VUV-diagnostics of inelastic collision processes in low temperature hydrogen plasmas J. Komppula & JYFL ion source group University of Jyväskylä Department of Physics Finland

Motivation ● Diagnostics challenging – Experimental plasma parameters – R i = n e n n i ∆T e = 1.5 eV → ∆R i = ±1000% ● This presentation – R i ∝ photon emission ∆T e = ±1.5 eV → ∆R i = ±50% – H - ion source vs. H + ion source ∝ R i τ ∝ n i Ion beam

Hot electrons ● Energy exceeds threshold of – Ionization (>15 eV) – Electronic excitation (>6-10 eV) photon H 2 (v>5), 2S, c 3 Π dissociation

Where is VUV from? Atom Molecule Balmer lines Lyman lines H2+H2+ H+H+ H2H2

Processes related to VUV emission 50% → H 2 (v>5) 15% → 2 H Ionization Lyman-band Molecular continuum ~100% → 2 H 30% → H 2 (c 3 Π) Proportional to

How to measure? ● Spectrometer – Good spectral resolution – Difficulties in calibration for absolute intensity ● Geometry! ● Diode + bandpass filtters – Lower spectral resolution – Easy to connect and use – ~15% accuracy for absolute values A. McPherson et al., Applied Optics, vol. 25, 1986,

Our apparatus 10 cm

Measurement of reaction rates ● Measure photodiode current + Simple equation → volumetric emission rate in line of sight volume + coefficient → volumetric reaction rate n e n n ionz n e n n Ly-Band = ~ constant min! Ionization30% v>520% Dissociation (singlet)20% Dissociation (triplet)50% Metastable states50% Order of magnitude ~20% ~50% ∆T e = 1.5 eV T e = 3-25 eV n e n n x10 x1000 Total uncertainty

There is a catch ● Monitor optical elements ● Check background signals ● Optical elements unstable in VUV range ● Significant background signal possible – e.g. filament

Benchmarking of simulations ● Diagnostics of the most significant electron impact processes – Included in simulations – High statistical noise in PIC-MCC simulations – Importance of validation H. C. Kim e al., J. Phys. D, 38 (2005) R238 M. J. Kushner, J. Phys. D, 42 (2009) M. M. Tuner, Plasma Sources Sci. Technol., 24 (2015)

New hybrid simulation? ● Reaction rates of hot electrons from experiments ● Dynamics of the cold (main) electron populations from simulation – PIC, fluid model, etc

Development of ion sources? ● Diagnostics about ion production ● Surface processes and diffusion are important but... – Feedback from mechanical modifications – Approve or exlude conclusions H 2, H + and H - ions +

Studied ion sources H - : filament driven multicusp arc H + : 2.45 GHz microwave Power: 1 kW

Results H2H2 H P(metal)~10% P(BN)~<<1% Filament arc superior driver for H - volume production Extremely poor energy efficiency in microwave discharge Microwave discharge a good proton source due to surface association H - (filament arc)H + (2.45 GHz) H 2 ionization x /cm 3 s v>5 production7-81-2x /cm 3 s Dissociation (singlet)20.3x /cm 3 s Dissociation (triplet)2-43-6x /cm 3 s Total photon emission>15-20%<8%of P heat Lyman-alpha/Lyman-band<15 Singlet/triplet~4~1 Reactions per moleculeSeveral tens2-20

Read more VUV diagnostics of electron impact processes in low temperature molecular hydrogen plasma J. Komppula and O. Tarvainen 2015 Plasma Sources Sci. Technol VUV irradiance measurement of a 2.45 GHz microwave-driven hydrogen discharge J. Komppula et al 2015 J. Phys. D: Appl. Phys My PhD thesis will be published in December PS. I need a job! (Contract will end in December)