Microwave Gun Research Articles

Thermionic microwave gun for terahertz and synchrotron light sources.

Conventional thermionic microwave and radio frequency (RF) guns can offer high average beam current, which is important for synchrotron light and terahertz (THz) radiation source facilities, as well as for industrial applications. For example, the Advanced Photon Source at Argonne National Laboratory is a national synchrotron-radiation light source research facility that utilizes thermionic RF guns. However, these existing thermionic guns are bulky, difficult to handle and install, easily detuned, very sensitive to thermal expansion, and due for a major upgrade and replacement. In this paper, we present the design of a new, more stable, and reliable gun with optimized electromagnetic performance, improved thermal engineering, and a more robust cathode mounting technique, which is a critical step to improve the performance of existing and future light sources, industrial accelerators, and electron beam-driven THz sources. We will also present a fabricated gun prototype and show results of high-power and beam tests.

High brightness field emission from printed carbon nanotubes in an S-band microwave gun

Printed carbon nanotubes (CNTs) were applied as cold cathode and placed into an S-band microwave gun operating at 2856 MHz with the pulse duration of 2.8 μs. High brightness field emission was demonstrated and the current density achieves the value more than 4.2 A/cm2. The emittance of field emission beam is calculated to be nearly 21 μm based on the beam profile of emission electrons monitored via yttrium aluminum garnet screen. The infrared image of printed CNTs confirms that the emitters in the center contributed more electrons and the heat generated during the large current density field emission. The results in the paper imply that randomly distributed printed CNTs have the potential to be applied as the high brightness electron sources for free electron lasers.

Emissivity of Lanthanum Hexaboride Thermionic Electron Gun Cathode

The emissivity of the thermionic electron gun cathode material lanthanum hexaboride (LaB $$_6$$ ) at an operating temperature of 1622 K has been measured for wavelengths from 550 nm to 2400 nm. The emissivity is calculated from scanning monochromator spectral measurements calibrated with a tungsten lamp and the published emissivity of 0.82 at 1600 K for LaB $$_6$$ at 650 nm. The results show a higher emissivity at shorter wavelengths (up to 0.86 at 729 nm) and a lower emissivity in the near-IR to mid-IR (down to 0.41 at 2146 nm). These measurements were motivated by the need to make fast, accurate optical measurements of the temperature of the LaB $$_6$$ cathode in our thermionic microwave gun, and to select the wavelength most readily absorbed by the cathode for manipulation of its surface temperature as a means of counteracting back-bombardment heating.

Use of inverse tapering to optimize efficiency and suppress energy spread in an RF-linac free-electron laser oscillator

We have studied the operation of tapered undulator free-electron lasers using a realistic numerical model which accurately accounts for short-pulse effects, mode pulling, and coupled electron-optical beam instabilities. Our simulations are based on the Maxwell-Lorentz equations of motion, incorporating realistic optical resonator modes and electron density fluctuations, and accurately track the phase and energy of the electrons throughout their entire interaction with the optical pulse. The studies assume a 2-m taperable undulator with a normalized vector potential of roughly unity, driven by an electron beam from either a thermionic or photocathode microwave gun. Inverse tapering was found to provide greater extraction efficiency and optical power than conventional tapering in moderate gain systems using thermionic injector technology, and yielded over four times the extraction efficiency of an untapered undulator with minimal effect on the energy spread of the electron beam. In contrast, little improvement in efficiency or power output was observed using a photocathode injector due to loss of coherence at high gain. The remarkable spectral stability, laser power output, and reduced energy spread achievable using inverse tapering in moderate gain systems are discussed with respect to applications in remote sensing and spectroscopy.

Theoretical study on the transient process in a microwave gun with a thermionic cathode

In this paper we treat the Schottky effect with a polynomial approximately, and obtain the stored energy differential equation of the transient process. On the basis of the result, the equations both the electron energy gain and the best coupling coefficient can be obtained. The rate of change of the energy spread versus time at the gun exit and the function of the building-up field during the transient process can also be given. Finally, several pictures of a transient process in the microwave gun with a thermionic cathode are also shown. The theory makes an important guide for the coupling design of a microwave gun with a thermionic cathode.

