Pierce-type Gun Research Articles

Zero-Compression, Laminar Electron Beams From Convex Cathodes in Uniform Magnetic Fields

The need for enhanced performance of high-power RF vacuum electron devices has led to the investigation of multiple-beam, sheet beam, and annular beam configurations. A key issue with such devices is the magnetic field shaping required producing high-power, laminar beams. Field shaping is difficult when Pierce-type gun geometries are employed. The development of high current density cathodes makes the necessary beam power achievable without compression. Such cathodes can operate within a uniform magnetic field yielding advantages for both single and distributed-beam RF devices. However, the quality of the resulting beams presents problems. A project to optimize beam quality in zero-convergence electron guns was undertaken by Calabazas Creek Research, Inc. (CCR) and North Carolina State University (NCSU). The surprising result was that high-quality electron beams can be generated in uniform magnetic fields using convex (dome)-shaped cathodes. The underlying physics involves perturbation of the beam cyclotron motion by a nonadiabatic radial electric field impulse. This article examines this physical mechanism and extends the initial result to additional diode and beam geometries.

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.

A long-pulse millimeter-wave free electron maser

An experimental free electron maser (FEM) facility is described which operates in the millimeter-wave regime. The experiment demonstrates that high gain and beam extraction efficiency can be obtained with low-current but high-quality electron beams. In the oscillator configuration, microwave powers of 110 kW were achieved with a 300-kV, 4-A beam, corresponding to a beam extraction efficiency of 9.2%. At low beam currents, with the system operating in the linear regime, the FEM gain curve was measured. The experiment uses a Pierce-type gun with a calculated beam emittance of 10.7-mm mrad at 300 kV and 4 A. The wiggler is of the iron permanent magnet hybrid design. The experiment is intended as a first step towards the development of continuous-wave (CW) devices that use beam energy recovery to make use of low-current, high-voltage power supplies. >

A computer program for electron gun design using second-order finite elements

A computer program has been developed for the analysis and design of electron guns. The program uses second-order, curved, isoparametric finite elements to obtain very accurate potential and field computation. Special features of the program include fast solution of the second-order finite element equations with a specially written incomplete Choleski conjugate gradient (ICCG) algorithm, accurate field computation using biquartic interpolation, and direct ray tracing with a third-order power series method. Space charge effects are taken into account, with an iterative solution of Poisson’s equation. The brightness is also computed. The second-order finite elements permit accurate simulation of curved cathodes and very large changes in geometrical scale factor. This allows the program to handle all types of electron guns, ranging from field emission guns with submicron radius cathodes, to high current Pierce-type guns. The computational techniques used are described and illustrated with typical examples.

Design of low velocity-spread cusp guns for axis encircling beams

The design of a novel electron gun suitable for intense beam cyclotron resonance devices is introduced here. An annular Pierce-type gun is used in conjunction with an unbalanced nonadiabatic field reversal and an adiabatic compression region to produce an axis-encircling beam. This beam is ideally suited for interaction with electromagnetic waves that have strong on-axis electric fields (e.g., the TE011 mode). Low velocity spreads are achieved by utilizing the beam self-fields in the compression region and by focusing in the Pierce gun.

Two electron guns with adjustable beam compression and perveance

Abstract Two electron guns have been designed by computer simulation, in order to produce high density electron beams focused by a superconducting solenoid. A spherical impregnated tungsten cathode is used in a high perveance Pierce type gun, whereas a Rogowski type gun of low perveance may be operated with a directly heated tantalum strip. By variation of the Wehnelt electrode bias both guns are adjustable in the compression—perveance relation with tolerable deterioration of the beam quality.

ＬａＢ?S６?Tカソードを用いた小型で安定なＰｉｅｒｃｅ型電子銃の試作

A stable and compact Pierce-type electron gun has been developed as a highly practical electron source of high beam intensity.This Pierce-type gun has LaB6 cathode of 2 mm in diameter and 7 mm in length with Ta ribbon heaters for electron bombardment. Using single-crystal LaB6 we have obtained beam current of 1.5 mA at 300 V with beam diameter 2.5 mm, by supplying only 15 watt for cathode heating. The gun assembly is so compact that no cooling facility is needed. Current stability within 1% can easily be obtained in vacuum better than low 10-6 Torr for single-crystal LaB6, however one needs much better vacuum for sintered LaB6 to get the same stability.The efficiency of 90% has been attained by the present Pierce-type gun with LaB6 cathode even at 300 V.

