H. Ishizuka

Smith–Purcell radiation experiment using a field-emission array cathode

We have recently started an experiment on visible Smith}Purcell (SP) radiation to examine practical applicability of a eld-emission array (FEA) cathode to compact free electron lasers, placing emphasis on safe operation of the cathode as well as beam quality. The electron beam was generated by a 5 cm long triode which employed either a single- or double-gated FEA. Accelerating voltages of up to !40 and !100 kV were applied to the cathode by a regulated power supply and a small Van der Graa! generator, respectively. A 25 lA beam of up to 45 keV was routinely produced and a5 lA 80 keV beam was also attained. The beam passed through a 1 mm wide slit in the anode and grazed the surface of a 2.5 cm long replica grating with a period of either 0.56 or 0.83 lm. The SP radiation has not been identied owing to irrelevant luminescence caused by the beam at the grating. Still it was conrmed that the FEA cathode is adequately durable and electron beams generated therefrom are su

Smith-Purcell experiment utilizing a field-emitter array cathode: measurements of radiation

ciently stable to be used for systematic measurements of radiation. ( 2000 Elsevier Science B.V. All rights reserved.

Laser-assisted electron emission from gated field-emitters

Abstract Smith–Purcell (SP) radiation at wavelengths of 350–750xa0nm was produced in a tabletop experiment using a field-emitter array (FEA) cathode. The electron gun was 5xa0cm long, and a 25xa0mm×25xa0mm holographic replica grating was placed behind the slit provided in the anode. A regulated DC power supply accelerated electron beams in excess of 10xa0μA up to 45xa0keV, while a small Van de Graaff generator accelerated smaller currents to higher energies. The grating had a 0.556xa0μm period, 30° blaze and a 0.2xa0μm thick aluminum coating. Spectral characteristics of the radiation were measured both manually and automatically; in the latter case, the spectrometer was driven by a stepping motor to scan the wavelength, and AD-converted signals from a photomultiplier tube were processed by a personal computer. The measurement, made at 80° relative to the electron beam, showed good agreement with theoretical wavelengths of the SP radiation. Diffraction orders were −2 and −3 for beam energies higher than 45xa0keV, −3 to −5 at 15–25xa0keV, and −2 to −4 in between. The experiment has thus provided evidence for the practical applicability of FEAs to compact radiation sources.

Design study of mm to nm diameter electron beams for submillimeter-wave to X-ray free-electron lasers

Abstract Enhancement of electron emission by illumination of gated field-emitters was studied using a 100xa0mW cw YAG laser at a wavelength of 532xa0nm, intensities up to 10 7 xa0W/m 2 and mechanically chopped with a rise time of 4xa0μs. When shining an array of 640 silicon emitters, the emission current responded quickly to on–off of the laser. The increase of the emission current was proportional to the basic emission current at low gate voltages, but it was saturated at ≈3xa0μA as the basic current approached 100xa0μA with the increase of gate voltage. The emission increase was proportional to the square root of laser power at low gate voltages and to the laser power at elevated gate voltages. For 1- and 3-tip silicon emitters, the rise and fall of the current due to on–off of the laser showed a significant time lag. The magnitude of emission increase was independent of the position of laser spot on the emitter base and reached 2xa0μA at a basic current of 5xa0μA without showing signs of saturation. The mechanisms of these phenomena are discussed.

Cross-Field Propagation of Intense Neutralized Proton Beam near a Metal Plate

Microfabricated field-emitter tips and arrays are promising candidates for an electron source of compact FELs. Examples are presented of moderate-energy high-brightness electron beams which are small in diameter and expected to be generated with the use of a Spindt cathode.

Propagation of Intense Pulsed Ion Beams across a Magnetic Field

An intense neutralized proton beam (70 keV, 18 A/cm2, 200–300 ns) is injected into a magnetic field of 2 kG across the magnetic lines of force where a thin metal plate is placed with its surface parallel to the beam axis and perpendicular to the magnetic lines of force. It is found that the polarization electric field in the beam is shorted out by the plate only when the beam is in contact with the plate, and then the beam is deflected according to gyro-motions of the beam protons; otherwise, the polarization electric field is retained and the beam penetrates into the magnetic field without deflection by polarization drift.

Experiments with field-emitter arrays for free electron micro-laser applications

The propagation of space-charge neutralized pulsed ion beams across the magnetic field was studied experimentally. The beam was generated by a reflex triode, and the ion energy peaked at 130 keV. A transverse magnetic field B ⊥ of up to 3 kG was applied to the drift tube over a distance of 20 cm. The maximum ion current density was approximately 10 A/cm 2 at the entrance of this region. The beam was not impeded by the magnetic field for (text{B}_{bot}{lesssim}2) kG in agreement with a theoretical prediction.

Cross-Field Propagation of Intense Pulsed Ion Beams

Abstract Propagation characteristics of beams extracted from double-gated field-emitter arrays (FEAs) were investigated. The beams were accelerated to several keV in a 15-mm long gun region and the whole beam traversed a distance of 1 m. Separate images of emitting tips were formed, when operating the focus lens of FEAs, on a fluorescent screen located at the downstream end. The images became clearer when the beam focusing was assisted by either an electrostatic or magnetic lens arranged in the transport system. A quadrupole lens was employed to determine the emittance diagram of the beam. It was verified that beamlets emitted from the tips were distributed on a straight line in the transverse trace plane. Normalized emittance of the beam thus obtained was a few times 10 −7 π m rad, independently of the beam current within a measured range of 10 nA–50 μA.

Simulation study on efficiency enhancement of 10–140 GHz FEL with wiggler tapering and a prebunched e-beam

The interaction of an intense ( n ∼10 11 cm -3 ), pulsed (200 ns), proton beam (∼80 keV and ∼380 A) with a transverse magnetic field B ⊥ produced by a pair of mirror coils has been studied experimentally. The beam is space-charge-neutralized and can travel across the magnetic field up to 3 kG by forming polarization electric field due to charge separation, which cancels out the deflection by the Lorentz force ν × B . It is observed that beam expansion along B ⊥ gives rise to the density reduction in the course of travelling. A model is proposed, which relates the outward force along B ⊥ to the density reduction and can explain the experimental results.

Analysis of chaotic behaviour in a Raman FEL

Abstract Efficiency enhancement with wiggler tapering and wiggler reductions with prebunching of e-beam in an induction linac-based FEL are assessed numerically in detail. The design of the wiggler with a prebunched beam is related to the upgrading of JAERI microwave FEL because it is crucial for the generation of extremely high-power RF pulse anticipated therein. Simulation studies to optimize the device parameters confirm that (1) an independent use of the prebunching is effective to reduce the rise distance of RF pulse along the wiggler, without significant enhancement of the saturation level, (2) wiggler tapering would lead to the extraction efficiency in energy as high as 50% when well optimized, and (3) the use of the optimized wiggler with the prebunched beam will realize a compact and high-power microwave FEL system. The tapering pattern depends on all of the FEL parameters and specifically on the initial distribution of the beam in phase space. The mechanisms involved such as the particle trapping in the ponderamotive potential well are discussed.