P. R. Menge

Effects of tapered tubes on long-pulse microwave emission from intense e-beam gyrotron-backward-wave-oscillators

Experiments are reported on tapered-tube versus uniform-tube, gyrotron-backward-wave-oscillators (4.5-6 GHz) driven by an intense electron beam with parameters: 0.8 MV, 1-4 kA, and pulselength (0.5-1 /spl mu/s). Results show that, compared to a uniform interaction tube, a gyro-BWO with a 10% downtapered tube produces the following effects: 1) highest microwave peak-power (up to about 100 MW in internal tube), a factor of 2 higher than the uniform tube, 2) more reproducible long-pulse (400-500 ns) emission, and 3) the largest inferred-integrated energy (factor of 2.5-3 increase). Experiments show high power microwave spikes with lower power plateaus. Experimental observations are in qualitative agreement with MAGIC code simulations of uniform and tapered-tube gyro-BWOs. >

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.

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.

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 magnitu...

Deflection of carbon dioxide laser and helium‐neon laser beams in a long‐pulse relativistic electron beam diode

Deflection of carbon dioxide and helium‐neon laser beams has been used to measure plasma and neutral density gradients during the operating mode and after the shorting time of a long‐pulse field‐emission electron beam diode. Plasma density gradients of (1014–1015) cm−4 were observed throughout the diode during the final microsecond of the 2–3 μs electron beam pulse. The neutral density gradient was less than 1×1018 cm−4 during the electron beam pulse. Upon diode shorting, neutral density gradients increased to (1018–1019) cm−4 over ∼1 μs, and decayed over many microseconds. Plasma density gradients of ∼1015 cm−4 were also observed after shorting. These experiments demonstrate the value of carbon‐dioxide laser and helium‐neon laser deflection for diagnosing plasma and neutral particles in long‐pulse electron beam diodes.

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 lowe...

Beam breakup instability experiments on long-pulse electron-beam transport through rf cavities

Experiments designed to investigate the beam breakup (BBU) instability have been performed using the long-pulse MELBA electron-beam generator (0.5 - 1.5 microsecond(s) , 0.7 - 0.8 MV, ldiode equals 1 - 15 kA, lextracted equals 0.1 - 0.5 kA). The experiment consists of 10 identical pillbox cavities each containing a small microwave loop antenna designed to detect the TM110 beam breakup mode. For our cavity design the TM110 resonant frequency occurs at approximately 2.5 GHz. The cavities are connected by small diameter tubes which attenuate the RF cavity-to-cavity crosstalk. The MELBA diode and subsequent cavity system are immersed in a solenoidal magnetic field (0.8 - 3 kG). Microwaves of 2.5 GHz (1 - 4 kW), whose pulselength exceeds the beam pulse, can be injected into the initial cavity in order to prime the BBU instability. BBU instability growth is measured through the growth of 2.5 GHz RF between the first (or second) and tenth cavities. The BBU growth is compared with predictions made by beam-cavity coupled-mode theory.

Experiments on the beam breakup instability in long-pulse electron beam transport through cavity systems

Experiments have been performed to investigate the beam breakup-up (BBU) instability in high current electron beams transported through RF cavity systems. Experiments utilized long-pulse electron beam accelerators operating with the following parameters: energy = 0.3-0.8 MeV, current=0.1-1 kA, and pulselength=0.3-1.5 mu s. The transport system consists of 10 RF cavities separated by tubes which are cut off to the RF. Each cavity has a microwave probe to detect growth of e-beam emission in the TM/sub 110/ mode at 2.5-GHz, corresponding to the BBU. Solenoidal magnetic fields of 0.8-5kG are applied. Experiments show that 40% of the injected current was transported through the cavity system. The growth of the 2.5-GHz RF was found to be 4.4 dB per cavity, which compares well with the theoretical growth of 3.9 dB per cavity.<<ETX>>

Tapered gyrotron-backward-wave-oscillators for high power, long-pulse microwave generation

Summary form only given. Experiments have been performed to investigate the effects of tapered interaction tubes on the gyro-BWO (backward wave oscillator) microwave power and pulse length for an intense microsecond electron beam. The MELBA microsecond electron beam accelerator is operated in these experiments at parameters: V = - 0.8 MV, I = 1-4 kA, and pulse length = 0.5-1 /spl mu/s. Peak microwave tube power is about 8 MW for the untapered case, with tunable frequency in the range from 4.5 to 6 GHz. MAGIC code simulations and experiments were conducted at a number of different tapered-tube magnitudes, 10%, 23% and 43%, and orientations (uptaper or downtaper). Of these, only the 10% downtapered tube was predicted by the MAGIC code to give a gyro-BWO microwave power enhancement for a flat voltage pulse. Experiments have shown significant increases in both microwave power and pulse length for a 10% downtapered tube.

Experimental reduction of electron beam breakup instability using external coupled cavities

Experiments on electron beam transport through 10 RF cavities have shown that BBU growth can be reduced by 6 dB when seven internal beam cavities are coupled by coaxial cable to seven external dummy cavities. The experiment consists of 10 brass pillbox resonant cavities immersed in a solenoidal field. The first cavity has its TM/sub 110/ mode primed at 2.5 GHz by a microwave pulse from an external magnetron. A 200 A e-beam is injected into the transport cavity system by the long pulse MELBA generator (t=0.5-1.5 /spl mu/s, V=-0.7 to