Vacuum Spark Ion Source: High Charge States Ion Beam E.M. Oks, G.Yu. Yushkov, A.G. Nikolaev, and V.P. Frolova High Current Electronics Institute, Siberian Branch of the Russian Academy of Science, Tomsk, Russia

“Mother” of vacuum arc metal ion source is Mevva-II (LBNL) The Metal Vapor Vacuum Arc (Mevva) High Current Ion Source", I.G. Brown. IEEE Trans. Nucl. Sci. NS-32, 1723 (1985). Invited Paper :Particle Accelerator Conference, Vancouver, B.C., May MEVVA-II (1985) MEVVA ion sources were created and developed by Dr. Ian Brown and co-workers from Berkeley Lab at mid of 1980 th for producing of high current metal ion beams.

Modern version of Mevva-V ion source (LBNL) Extractor voltage: kV. Arc current: A Pulse duration:  s. Pulse repetition rate: single – 50 pps. Ion beam current: mA. Beam area – 100 cm. sq MEVVA –V is the last and widely used version of “MEVVA ion sources family”. It provides broad cross- section beam of metal ions with mean ion charge state from 1,0+(C) up to 2.6+(U). Highest ion charge states (for example) are following: C+, Mg2+, Ti3+, Fe3+, Zr4+, Mo4+, Ag3+, W5+, Pt3+, Au3+, Pb2+, U4+.

Elevation of ion charge state of metal ion beams The following methods for elevation of ion charge state of metal ion beams extracted from MEVVA ion source were used: Strong magnetic field : collaboration with GSI ( Darmstadt) and LBNL (Berkeley). Current spikes : collaboration with LBNL (Berkeley). Additional ionization of vacuum arc plasma by e-beam : collaboration with BNL (Brookhaven) and ITEP (Moscow). ECR heating of vacuum arc plasma – 2002: collaboration with IAP (Nizhny Novgorod, Russia). Spark mode of high current vacuum discharge – 2008: collaboration with LBNL (Berkeley).

Comparison of different methods MethodFeaturesEnhance of Increase of Highest ion charge state Strong magnetic field Magnetic field > 1 TFrom 2.1+ to 3.4+From Pt 3+ to Pt 4+ Current spike μs spike with current of A From 2.1+ to 2.4+From Pt 3+ to Pt 4+ E-beam ionization 100 A, 20 kV, 50 μs electron beam From 1.2+ to 3.5+From Bi 2+ to Bi 7+ Microwave heating Magnetic trap (5 T) and 100 kW, 75 GHz gyrotron From 2.1+ to 7.1+From Pt 3+ to Pt 10+ High Current Spark 4 μs, 3 кА discharge and low impedance capacitor and circuit From 2.1+ to 6.7+From Pt 3+ to Pt 10+

Experimental setup of current spark Current spark is initial phase of vacuum arc, or phase of vacuum arc development, when both high current and high voltage exist on the discharge gap. Capacitor charging voltage is 15 kV. Pulse duration of discharge is less 10 μs. Spark current is 15 kA.

Spark version of ion source and Mevva-V.Ru test bench (HCEI, Tomsk) TOF Spectrometer Vacuum Chamber Dry Pumping 60kV HV Terminal Computer Control Valeria Frolova Mevva-V.Ru

Spark discharge system and spark version of Mevva-V.Ru ion source

Examples of discharge current 10μF and 1μF are special pulse low-inductance capacitors with inductivity less then 20μН

Discharge current and ion beam current Pulse duration of extracted beam current is longer then discharge current due velocity and angle distribution of ions in discharge plasma.

Influence of magnetic field Magnetic field leads to increasing of mean charge state of metal ions, but the same time it leads to increase background gas ion fraction.

Influence of spark current For 3 μs pulse beams with ions up to Bi12+ where extracted. Exceeding current more than optimal leads to increase gas ion fraction and decrease of metal ion charge state.

New version of spark discharge system of Mevva-V.Ru ion source Distance cathode-anode was increasing from 1 to 4 cm Background pressure was decreased from 1E-6 to 3E-7 Torr

Spark current, voltage and pulse power for single pulse For beginning of current pulse discharge voltage is still high and power input in discharge may reach up to 4 MW! Optimal current for high metal ion charge state is about 4 kA.

Extracted ion beam current measured by Faraday cup (total ion beam current is about 1A) for a spark peak current 4kA and ion beam extraction voltage 30kV. Spark current and extracted ion beam

Spark current low then optimal current Mean ion charge state for bismuth ions is Highest charge state bismuth ions is 5+.

Spark current nearly the optimal current Mean ion charge state for bismuth ions is Highest charge state for bismuth ions is 15+.

Spark current higher then optimal current Mean ion charge state for bismuth ions is Highest charge state bismuth ions is 9+. Very high gaseous ion fraction.

Conclusion 1. Extractable high ion charge state ion beam was generated by Mevva-V.Ru ion source on spark mode. 2. Highest ion charge state Bi15+, with mean charge state Exceed the optimal current leads to decreasing ion charge state and gaseous contamination of extracted ion beam. These results open up the possibility of generating high current ion beams in the MeV energy range using a simple ion source with only 100 kV extraction voltage. Alternatively, for extraction voltage of less than 20 kV, it is possible to design an ion source that is very compact, X-ray free with a metal ion beam of up to 200 keV.

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