Low Emittance Electron Gun Research Articles

Design and performance of a low-emittance electron gun for electron beam probe

An Electron Beam Probe (EBP) is preferable to provide a non-interceptive measurement of the transverse distribution of a dense bunched ion beam. During EBP operation, it was observed that the electron beam spot subjects to expansion when scanned across the ion beam due to the strong space charge field of the ion bunch, leading to significant degradation of spatial resolution. To mitigate this effect, a small electron beam spot is beneficial as demonstrated in previous research. This paper presents design details of a low-emittance, small-spot, and long-work-distance electron gun. Geometry optimization of the gun and focus lens is conducted using a multiobjective genetic algorithm (MOGA) interfaced with space charge simulation codes to maintain normalized thermal emittance within an acceptable increase and achieve a small spot at a long work distance. The emittance is measured using a slit-screen technique, resulting in a norm. rms emittance of 0.046(+-0.005) um*rad under low beam current conditions (50 nA). In high-resolution mode, a beam spot size of 0.6 mm (rms) is achieved at a work distance of 1.2 m with the combination of Einzel lens and solenoid. The gun has been successfully operated as part of EBP experiments for accurate and high-resolution measurement of ion beam width, demonstrating excellent agreement with wire scanner measurements with relative deviation not exceeding 1%.

Multilateral surface analysis of the CeB6 electron-gun cathode used at SACLA XFEL.

The CeB6(001) single crystal used as a cathode in a low-emittance electron gun and operated at the free-electron laser facility SACLA was investigated using cathode lens electron microscopy combined with X-ray spectroscopy at SPring-8 synchrotron radiation facility. Multilateral analysis using thermionic emission electron microscopy, low-energy electron microscopy, ultraviolet and X-ray photoemission electron microscopy and hard X-ray photoemission spectroscopy revealed that the thermionic electrons are emitted strongly and evenly from the CeB6 surface after pre-activation treatment (annealing at 1500°C for >1 h) and that the thermionic emission intensity as well as elemental composition vary between the central area and the edge of the old CeB6 surface.

Low-emittance Electron Gun for X-ray Free Electron Laser at SPring-8

A low-emittance electron gun has been developed for the x-ray free electron laser facility at SPring-8 that was named as SACLA. The gun uses a single-crystal cerium hexaboride (CeB6) cathode as a thermionic emitter to produce a low-emittance electron beam. A pulsed high-voltage of 500 kV is applied on the cathode to obtain more than 1 A peak current without emittance degradation. A normalized emittance of the beam was measured to be 0.6 mm mrad. This value is satisfied with a requirement of SACLA. The CeB6 electron gun is briefly reviewed in this paper.

Recent Developments of Low-emittance Electron Gun for Accelerator

Recent developments of low-emittance electron guns for accelerator are reviewed. In the accelerator field, DC biased triode thermionic gun (Pierce type gun) has been widely used and is still conventional. On the other hand, because of strong demands on the high brightness electron beam by FEL and other advanced accelerator concepts based on linear accelerator, the low emittance beam generation becomes one of the most important issue in the accelerator science. The R&D effort is “accelerated” by two technological innovations, photo-cathode and RF gun. They made a large improvement on the beam emittance. After the explanations on the technical and physical aspects of the low emittance electron beam generation, advanced electron sources for accelerators are reviewed.

Design of a low emittance electron gun with GV/m high field

The design of the electron gun for generating a low-emittance beam is presented. This gun employs a diode for generating the beam and a 1.6-cell RF cavity for accelerating up to relativistic energies. The goal of this gun is to generate a beam of ≈ 1 nC and 100 A peak current with a normalized rms transverse emittance of below 1 π μ m . In the diode region, pulsed voltage with a pulse duration of ≈ 1 ns and voltage of 2 MV is applied to the 2-mm-gap between cathode and anode, generating a field gradient of ≈ 1 GV / m . The combination of the high-field diode and the RF cavity can reduce space charge effects on emittance to a greater extent than an RF gun and solenoid field system. A Cs–Te was selected to match the requirement of a photocathode with high quantum efficiency (QE) since the laser power available to us at KEK-ATF gives ≈ 5 μ J / pulse at a UV wavelength of 266 nm. A diamond is used for the substrate to allow UV light to be injected from the back of the cathode. To estimate the generated beam emittance, a simulation was performed using the general particle tracer (GPT) code. Details of the design and simulations are presented.

Test of a High-gradient Low-emittance Electron Gun

A maximum electric field E = 2.65 GV/m with an accelerated electron current of 1 KA has been obtained, for pulse lengths of 130 ps, in an electron gun based on Pulse Power Technology. This is the highest accelerating field ever achieved in the presence of such a large current. Measurements of beam emittance and energy from 0.4 to 2.65 MeV show that the scaling of the invariant emittance with electric field and with beam current is consistent with theoretical predictions. A few applications of high-gradient acceleration are discussed.

A very low emittance electron gun for crystallized beam

This paper illustrates the project for a very low emittance electron beam. The beam so produced will be used for a feasibility study on ion beam crystallization using the electron cooling technique. The electron beam is generated by a GaAs photocathode and accelerated adiabatically in order to maintain the longitudinal low temperature and to prevent relaxation. Experimental results regarding the GaAs source and the principle of adiabatic acceleration are herein presented and discussed.

X-Ray Source without Moving Parts for Ultra-High Speed Tomography

A method for producing a sequence of intense X-ray focal sources, by azimuthally sweeping apulsed electron beam around a large diameter target path circumscribing the patient, was demonstrated during the Spring of 1977 in laboratory tests at Palo Alto using the full scale geometry of a whole body scanner, at electron energies in the range of 130 to 200 kV and with peak currents of 200 to 400 mA. A low emittance electron gun together with a multi-element beam handling system was employed to overcome a variety of beam optic problems and to provide small X-ray focal sources of controllable and reproducible shape and position. Some of the initial test results illustrating the effectiveness of this technique will be discussed. Also discussed are alternative, more recently investigated methods, in which immediately prior to executing an X-ray scan, the HV system is energy stabilized by loading the azimuthally fast sweeping high power electron beam into a radiation shielded, annular collector located contiguous to the target. With this method, a rapid sequence of X-ray sources is produced (a) in the case of a pulsed electron beam, by using a low intensity dc field to kick the beam onto the annular target, and (b) in the case of a dc electron beam, by using a programmed low perturbing field to radially oscillate the beam on and off the target during the azimuthal sweep.