High-Current Carbon-Epoxy Multi-Capillary Cathode Tal Queller, Jozeph Z. Gleizer and Yakov E. Krasik Plasma and Pulsed Power Laboratory Physics Department, Technion, 32000 Haifa, Israel Vladimir Bernshtam Department of Physics, Weizmann Institute of science, 61000 Rehovot, Israel

Outline A brief review of research of plasma cathodes for generation of high-current relativistic electron beam in our lab in the last 12 years Active plasma cathodes – Plasma is produced prior to the accelerating pulse application [Ferroelectric Plasma Source (FPS, Hollow Anode with incorporated FPS] Passive plasma cathodes – Plasma is produced by the accelerating pulse (metal-ceramic, velvet, carbon fibers, multi-capillary dielectric) The recent carbon epoxy multi-capillary cathode Experimental setup and cathode configurations Diode and plasma parameters Electron beam parameters Conclusions

Expectations from an ideal Pulsed Electron Source Instantaneous turn-on (low threshold electric field for electron emission initiation) Ethresh<10 kV/cm Low sensitivity to electric field rise time dE/dt<1011 V/(cms) Long electron beam duration t> 10-6 s High electron beam current density jbeam = 10 – 1000 A/cm2 Quasi-constant perveance of the diode Electron beam cross-sectional current density uniformity Electron beam arbitrary cross-sectional Repetition rate compatibility compatibility with vacuum P =10-4 – 10-5 Torr long lifetime # of pulses >107

Diagnostic techniques Fast framing light imaging – plasma initiation time and spatial distribution Time– & space-resolved plasma composition, density, temp. spectral measurements – & expansion velocity Thomson scattering – plasma electron energy distribution Microwave cut-off – plasma electron density Laser Induced Fluorescence – plasma ion temperature X-Ray imaging – electron beam cross-sectional uniformity Electrical measurements – Integrated & collimated arrays of FCs VDs & self- integrated RCs-CVR Single & Double floating probes – plasma density, electron temp. and plasma potential Retarding spectrometer – plasma electron & ion energy Fast Penning probe – background pressure dynamics Pin-hole cameras – electron beam macro- and micro-divergence

Active Cathodes Ferroelectric Plasma Source Light emission 10ns rear electrode ceramic front Mechanism – Incomplete surface discharge along the surface of the ferroelectric, initiated by the driving pulse in the metal-ceramic-vacuum triple point grid 120mm Diode & Electron beam parameters X-Ray 20ns Plasma parameters 5mm Advantages Arbitrary cross-section Uniform electron beam cross-sectional density Instantaneous electron beam formation E, 106 V/m 5 Metal Dielectric Vacuum 10 Drawbacks Requires an additional driving pulse system Duration ≤510-7s Deterioration of vacuum Current density ≤100 A/cm2

Active Cathodes FPS-assisted HA Diode & Electron beam parameters FPS igniter Mechanism – Electrons from the plasma formed by the FPS are accelerated and ionize background and desorbed from FPS gas inside the anode. Depending on the discharge current Light 120mm 20ns Plasma parameters X-ray 10ns Diode & Electron beam parameters Advantages Arbitrary cross-section area Instantaneous electron emission Long (10-6 – 10-5 s) pulse duration Drawbacks Electron current density <50 A/cm2 Additional plasma source Plasma penetrates into the accelerating gap Early and Sharp current rise because of the plasma pre-filling of the hollow anode space

Passive Cathodes Metal-Ceramic Diode & Electron beam parameters Light 120mm Light 20ns Plasma parameters X-ray 10ns Diode & Electron beam parameters Mechanism - Incomplete surface discharge initiated in triple points Dielectric constant of ~5 Drawbacks High plasma expansion velocity (5106 cm/s) High transverse electron beam velocity – beam divergence (120) High E ≥ 105 V/cm & dE/dt≥ 1013 V/(cm∙s) are required for uniform and fast plasma ignition Advantages Instantaneous plasma generation – a few ns (E ≥ 105 V/cm, dE/dt≥ 1013 V/cm∙s) Long lifetime - 107-108 shots Repetition rate compatibility for short pulses (tens of ns)

Passive Cathodes Velvet \ Carbon fiber 80mm Mechanism – a surface flashover along the fibers creates plasma 70mm Plasma parameters Light 20ns Diode & Electron beam parameters X-Ray 10ns Advantages Low electric field threshold – E < 104 V/cm (large field enhancement) Cross-sectional electron beam uniformity Quasi-constant impedance behavior due to slow (106 cm/s) plasma expansion velocity Long pulse duration (1µs) Drawbacks Short cathode lifetime < 10 shots at je>100A/cm2 Repetition rate incapability (relatively large outgassing due to the surface flashover mechanism)