The Mark III IR FEL: improvements in performance and operation

The Mark III has been improved both by the installation of a new thermionic microwave gun and added capabilities. The new electron gun yields a reduced emittance and allows operation at a higher repetition rate and an increased electron macropulse length. We have added the capability to tailor the input microwave pulse by adding a ramp or a step onto the macropulse. Phased-locked operation has been used to produce tunable optical radiation with filtered spectral widths of 200 MHz. Both phase-locking and hole-coupling have been used to increase the outcoupling efficiency. The RF system of the Mark III is now phase-locked to the RF system of the adjacent storage ring driver for the short-wavelength FEL sources of the laboratory, making possible two-color UV-IR pump-probe experiments.

The saturation of the Beijing FEL

The first lasing of the Beijing FEL was reported at the 1993 FEL Conference. After improving the performance of some components, saturation of the system was achieved in December 1993. The main measures adopted will be discussed in this report. First, the original undulator was replaced with a new one with better characteristics. Second, back-bombardment of the thermionic cathode microwave gun was reduced by deflecting magnet. With these and other modifications, saturated power output of 108 over the spontaneous emission was obtained. Saturation over the wavelength range 9–11 μm was achieved.

Measurements on photoelectrons from a magnesium cathode in a microwave electron gun

The performance of a magnesium cathode in a one-and-half cell photocathode microwave gun was measured at the Brookhaven Accelerator Test Facility (ATF). The frequency quadrupled Nd:YAG laser (266 nm) was used to stimulate the photoelectron emission. For a normal incident laser pulse, the quantum efficiency of the magnesium at 10 −7 Torr was measured to be 5 × 10 −4, which is more than 20 times the value for copper under similar conditions. After more than 5000 h operation, and despite some optical damage on the cathode surface, no decrease in the quantum efficiency of the magnesium cathode has been observed. The Schottky effect of photoelectron emission was also studied. No electron saturation effect was observed for a train of laser pulses. The photoelectron beam bunch length was measured to be 11 ps FWHM using a microwave kicker cavity, in agreement with a streak camera measurement of the laser pulse length. The measured rms normalized emittance is 2.5 mm mrad for a bunch charge of 80 pC, and rms radius of the laser spot 0.2 mm, in agreement with a simple analytical model predictions.

Backbombardment, phase length and transport process for BFEL

The backbombardment effect for a microwave gun with thermionic cathode is studied by using the approximate formula fitted with the experimental data of transverse magnetic fields. The variation of phase length, current, energy width and emittance in the transport process are investigated in detail for many transport system structures. The prediction will be useful in the experiments of our FEL.

A self-adaptive feedforward rf control system for linacs

The design and performance of a self-adaptive feedforward rf control system are reported. The system was built for the linac of the Accelerator Test Facility (ATF) at Brookhaven National Laboratory. Variables of time along the linac macropulse, such as field or phase are discretized and represented as vectors. Upon turn-on or after a large change in the operating-point, the control system acquires the response of the system to test signal vectors and generates a linearized system response matrix. During operation an error vector is generated by comparing the linac variable vectors and a target vector. The error vector is multiplied by the inverse of the system's matrix to generate a correction vector is added to an operating point vector. This control system can be used to control a klystron to produce flat rf amplitude and phase pulses, to control a rf cavity to reduce the rf field fluctuation, and to compensate the energy spread among bunches in a rf linac. Beam loading effects can be corrected and a programmed ramp can be produced. The performance of the control system has been evaluated on the control of a klystron's output as well as an rf cavity. Both amplitude and phase have been regulated simultaneously. In initial tests, the rf output from a klystron has been regulated to an amplitude fluctuation of less than ±0.3% and phase variation of less than ±0.6°. The rf field of the ATF's photo-cathode microwave gun cavity has been regulated to ±0.5% in amplitude and simultaneously to ±1° in phase. Regulating just the rf field amplitude in the rf gun cavity, we have achieved amplitude fluctuation of less than ±0.2%.

