Effect of x‐y coupling on the beam breakup instability

In solenoidal beam transport systems, motions in the x and y directions are coupled by the v×B force. A two‐dimensional coupled mode description of the beam breakup (BBU) instability is presented; its dispersion relation is derived and compared with the one‐dimensional BBU dispersion relation. In the two‐dimensional description, instability growth is doubled and two additional wave modes are found in the regime of strong focusing. In the weak focusing regime, the two‐dimensional description gives the same dispersion relation as the one‐dimensional model.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69723/2/APPLAB-58-7-699-1.pd

Microwave growth from the beam breakup instability in long‐pulse electron beam experiments

The beam breakup (BBU) instability has been investigated in high‐current, long‐pulse electron beams propagating through microwave cavities. Experiments are performed using a relativistic electron‐beam generator with diode parameters: 0.7–0.8 MV, 1–15 kA, and 0.5–1.5 μs. The magnitude of the solenoidal magnetic field places these experiments in an intermediate regime between strong focusing and weak focusing. The electron‐beam transport system consists of ten identical pillbox cavities each containing a small microwave loop antenna designed to detect the TM110 beam breakup mode. The TM110 microwave mode is primed in the first cavity by a magnetron tuned to the resonance frequency of 2.5 GHz. The BBU instability growth is measured through the amplification of the 2.5 GHz microwaves between the second and tenth cavities. Strong growth (25–38 dB) of the TM110 microwave signal is observed when the initial cavity is primed exactly on resonance, with a rapid decrease of the growth rate off‐resonance. The magnitude of microwave growth is consistent with the predictions of BBU theory.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69350/2/APPLAB-61-6-642-1.pd

Thin film contact resistance with dissimilar materials

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/98706/1/JApplPhys_109_124910.pd

The beam breakup instability in quadrupole and solenoidal electron‐beam transport systems

Dispersion relations are derived to determine the growth rate, dominant wavelength, and group velocity of disturbances caused by the beam breakup instability. Considerations include weak and strong focusing, x‐y coupling in solenoidal transport, the spacing of accelerator cavities, and periodically pulsed beams. Beam breakup growth is minimum when the cavity spacing equals an integral number of half‐betatron wavelengths for quadrupole focusing, and an integral number of betatron wavelengths for solenoidal focusing. Minimum growth is also found for periodic pulses separated by an integral number of half‐periods of the TM110 cavity mode. Expressions for beam breakup growth at the minima are obtained.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71286/2/JAPIAU-71-7-3091-1.pd

Multipactor susceptibility on a dielectric with a bias dc electric field and a background gas

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/98753/1/PhysPlasmas_18_053508.pd

Beam breakup growth and reduction experiments in long‐pulse electron beam transport

The results of an experimental program whose sole objective is to investigate the cumulative beam breakup instability (BBU) in electron beam accelerators are presented. The BBU growth rate scalings are examined with regard to beam current, focusing field, cavity Q, and propagation distance. A microwave cavity array was designed and fabricated to excite and measure the cumulative BBU resulting from beam interactions with the deflecting TM110 cavity mode. One phase of this experiment used high Q(≊1000) cavities with relatively large frequency spread (Δf/f0≊0.1%). The observed TM110 mode microwave growth between an upstream (second) and a downstream (tenth) cavity indicated BBU growth of 26 dB for an electron beam of kinetic energy of 750 keV, 45 A, and focused by a 1.1 kG solenoidal field. At beam currents of less than 100 A the experiments agreed well with a two‐dimensional continuum theory; the agreement was worse at higher beam currents (≳100 A) due to beam loading. The second‐phase experiments used lower Q(≊200) cavities with relatively low frequency spread (Δf/f0≊0.03%). Theory and experiment agreed well for beam currents up to 220 A. Distance scaling experiments were also performed by doubling the propagation length. Instability growth reduction experiments using the technique of external cavity coupling resulted in a factor of four decrease in energy in BBU growth when seven internal beam cavities were coupled by microwave cable to seven identical external dummy cavities. A theory invoking power sharing between the internal beam cavities and the external dummy cavities was used to explain the experimental reduction with excellent agreement using an equivalent circuit model.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71080/2/JAPIAU-75-3-1258-1.pd

Leak widths resulting from plasma diffusion in a magnetic cusp

Several formulas describing cusp leak widths are derived using different assumptions on the natures of the crossfield diffusion and the plasma flow parallel to the magnetic field.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/27331/1/0000356.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

Multipactor discharge on metals and dielectrics: Historical review and recent theories

This paper reviews the history of multipactor discharge theory, focusing on recent models of multipactor accessibility and saturation. Two cases are treated in detail: That of a first-order, two-surface multipactor, and that of a single-surface multipactor on a dielectric. In both cases, susceptibility curves are constructed to indicate the regions of external parameter space where multipactor is likely to occur, taking into account the dependence on surface materials, and the effects of space charge and cavity loading. In the case of a dielectric, multipactor is found to deliver about 1% of the rf power to the surface. The two cases are contrasted in light of experimental observations. © 1998 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71019/2/PHPAEN-5-5-2120-1.pd

Measurement of long‐pulse relativistic electron beam perpendicular‐ to‐parallel velocity ratio by Cerenkov emission and radiation darkening on a glass plate

We report measurements of the ratio of the perpendicular velocity to the parallel velocity, α= v⊥ /v∥, of a relativistic electron beam gyrating in a magnetic field by the use of (1) Cerenkov emission from a glass plate, detected by a gated microchannel plate image intensifier camera, and (2) electron‐beam‐induced radiation darkening pattern on the same glass plate. The measurements are based on a direct determination of the Larmor radius of an electron beam of known energy. Experiments were performed on a long‐pulse electron beam accelerator with e‐beam diode parameters: VD = 0.6–0.9 MV, pulse length=0.5–1 μs, ID = 1–10 kA. The experimental value of α agrees with simulation results from particle trajectory codes as well as theoretical predictions from Busch’s theorem and adiabatic theory.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70360/2/RSINAK-63-2-1671-1.pd