S. Ivkovic

Nanosecond pulsed field emission from single-gate metallic field emitter arrays fabricated by moldinga)

Electrically gated pulsed field emission from molybdenum field emitter arrays was studied. Single-gate field emitter array devices supported by metallic substrates were fabricated by a combination of molding and a self-aligned gate process. Devices were tested in a low-inductance cathode holder compatible with the high-acceleration electric field of a pulsed diode gun. Pulsed field emission down to 1.1 ns was observed for single-gate devices with 1.2×103–1.2×105 emitter tips with 5 μm array pitches. Integrating the field emitter arrays in a high-voltage pulsed diode gun, the authors demonstrated nanosecond field emission at an acceleration field of 30 MV/m at the cathode surface and acceleration of the field emission electron beam up to 300 keV. In addition, transverse beam emittance of the single-gate devices was measured with two different array sizes.

Vacuum breakdown limit and quantum efficiency obtained for various technical metals using dc and pulsed voltage sources

For the SwissFEL project, an advanced high gradient low emittance gun is under development. Reliable operation with an electric field, preferably above 125 MV/m at a 4 mm gap, in the presence of an ultraviolet laser beam, has to be achieved in a diode configuration in order to minimize the emittance dilution due to space charge effects. In the first phase, a dc breakdown test stand was used to test different metals with different preparation methods at voltages up to 100 kV. The authors show that gradient achieved for rough machined (Ra<200 nm) metal electrodes followed by an argon glow plasma are similar to the one obtained using a mirrorlike electrode (Ra<40 nm). In addition, high gradient stability tests were also carried out over several days in order to prove reliable spark-free operation with a minimum dark current. In the second phase, electrodes with selected materials were installed in the 250 ns full width at half maximum, 500 kV electron gun and tested for high gradient breakdown and for quantu...For the SwissFEL project, an advanced high gradient low emittance gun is under development. Reliable operation with an electric field, preferably above 125 MV/m at a 4 mm gap, in the presence of an UV laser beam, has to be achieved in a diode configuration in order to minimize the emittance dilution due to space charge effects. In the first phase, a DC breakdown test stand was used to test different metals with different preparation methods at voltages up to 100 kV. In addition high gradient stability tests were also carried out over several days in order to prove reliable spark-free operation with a minimum dark current. In the second phase, electrodes with selected materials were installed in the 250 ns FWHM, 500 kV electron gun and tested for high gradient breakdown and for quantum efficiency using an ultra-violet laser. PACS numbers: 68.37.Vj, 29.25.Bx, 32.80.-t, 32.00.00 ∗Electronic address: frederic.le.pimpec@psi.ch

Tesla Coil Design for Electron Gun Application

The current project is to build an electron gun for X-ray free electron laser (XFEL) application. The electron gun will utilize field emission and extreme accelerating gradient to achieve very low emittance. However for long-term study of cathode characteristics, a stable pulsed voltage in the megavolt range is needed. The first project phase is to design and test a 500 kV pulser using a resonant air-core transformer (Tesla coil). Detailed results of simulations with Microwave Studioreg and PSpicereg for various coil geometries, tuning and coupling factors are given, and the optimum values for this application are given. In addition, experimental results are given for the most promising geometries.

Experimental study of Diamond Like Carbon (DLC) coated electrodes for pulsed High Gradient electron gun

For the SwissFEL Free Electron Laser project at the Paul Scherrer Institute, a pulsed High Gradient (HG) electron gun was used to study low emittance electron sources. Different metals and surface treatments for the cathode and anode were studied for their HG suitability. Diamond Like Carbon (DLC) coatings are found to perform exceptionally well for vacuum gap insulation. A set of DLC coated electrodes with different coating parameters were tested for both vacuum breakdown and photo electron emission. Surface electric fields over 250MV/m (350–400kV, pulsed) were achieved without breakdown. From the same surface, it was possible to photo-emit an electron beam at gradients up to 150MV/m. The test setup and the experimental results are presented.

Sub-nanosecond electron emission from electrically gated Field Emitting Arrays

Field Emitting Arrays (FEAs) are a promising alternative to the conventional cathodes in different vacuum electronic devices such as traveling wave tubes, electron accelerators and etc. Electrical gating and modulation capabilities, together with the ability to produce stable and homogeneous electron beam in high electric field environment are the key requirements for their practical application. Due to relatively high gate capacitance, fast controlling of FEA emission is difficult. In order to achieve sub-nanosecond, electrically controlled, FEA based electron emission a special pulsed gate driver was developed. Bipolar high voltage (HV) pulses are used to rapidly inject and remove charge form FEA gate electrode controlling quickly electron extraction gate voltage. Short electron emission pulses (&#60;600 ps FWHM) were observed in low and high gradient (up to 12 MV/m) environment. First attempts were made to combine FEA based electron emission with radio frequency acceleration structures (1.5 GHz) using pulsed preacceleration. The gate driver design together with low inductance FEA chip contact system is described. The results obtained in low and high gradient experimental setups are presented and discussed.

Status of 500kV Low Emittance Electron Gun Test Facility for a Compact X-ray Free Electron Laser at Paul Scherrer Institute

The Paul Scherrer Institute (PSI) is developing an X-ray free electron laser (XFEL) [1] which relies on photo-assisted field emission and high gradient acceleration in a low emittance electron gun (LEG). The first phase of the LEG design incorporates a sub-microsecond 500 kV pulser, a high gradient acceleration diode structure and a diagnostic electron beam line. An air core (Tesla) transformer, in critically coupled mode [2], is generating 250 ns FWHM voltage pulse to give a high gradient between the mirror finished surfaces of the cathode and anode. The emitted electron beam passes to the diagnostic beam line through a 1.5 mm hole in the anode. The measured results are compared with numeric simulations. The timing and the amplitude stability of the pulser are discussed since they are sensitive parameters for this application.

Low Emittance Electron Gun for XFEL Application

The Paul Scherrer Institute (PSI) in Switzerland is planning to build a cost‐effective X‐ray Free Electron Laser (XFEL) facility for 0.1–10 nm wavelength and 10–100 fsec pulse length, requiring only 6 GeV electron energy. The facility will consist of a Low Emittance electron Gun (LEG) with high gradient acceleration and advanced accelerator technology for preserving the emittance during acceleration and bunch compression. To demonstrate feasibility of the project, a 4 MeV test stand followed by a new 250 MeV test stand will be used at PSI. An emittance of 200 pC, with >700 keV energy and >125 MV/m gradient.