A compact free-electron laser design for extremely broadband harmonic operation in the infrared

Abstract Warren has suggested that operational on very high harmonic numbers using electromagnetic undulator may be an appropriate method to achieve short wavelength lasing with a low energy beam of very high brightness. If the short wavelength requirement is relaxed but the need for extreme broadband tuning is added, one finds that higher harmonic lasing is an attractive method to achieve lasing over the entire infrared range in a single device. The undulator required is now a very inexpensive hybrid device with a moderate period. This paper will describe the design of a device utilizing a photocathode equipped microwave gun, and a short accelerator and undulator. The accelerator will have a maximum energy of 25 MeV and a peak current of 125 A in 4 ps bunches. The undulator is a fixed strength undulator with only eight periods. Gain in such a device is in the range of 20% to 50% over the range of 3 to 100 μm via harmonic lasing on harmonics up to the ninth. A ring cavity design is proposed to ease output coupling and diffraction problems. A prototype version of this system could be installed on the Mark III accelerator at Duke.

Compact far-IR FEL design

Abstract A compact size far-infrared free-electron laser (FIR FEL) is currently being built at Stanford. A microwave gun products 1–3.3 MeV electrons, which are sent into a 50 cm long wiggler of 1 cm period through a hole on the upstream mirror to generate radiation at a wavelength of 100 to 1000 μm. A superconducting solenoid along with an array of permeable material is used to generate a 9.6 kG rms wiggler field with a 2.0 mm gap. The electron beam consists of 10 ps micropulses with 10 A peak current, 1% energy spread and unnormalized emittance for 90% of the particles of 2π mm mrad. A 10 dB small signal gain has been calculated with the parameters mentioned above. An overview of the design details as well as a discussion on the uniqueness of our wiggler are presented.

Simulation of microwave gun injector RF linac driving infrared free electron lasers

In contrast with the conventional injector consisting of dc gun, chopper, subharmonic buncher and/or fundamental buncher, a microwave gun can cooperate with an alpha magnet as a more compact high brightness injector for the dedicated rf linear accelerators driving infrared free electron lasers (FELs). Such a concept has been put into practice in the MARK III FEL and will be used in the Beijing Free Electron Laser (BFEL) as well. In this paper, the phase spread, energy spread, energy spread, emittance growth and intensity variation of a single electron microbunch in such an FEL accelerator system are investigated using SUPERFISH and PARMELA.

A review of the Stanford Mark III infrared FEL program

The performance of the Mark III infrared FEL with a new microwave gun will be reviewed. Operation of the accelerator is now close to design values. The Mark III has provided over 2000 hours of laser time to experiments in FEL physics, materials science and medical physics. Highlights of the experimental program will be presented and the new facility at Duke will be described.

Status report on the Stanford Mark III infrared free electron laser

We review recent experimental results from the Stanford Mark III infrared FEL. More than 900 h of lasing operation have achieved from 2.0 to 5.5 μm and continuous gap tuning has been achieved over the design range of 1.7:1. Micropulses with peak powers as high as 2 MW and pulse lengths as short as 500 fs have been measured. A multiple output coupler arrangement has been tested which allows variation of the output coupling from 0.1% to 7% in a matter of a few minutes. Accelerator performance will also be discussed and the results of recent improvements to the microwave gun will be presented. The measured brightness of this gun is in excess of 1012 A/cm2 which is sufficient for operation in the UV.

Design considerations of the Beijing free electron laser project

This paper first mentions the rationale to start a Compton regime, rf linac-based FEL research project in IHEP. Then the physical design parameters are given based on both the analytical formulae and a one-dimensional numerical computation. Considerations are given in sequence regarding the expected tuning range, the microwave gun performance, the longitudinal phase space, factors affecting the accelerated beam energy spread, beam transport system, optical resonator, and undulator design.