Beam breakup instability in an annular electron beam

It is shown that an annular electron beam may carry six times as much current as a pencil beam for the same beam breakup (BBU) growth. This finding suggests that the rf magnetic field of the breakup mode is far more important than the rf electric field in the excitation of BBU. A proof‐of‐principle experiment is suggested, and the implications explored.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71057/2/JAPIAU-74-9-5877-1.pd

Effects of a series resistor on electron emission from a field emitter

Universal curves are constructed that provide an immediate determination of the effect of a series resistor on the electron emission from a field emitter. These curves are applicable to both the low current and high current regime. The effects of space charge and of the series resistor are apparent from these curves, which are applicable to a large class of materials. An example is given to illustrate their use. © 1996 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70468/2/APPLAB-69-18-2770-1.pd

Beyond the Child–Langmuir law: A review of recent results on multidimensional space-charge-limited flow

Space-charge-limited (SCL) flows in diodes have been an area of active research since the pioneering work of Child and Langmuir in the early part of the last century. Indeed, the scaling of current density with the voltage to the 3/2’s power is one of the best-known limits in the fields of non-neutral plasma physics, accelerator physics, sheath physics, vacuum electronics, and high power microwaves. In the past five years, there has been renewed interest in the physics and characteristics of SCL emission in physically realizable configurations. This research has focused on characterizing the current and current density enhancement possible from two- and three-dimensional geometries, such as field-emitting arrays. In 1996, computational efforts led to the development of a scaling law that described the increased current drawn due to two-dimensional effects. Recently, this scaling has been analytically derived from first principles. In parallel efforts, computational work has characterized the edge enhancement of the current density, leading to a better understanding of the physics of explosive emission cathodes. In this paper, the analytic and computational extensions to the one-dimensional Child–Langmuir law will be reviewed, the accuracy of SCL emission algorithms will be assessed, and the experimental implications of multidimensional SCL flows will be discussed. © 2002 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69652/2/PHPAEN-9-5-2371-1.pd

Limiting current in a relativistic diode under the condition of magnetic insulation

The maximum emission current density is calculated for a time-independent, relativistic, cycloidal electron flow in a diode that is under the condition of magnetic insulation. Contrary to conventional thinking, this maximum current is not determined by the space charge limited condition on the cathode, even when the emission velocity of the electrons is assumed to be zero. The self electric and magnetic fields associated with the cycloidal flow are completely accounted for. This maximum current density is confirmed by a two-dimensional, fully electromagnetic and fully relativistic particle-in-cell code. © 2003 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71144/2/PHPAEN-10-11-4489-1.pd

A novel two‐beam accelerator (twobetron)

A new configuration is analyzed wherein a low current beam is accelerated to high energies (10’s of amps, 10’s of MeV) by a driver beam of high current and low energy (a few kiloamps, <1 MeV). The annular driver beam excites the TM020 cavity mode of an accelerating structure which transfers its rf power to the on‐axis secondary beam. Systematic variation of the driver beam radius provides the secondary beam with phase focusing and adjustable acceleration gradient. A proof‐of‐principle experiment is suggested. Various issues, such as the scaling laws, transverse and longitudinal instabilities, rf coupling among cavities, etc., are examined. © 1995 American Institute of PHysics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87548/2/451_1.pd

Mode Competition in Relativistic Magnetrons and Injection Locking in KW Magnetrons

Both relativistic and nonrelativistic magnetrons are under experimental and theoretical investigation at U of M. Relativistic (Titan‐6‐vane) magnetron experiments (300–400 kV, 1–10 kA, 0.5 microsecond) investigate mode control with various output coupling geometries. Mode competition between the pi mode and the 2/3 pi mode has been characterized for two‐versus‐three output extractors for comparison with particle in cell simulations. Phase measurements and time‐frequency‐analysis are performed for mode identification. Peak microwave output power on the order 0.5 GW has been measured, assuming equal output from 3 waveguides. Nonrelativistic (4 kV, <1A, kW microwave power) magnetron experiments are performed on commercial oven magnetrons for an in‐depth investigation of crossed‐field injection‐locking and noise. Injection‐locking is demonstrated by utilizing an oven magnetron as a reflection amplifier. Noise generation is explored as a function of injected signal and cathode conditions. © 2003 American Institute of PhysicsPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87505/2/301_1.pd

Emittance, surface structure, and electron emission

The emittance of high brightness electron sources, particularly field emitters and photocathodes but also thermionic sources, is increased by surface roughness on the emitter. Such structure causes local field enhancement and complicates both the prediction of emittance and the underlying emission models on which such predictions depend. In the present work, a method to find the emission trajectories near regions of high field enhancement is given and applied to emittance predictions for field, photo, and thermal emission for an analytically tractable hemispherical model. The dependence of the emittance on current density, spatial variation, and acceleration close to the emission site is identified and the impact of space charge discussed. The methodology is extensible to field emission from close-spaced wirelike structures, in particular, and extensions to that configuration are discussed. The models have application to electron sources for high frequency vacuum electronics, high power microwave devices, and free-electron lasers