M. Paraliev

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

Nanoseconds field emitted current pulses from ZrC needles and field emitter arrays

The properties of the electron source define the ultimate limit of the beam quality in linear accelerators such as free electron lasers (FELs). The goal is to develop an electron gun delivering beam emittance lower than the current state of the art. Such a gun should reduce the cost and size of an x-ray FEL (XFEL). In this article we present two concepts of field emitter cathodes which could potentially produce low emittance beam. The first challenging parameter for such cathode is to emit peak current as high as 5 A. This is the minimum current requirement for the XFEL concept from Paul Scherrer Institut (http://leg.web.psi.ch). Maximum currents of 0.12 and 0.58 A have been reached, respectively, with field emitter arrays and single needle cathodes. Laser assisted field emission gave encouraging results to reach even higher peak current and to prebunch the beam.

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.

Picosecond electrical switching of single-gate metal nanotip arrays

Electrical switching of single-gate all metal field emitter arrays is studied to generate subnanosecond electron pulses. By applying a bipolar current pulse method to the metal nanotip array, electron pulses with the duration down to 210 ps were generated. To explore the short-pulse limit of the proposed switching method, the propagation of the gate potential across the array was analyzed by numerical simulation. The result shows that single-gate field emitter arrays with the array diameter of 1 mm can be electrically switched with the duration down to ∼5 ps.

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.

Development of high performance electron beam switching system for Swiss Free Electron Laser at PSI

A compact X-ray Free Electron Laser (SwissFEL) is under development at the Paul Scherrer Institute. To increase facility efficiency the main linac will operate in two electron bunch mode. The two bunches are separated in time by 28 ns and sent to two undulator lines. The combination of two beam lines should produce short X-ray pulses covering wavelength range from 1 to 70 Å with submicron position stability. To separate the two bunches, a novel electron beam switching system is being developed. The total deflection is achieved with a combination of high Q-factor resonant deflector magnet, followed by a DC septum magnet. The shot-to-shot deflection stability of the entire switching system should be <;+/-10 ppm in amplitude and +/-100 ps in time, values which present severe measurement difficulties. Deflection magnets requirements, development and results of the kicker prototype are presented.

Development of high stability resonant kicker for Swiss Free Electron laser

A high stability resonant beam deflector system is being developed as part of the beam switching of the X-ray Free Electron Laser (SwissFEL) at Paul Scherrer Institute. To increase facility efficiency the main linac will operate with two electron bunches in one RF pulse. Three identical resonant kickers followed by a DC septum should separate the two bunches, that are 28 ns apart, and send them to two separate undulator lines. Shot-to-shot amplitude stability of each kicker should be better than +/- 82.5 ppm peak-to-peak, assuming the stability budget is equally split between phase and amplitude jitter. Development of the resonant kicker prototype and the precision measurement system is described. Preliminary measurements 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.

Electron beam collimation from an all-metal double-gate 40 000 nanotip array: Improved emission current and beam uniformity upon neon gas conditioning

In this study, the authors characterized field emission for stacked-double-gate all-metal field emitter arrays (FEAs) consisting of 40 000 nanotips. After careful conditioning of the FEAs under ultrahigh vacuum and in low-pressure neon gas ambient, the authors were able to produce a highly collimated beam with a current of ∼50 μA which showed an improved beam homogeneity. The beam rms radius reduced by a factor 10 and the transverse energy spread was reduced to well below 1 eV.