Parallel flow of cylindrical beams of charged particles

The usual geometry of an axially symmetric parallel flow Pierce-type gun is criticized and some shortcomings underlined. The general equation defining the family of field forms ensuring parallel flow is carried out and a particular solution selected. The shape of electrodes providing the chosen field form is calculated in a simple and reliable way.

A High Current Electron Gun Suitable for Use down to 1 Nanosecond Pulse Length

A triode electron gun is described which has been developed to meet the increasing interest in short pulse high current operation. The gun is of variable perveance capable of injecting at least 15 amps under short pulse conditions (1 nanosecond) and 1.5 amps under long pulse conditions (5 microseconds) with a beam diameter of 1 cm. Considerations of beam geometry have lead to the choice of a convergent flow Pierce-type gun incorporating an intercepting grid giving an amplification factor ?o of 100 and a maximum beam perveance of 0.7?P for the high current condition. A conventional oxide cathode is used. The cathode-grid structure, which is plug-in replaceable, forms part of a 50 ohm co-axial system mounted on the gun base from which a matched 50 ohm co-axial feeder is taken to the modulator. Some experimental results for long and short pulse operation are given.

Computer experiments on electron guns

Computer techniques for analyzing an axially symmetric or two-dimensional electrode system with an emitting surface can be used to obtain cathode current density distribution, beam minimum radius and its location, as well as possible electrode current interception. Here, comparison is made of available experimental data from two axially symmetric Pierce-type guns. The choice of the two axially symmetric Pierce-type guns was made for their different value of PV/T ratio, an invariant for transverse scaling. One of them has a low ratio of PV/T (\approx 0.1 \times 10^{-6}) , which signifies considerable beam spread due to the effects of the Maxwellian velocity distribution of the thermal electrons leaving the cathode. The other gun has a high perveance of 2 \times 10^{-6} , and thus relatively high PV/T ratio where beam spreading is mainly due to space-charge forces alone. Good agreement with experimental data is shown. Computer techniques of this type can be a valuable tool for diagnostic purposes of any type of space-charge-flow devices (e.g., electron guns, ion guns for space propulsion, linear accelerators, etc.) and enable the engineer-designer to arrive quickly and cheaply at an optimum configuration.

Convergence and accuracy criteria of iteration methods for the analysis of axially symmetric and sheet beam electrode shapes with an emitting surface

Convergence and accuracy criteria have been determined for computer techniques analyzing an axially symmetric or two-dimensional electrode system with an emitting surface. The convergence criterion has been drawn from analytical studies of the one-dimensional model and verified numerically by the computer program for an analytically designed, axially symmetric Pierce-type electrode system. The accuracy of the computer program is illustrated with two electron gun configurations of known analytical solutions, and comparison of cathode current emission, beam minimum radius, and its location is made with experimental data of a high perveance Pierce-type gun.

The grid lens effect in convergent Pierce guns

In a conventional Pierce-type gun, the anode aperture causes a potential reduction in the cathode-anode region from the ideal Langmuir potential distribution. For low-voltage gating of the electron beam, a mesh grid of spherical shape (conforming to an equipotential surface) is used in front of the cathode. When this grid is operated at the Langmuir potential depicted by its relative position, there is a difference in the potential gradients on its two sides. This difference causes a lens action at each mesh element which results in a displacement of the actual electron trajectory from the ideal laminar trajectory in the region beyond the anode. A means for calculating these displacements as a function of distance along the axis is developed. As the grid lenses are divergent, the images of the mesh elements in any plane beyond the anode are larger than those for ideal laminar flow, resulting in a current density distribution which differs from that of the ideal beam. A means of calculating the current density profile by summing the effects of the grid lenses is devised, and the method is applied to a sample gun design to illustrate the effect on the current density distribution.

Transverse Scaling of Electron Beams

Under certain conditions an electron beam can be subjected to a process of scaling in which the transverse components of position, velocity, and force are scaled by a common factor while longitudinal components remain unchanged. In analyzing beams produced by electron guns under conditions where thermal velocity effects have to be taken into consideration, the application of transverse scaling simplifies the treatment by allowing a reduction in the number of independent parameters required. Design curves for Pierce-type guns are reproduced in normalized and condensed form. These curves may be used in choosing design parameters to produce a prescribed beam.