William C. Guss

Experimental investigation of a 140 GHz gyrotron-backward wave oscillator

We report the experimental operation of a voltage tunable gyrotron backward wave oscillator (gyro-BWO) in the frequency range near 140 GHz. Voltage tunability is an important feature of the gyro-BWO for application as a fast tuning source for driving high power free electron lasers or cyclotron autoresonance maser amplifiers. The gyro-BWO operated in an overmoded cylindrical waveguide structure in the TE1,2 mode. The electron beam source was a Pierce-wiggler gun producing an 80 kV, 6.2 A beam. Frequency tuning with voltage between 134 and 147 GHz was achieved in the TE1,2 mode with constant magnetic field. However, this tuning was found to be discontinuous. Output powers of up to 2 kW and 2% efficiency were found, significantly below theoretical predictions for a cold beam. The theoretical beam velocity spread was modeled by a 3D beam transport code. The code results show that space charge forces, coupled with the wiggler-induced helical motion and the short cyclotron wavelength of the beam, produce large increases in velocity spread in the magnetic compression region. A beam with smaller velocity spread would be needed to make the gyro-BWO operate at the desired efficiency.

Velocity ratio measurements of a gyrotron electron beam

A 140 GHz high‐power gyrotron has recently been operated in a 14 T Bitter magnet to characterize emission as a function of magnetic field and beam current. The velocity ratio (or pitch angle) α=〈v⊥〉/〈v∥〉 of the beam electrons is a critical parameter for high‐efficiency gyrotron operation and was measured using a capacitive probe located in the beam tunnel before the cavity. The observed velocity ratio decreased as the beam current increased while the beam voltage and magnetic fields were held fixed. This decrease in α partially explains the reduced gyrotron efficiency observed at high‐beam currents. The velocity ratio exhibited saturation effects as a function of both the beam current and the control‐anode voltage, at low cathode magnetic field values. Particle code results show a decrease in α as a function of beam current that is consistent in magnitude with the observed values.

Electron density and gas density measurements in a millimeter-wave discharge

Electron density and neutral gas density have been measured in a non-equilibrium air breakdown plasma using optical emission spectroscopy and two-dimensional laser interferometry, respectively. A plasma was created with a focused high frequency microwave beam in air. Experiments were run with 110 GHz and 124.5 GHz microwaves at powers up to 1.2 MW. Microwave pulses were 3 μs long at 110 GHz and 2.2 μs long at 124.5 GHz. Electron density was measured over a pressure range of 25 to 700 Torr as the input microwave power was varied. Electron density was found to be close to the critical density, where the collisional plasma frequency is equal to the microwave frequency, over the pressure range studied and to vary weakly with input power. Neutral gas density was measured over a pressure range from 150 to 750 Torr at power levels high above the threshold for initiating breakdown. The two-dimensional structure of the neutral gas density was resolved. Intense, localized heating was found to occur hundreds of nano...

VELOCITY SPREAD MEASUREMENTS ON A MAGNETRON INJECTION GUN BEAM

The parallel electron velocity distribution function has been measured on an electron beam from a magnetron injection gun (MIG) using an E∥B energy analyzer. The magnetic field, beam voltage, and beam current were scaled down from normal operating parameters. The perpendicular velocity spread, inferred under the assumption that the electrons are monoenergetic, is relatively constant for electron velocity ratios β⊥/β∥≳0.7 and increases approximately linearly with the beam current. The current scaling of the perpendicular velocity spread is also consistent with electron‐loss currents measured at the control anode of the MIG. Observed perpendicular velocity spreads for the gun design parameters are substantially larger than computational values.

Influence of sideband oscillations on gyrotron efficiency

We report the observation of sideband mode effects on the efficiency of overmoded gyrotron operation. Two cavities were designed and operated which differed in the presence of sideband modes. In one version of the cavity, parasitic backward waves modes were observed and efficiencies were approximately 22% at 40 A beam current. With the use of a multi-mode self-consistent nonlinear code, a modified design was generated which eliminated the sideband modes. Experiments were conducted with this new cavity which produced efficiencies of approximately 33% at 30 A and 27% at 40 A beam current, but with a slightly higher velocity ratio than seen with the earlier cavity. An additional cavity, also with no sideband modes but with a longer cavity length and therefore higher Q obtained powers up to 1.3 MW with an efficiency of 39% at a 40 A beam current. >

