Ongoing, Upcoming and Planned Experiments in the DC Spark Lab

1. Ongoing, Upcoming and Planned Experiments in the DC Spark Lab Anders Korsbäck, Tomoko Muranaka, Nicholas Shipman, Iaroslava Profatilova

2. Overview • The research cases at the DC Spark Lab serve two purposes: • Provide a low-cost, low-complexity, high rep rate alternative to RF testing • Advance fundamental, “basic physics” understanding of breakdown • DC equivalents of RF measurements • Pulse width dependence of breakdown rate • Structure conditioning studies • Fundamental breakdown studies • Dark current fluctuation measurement • Temperature dependence of breakdown rate • Optical spectroscopy of breakdown plasma • Effect of subsurface voids on breakdown rate Anders Korsbäck, Tomoko Muranaka

3. Pulse width dependence of breakdown rate • DC Spark System II has been upgraded with the capability to vary pulse length • Objective is to reproduce the BDR ~ t6 dependence seen in RF, early results are positive Pulse Length (μs) BDR [#BDs/pulse] Anders Korsbäck, Tomoko Muranaka

4. Structure conditioning studies • Fixed-Gap DC Spark System turned out to be useful for the study of conditioning of electrodes when subjected to repeated pulses • Plan to implement pulsing feedback control algorithm used in XBox into DC for direct comparison • Rep rate of 1 kHz allows for much faster, cheaper data collection than XBox, possible to vary conditioning method, might give indications to the nature of the conditioning process (pulses or breakdowns question) Anders Korsbäck, Tomoko Muranaka

5. Dark current fluctuation measurement • Hypothesis originally advanced by Yinon Ashkenazy from the University of Jerusalem: Electric field causes (quasi-cyclical) microstructural processes to happen in the cathode, breakdown is triggered by a critical phenomenon (MeVArc 2014) • When an applied DC voltage causes a dark current across the gap, might current fluctuations be caused by such microstructural processes? • Approach: Measure fluctuations at different field strengths; if hypothesis true, expect fluctuations to grow when approaching breakdown voltage • First measurement attempts made in DC Spark System II, largest challenge is achieving necessary time- and current-resolution Anders Korsbäck, Tomoko Muranaka

6. Temperature dependence of breakdown rate • Theory developed at the University of Helsinki (Nordlund, Djurabekova) where breakdown is triggered by convergence of crystallographic defects at breakdown site • Due to effect of temperature on defect mobility, it would follow that breakdown rate is dependent on temperature. Thus, a study of the effect of temperature on breakdown rate might yield theoretical understanding of the breakdown process. • Doing the measurement requires temperature control, as well as gap control to offset the change in gap size caused by thermal expansion. DC Spark System II is being upgraded with these capabilities. Anders Korsbäck, Tomoko Muranaka

7. Optical spectroscopy of breakdown plasma • Composition and evolution of breakdown plasma is of interest for providing input for plasma dynamics simulations • Light emission spectroscopy used to measure time-evolution of relative abundances of different Cu ions in plasma • In early planning stage: Attempt to use light absorption spectroscopy developed at University of Helsinki (Meriläinen) for ultra-low-pressure manometry to monitor time-evolution of plasma density and matter discharge from cathode Anders Korsbäck, Tomoko Muranaka

8. Effect of subsurface voids on breakdown rate • Theoretical and simulational studies at the University of Helsinki (Djurabekova, Pohjonen, Parviainen) have suggested voids (diameter: few nm) near the cathode surface as possible breakdown sites • To test this hypothesis, void-filled cathodes have been manufactured through He+ ion bombardment of samples, which are to be annealed to make the He+ form bubbles and evaporate, leaving empty voids • Annealing to be done in-situ by temperature controller of DC Spark System II, after which breakdown measurements will be conducted Anders Korsbäck, Tomoko Muranaka