Passive Cathodes Multi-Capillary Diode & Electron beam parameters Mechanism – Surface flashover plasma inside the cylindrical dielectric capillaries 90mm Plasma parameters Light 20ns X-Ray 10ns Diode & Electron beam parameters Drawbacks Fast plasma expansion >3106cm/s at je≥50 A/cm2 Outgassing Advantages Satisfactory uniform cross-sectional beam current density Long life-time Low-voltage threshold

Multi-Capillary Carbon-Epoxy Passive Cathodes The Recent : Multi-Capillary Carbon-Epoxy

Passive Cathodes Multi-Capillary Carbon-Epoxy Experimental Setup

Passive Cathodes Multi-Capillary Carbon-Epoxy Cathode geometries 1.2mm 30mm 25mm 7mm 0.7mm 75 capillaries 9 capillaries single capillary Voltage  300 kV Current  20 kA jav  600 A/cm2 jcap  17 kA/cm2 Beam duration 700ns Planar 70mm Voltage  150 kV Current  3 kA jav  400 A/cm2 jcap  10 kA/cm2 Beam duration 500ns Planar 30mm Strip 70mm Voltage  250 kV Current  6 kA jav  12 kA/cm2 jcap  170 kA/cm2 Beam duration 1s Voltage  40 kV Current  50 A jav  3 kA/cm2 jcap  13 kA/cm2 Beam duration 100ns Single Capillary Cathode geometries X-ray image Frame Duration 20ns

Passive Cathodes Multi-Capillary Carbon-Epoxy Typical diode voltage, current and impedance Plasma expansion velocity across the gap is slow (≤1.5∙106 cm/s) Long pulse duration Low electric field and dE/dt threshold for electron emission beginning Plasma Light Images Front view Time Frame Duration 20ns 50 ns 80 ns 130 ns 180 ns Plasma originates in almost all individual capillaries

Passive Cathodes Multi-Capillary Carbon-Epoxy Spectral emission intensity Light emission – side view Dense plasma remains in the cathode vicinity throughout the entire HV pulse Current density  650A/cm2 Frame Duration: 20ns

Passive Cathodes Cathode plasma expansion velocity Multi-Capillary Carbon-Epoxy Cathode plasma expansion velocity Hydrogen expansion velocity Carbon ions

Passive Cathodes Plasma density and ion temperature Multi-Capillary Carbon-Epoxy Plasma density and ion temperature (Stark and Doppler analysis) Spatial distributions (HV generator #2: 650 A/cm2) Hydrogen temperature Plasma electron density Plasma density in the vicinity of the cathode does not exceed 71014 cm-3 Relatively low plasma density indicates strongly on its flashover origin, e.g., inside the capillary (explosive emission plasma is characterized by n ≥ 1016cm-3)

Passive Cathodes Plasma parameters - Temporal distributions Multi-Capillary Carbon-Epoxy Plasma parameters - Temporal distributions B = 0.86 Tesla B=0 H atoms CII ions There is no significant change in the temperature during the accelerating pulse, and with\wo a magnetic field Electron temperature LINES – Time dependent CR Model simulation DOTS – Experimental data CR modeling indicates of high electron temperature : 11 eV.

Passive Cathodes Multi-Capillary Carbon-Epoxy Single capillary Frame duration : 20ns Single capillary Generator #3: I = 50 A ; je = 3 kA/cm2 Time Planar 30mm Generator #2 : I = 3 kA ; je = 400 kA/cm2 Anode plasma Plasma parameters The plasma is formed as a result of flashover inside the capillary !

Passive Cathodes Conclusions Multi-Capillary Carbon-Epoxy The source of electrons is a low-density and slow expanding plasma, typical of flashover process Plasma is formed as result of flashover inside the capillaries Long life time (>104 shots – no damages) This type of cathode could be used for applications requiring simultaneously: Long duration beam (10-7 – 10-6s) High-current density beam (102 – 104 A/cm2) Large total beam current (103 – 104 A) High energy beam electrons (>100 keV) Long lifetime Specific cross-sectional area of the beam

Acknowledgements Prof. Yakov Krasik Prof. Joshua Felsteiner Prof. Victor Gurovitch Dr. Anatoli Shlapakovski Dr. Joseph Gleizer Dr. Arkadi Sayapin Dr. Yoav Hadas Dr. Vlad Vekselman Dr. Dima Yarmolich Dr. Kostantin Chirko Dr. Vladimir Bernshtam Dr. Alexander Krohkmal Dr. Yuri Bliokh Dr. Alexander Dunaevskii Shurik Yatom Vladi Raikhlin Andrei Levin Kalman Gruzinski Hila Sagi Or Peleg Galina Bazalitski Svetlana Gleizer