Efficiency of Cs-free Materials for H /D Production

Transcription

1 AG Experimentelle Plasmaphysik (EPP) University of Augsburg Efficiency of Cs-free Materials for H /D Production Roland Friedl 1, Uwe Kurutz 1,2, and Ursel Fantz 1,2 1 EPP Uni Augsburg, 2 IPP Garching 5 th NIBS conference ThuO6 12 th - 16 th Sept 2016 Oxford, UK

2 1. Introduction 1/12 Negative Hydrogen Ion Sources H 2 plasma: H, H 2, H x+, e Volume production H Surface production H H H H Converter surface High performance high current dens. reliable & reproducible stable (long pulses) homogeneous (large beams) Caesiation low WF Cs chemical reactive Complex redistribution dynamics Issues Careful conditioning Cs consumption Diagnostics Alternatives to Cs?

3 1. Introduction 2/12 Surface Assisted H Formation - Effect Materials Surface reformation Volume production (DA) Supporting volume formation Refractory metals (W, Ta) H + H ads H 2 (ν) H 2 (ν) + e H + H Caesiated surfaces Direct conversion Inherent low WF (LaB 6, MoLa) Direct e capture Sputtering of ads. H Carbon materials (Diamond, BDD) H, H x+, H ads + e (surface) H Low work function beneficial Comparative studies under identical and controlled conditions close to ion source parameters

4 2. Experimental setup 3/12 Laboratory Experiment HOMER (ECR) Microwave f = 2.45 GHz P max = 1 kw 31 cm Coil 15 cm Driver Meshed grid Nd:YAG λ = 1064 nm Coil Langmuir probe Downstream p 0.3 Pa H 2 /D 2 T e 1 2 ev n e m -3 n H m -3 Sample surface (floating or biased, heatable) Sample holder (height adjustable)

5 2. Experimental setup 4/12 Investigated Materials Effect Material group Materials Work function Supporting volume formation Refractory metals Caesiated surfaces Bulk Tantalum (Ta), Bulk Tungsten (W) Stainless steel + Cs (in-situ) 2.1 ev (bulk) Direct conversion Inherent low WF Lanthanum-doped molybdenum (0.7 % La, MoLa), Lanthanum hexaboride (LaB 6 ) < 3 ev < 3 ev Carbon materials Non-doped diamond, Boron-doped diamond (BDD) Investigations of H density depending on: Pressure: Pa Distance: cm Sample bias: V Temperature: C

6 2. Experimental setup 5/12 Rating of Cs-free Materials W, Ta, diamond, BDD, LaB 6, MoLa Cs-free material Laser photo detachment Influence on H density Influence on plasma parameter Permanent offset Influence on H volume production Efficiency of H formation vs. Stainless steel (V2A) reference In-situ caesiation 0-dimensional modeling YACORA H Diagnostics Langmuir probe n ion, EEDF, φ pl, φ fl OES n H, T gas, T vib

7 3. Results Cs 6/12 3x x10 16 Target for Cs-free materials - Cs Positive ion density Cs dispenser Density [m -3 ] 1x x x x10 15 Negative ion density Cs evaporation Cs emission Time [min] Laser Cs emission [a. u.] OES V2A χ Cs 2.1 ev (confirmed at lab exp. ACCesS Poster MonP18) H density increased by a factor of 2.5 Goal for Cs-free materials

8 3. Results Cs-free 7/12 Negative ion density [10 15 m -3 ] Cs-free Materials LaB 6 & BDD V2A Bias voltage [V] Bias between sample and vessel walls Influence on energy of charged particles impinging/leaving the surface Influence on plasma parameters Plasma potential always higher Particle energy varies between 20 ev and 1 ev

9 3. Results Cs-free 8/12 Negative ion density [10 15 m -3 ] Cs-free Materials LaB 6 & BDD T vib,low [10 3 K] 5.5 V2A V2A 2 V2A H (ν) + e H + H DA vs. AD H + H H 2 + e Bias voltage [V] Bias voltage [V] Bias voltage [V] n H / n H2 [%] V2A: H volume formation: absolutely determined by H 2 (ν) and H/H 2 relative variation due to EEDF (T e eff ) BDD: enhanced volume formation expected but no enhancement measured and severe plasma-induced erosion detected LaB 6 : similar volume formation expected but enhancement measured (χ < 3 ev)

10 3. Results Cs-free 10/12 Cs-free Materials MoLa in H 2 /D 2 Negative ion density [10 15 m -3 ] MoLa H MoLa D 2 2 V2A H 2 Positive ion density [10 16 m -3 ] MoLa D 2 MoLa H Atomic-to-molecular density ratio Bias voltage [V] Bias voltage [V] Bias voltage [V] MoLa (χ < 3 ev): 60 % enhancement comp. to V2A D 2 : enhanced volume formation expected (n e ) enhanced volume destruction expected (D/D 2 ) compensating Similar H /D density

11 3. Results Cs-free 11/ Cs-free Materials Overview Volume production Carbon materials Low work function Cs H /e density ratio Reference V2A Ta W Diamond BDD MoLa LaB 6 V2A + Cs No enhancement by: bulk Ta & W, diamond & BDD Only low work function materials enhance beyond volume formation No material comparable to Cs lowest work function

12 4. Conclusion 12/12 Conclusion Cs-free materials investigated under identical and ion source parameters Volume production Carbon materials Low work function Cs Compared to pure volume formation and caesiation H /e density ratio Reference No material comparable to Cs (χ bulk 2.1 ev, increase by 150 %) Carbon materials show severe erosion (none of the other materials) Most promising: materials with inherent low work function (LaB 6, MoLa) (χ < 3 ev, increase by 50 %) Comparable negative ion densities for H 2 /D 2 operation V2A Ta W Diamond BDD MoLa LaB 6 V2A + Cs