Experimental Study of the Start-Up Scenario of a 1.5-MW, 110-GHz Gyrotron

We present experimental results of the modes excited during the voltage rise of a 1.5-MW, 110-GHz gyrotron operating in the TE22,6,1 forward-wave mode. Results were obtained by two different experimental techniques: measurements with a time-gated heterodyne receiver and measurements during the flat-top portion of the voltage pulse with a sequence of increasing voltages. Two operating points were selected: a high-efficiency 1.2-MW power-level point at 4.38 T and a highly stable 600-kW point at 4.45 T. In the former case, the TE21,6,3 and TE21,6,4 backward-wave modes far from cutoff were excited during the voltage rise of the pulse before the desired TE22,6,1 operating mode was excited; in the latter case, the excitation of a TE22,6,2 backward-wave mode dominated the voltage rise before eventually exciting the desired operating mode. Analysis of the microwave output beam spatial pattern and the frequency and power levels recorded indicate that these modes are indeed excited within the cavity. Single-mode MAGY simulations provide further evidence that such modes can exist in the gyrotron during the voltage rise. Knowledge of the modes excited during start-up is important for achieving high efficiency and avoiding power at unwanted frequencies.

Novel linear analysis for a gyrotron oscillator based on a spectral approach

With the aim of gaining a better physical insight into linear regimes in gyrotrons, a new linear model was developed. This model is based on a spectral approach for solving the self-consistent system of equations describing the wave-particle interaction in the cavity of a gyrotron oscillator. Taking into account the wall-losses self-consistently and including the main system inhomogeneities in the cavity geometry and in the magnetic field, the model is appropriate to consider real system parameters. The main advantage of the spectral approach, compared with a time-dependent approach, is the possibility to describe all of the stable and unstable modes, respectively, with negative and positive growth rates. This permits to reveal the existence of a new set of eigenmodes, in addition to the usual eigenmodes issued from cold-cavity modes. The proposed model can be used for studying other instabilities such as, for instance, backward waves potentially excited in gyrotron beam tunnels.

Effect of velocity spread on operation of high power gyrotrons

Summary form only given, as follows. The effect of velocity spread on the operation of 140 GHz gyrotrons has been studied. The performance of two cavities, with low and high Q, has been examined experimentally and theoretically. The simulation code MAGY was modified to include the measured velocity distribution function and the measured pitch angle in order to compare the measured efficiencies with the predicted efficiencies. Based on measurements, the inferred velocity spread at a beam current of 40 A is given by (/spl delta/v/sub /spl perp///v/sub /spl perp//) RMS=15%. Simulations with this spread produced efficiencies lower than those measured. However, it was found that assuming /spl delta/v/sub /spl perp///v/sub /spl perp//=10% for 40 A current and using the experimentally determined dependence of the spread on the current the calculated efficiencies agree well with the measured efficiencies for the low 8 cavity. The efficiency of the low Q gyrotron at 40 A beam current is only 27%. For the same beam current and velocity spread the efficiency of the high Q gyrotron was predicted to be 40% which agrees well with the measured efficiency of 39%.

The operation of a megawatt gyrotron in the submillimeter wave region

The operation of a high-power, tunable gyrotron oscillator at wavelengths extending into the submillimeter-wave regime is reported. Using a 14-T Bitter magnet, frequencies from 141 GHz (TE/sub 15,2,1/ mode) up to 328 GHz (TE/sub 27,6,1/ mode) have been measured. As in earlier experiments, it was possible to step tune through a sequence of TE/sub m,p1/ modes over this range. From 198 GHz to 328 GHz, these modes corresponded to p=4, 5, and 6. Even though the cavity is highly overmoded at 328 GHz, output powers remain quite high, with a peak output power of 430 kW at 80 kV and 35 A. This corresponds to an efficiency of 15%. This is the highest power generated by a gyrotron in the submillimeter region. Even better results were obtained in the TE/sub 22,5,1/ mode at 267 GHz. A peak power of 600 kW was produced when operating at 80 kV and 35 A, for an efficiency of 21%. A detailed study of the TE/sub 16,2,1/ mode at 148 GHz has been made, and the beam parallel velocity was measured with a capacitive probe.<<ETX>>

Operation Of A 140 GHz Tunable Backward Wave Gyrotron Oscillator

A tunable backward-wave oscillator (BWO) gyrotron is currently being operated at MIT which is a prototype for a FEL driver at high frequency. Novel features of this design, are the overmoded TE12 cylindrical cavity, a wide band moth-eye window, and the use of a Pierce-wiggler gun. The design objectives are voltage tuning from 130-140 GHz with 10 kW output power. The interaction region is 10λο long where λο is the free space wave length. A linear 2° uptaper is used to maintain mode purity and a broadband motheye window is used for maximum transmission.