, n e ↓) Cs oven extraction apertures CRDS CRDS LOS PG H − (volume) H − (surface) 2.2 cm

Cs conditioning: negative ions 11 Timetrace of n H − : Strong influence of beam extraction on n H − ! Removal of H − due to extraction (deep extraction meniscus) or change of plasma parameters? During conditioning: Reduction of n H − becomes lower! → Extraction meniscus does not penetrate into CRDS LOS!

Conclusions 12 BATMAN: Cs conditioning first time at 0.3 Pa, constant operational parameters followed by several diagnostics close to PG Langmuir probe characteristics: transition from electron-ion plasma to ion-ion plasma  Extracted currents: j H − reacts relatively stronger (x 9.7) than j e (/ 1.8)  Potentials: plasma potential & bias voltage (const. I bias = 5 A) is lowered  Potential difference in PG sheath (U bias – Φ pl ) almost constant  Comparison n H − ↔ j H − : linear dependence after reaching a certain conditioning stage  deviation close to pure volume production  reduction of n H − (extraction ↔ RF only) weaker with better conditioning 