S. A. Naqvi

Axial extraction of high‐power microwaves from relativistic traveling wave amplifiers

We report theoretical and experimental results from research into coaxial extraction of high‐power microwaves from X‐band traveling wave tube amplifiers. Power levels exceeding 60 MW have been measured at 9.1 GHz. The output level is relatively constant for the full 70 ns duration of the 700 kV, 500 A electron beam pulse. Results indicate that this coaxial geometry is broadband when compared to traditional, highly tuned radial extraction and may thus have applications in a range of high‐power microwave devices.

High-efficiency TWT design using traveling-wave bunch compression

A bunch compression scheme designed to obtain high efficiency in relativistic traveling-wave tube (TWT) amplifiers is reported. Bunch compression is achieved by making the bunches stay in more positive slopes of the axial electric field in the amplifier. The resulting momentum gradient across the bunch tends to oppose space-charge spreading and helps to sustain short bunch lengths for long distances. A faster than light structure is employed to produce the required bunching. At the optimal bunching point, a transition is made to a lower phase velocity structure where the narrow bunches are decelerated. An RF conversion efficiency of >50% in an X-band amplifier is achieved in particle-in-cell (PIC) simulations. This contrasts strongly with the 20%-30% efficiencies achievable from typical relativistic TWTs, where the bunching is not optimized.

Efficient operation of a high-power X-band traveling wave tube amplifier

We report experimental results demonstrating 54% power conversion efficiency (43% energy conversion efficiency), from a two-stage X-band traveling wave tube amplifier designed for high-power operation. The first stage of the amplifier is a 12-cm-long Boron Nitride dielectric section used to modulate the electron beam. The second stage consists of a long high-phase-velocity bunching section followed by a short low-phase-velocity output section. Output powers of up to 78 MW with narrow spectrum width were obtained with ∼700 kV, ∼200 A beam.

Progress in high power, high efficiency relativistic traveling wave tube amplifiers

We present an overview of recent research at Cornell University on the use of relativistic traveling wave tube amplifiers for high power microwave generation. We consider three topics namely the dependence of the amplifier gain on the beam energy, axial energy extraction using a TM to TEM mode converter, and techniques for enhancing the efficiency of the amplifier to at least 50%.

Studies of high efficiency interaction in traveling wave structures

We review the main experimental results on high power traveling wave amplifiers performed at Cornell; the highest efficiency achieved was 40% in a uniform periodic structure. For a higher efficiency, it is necessary to taper the output section. An analytical method to analyze and design quasi‐periodic output structure, is presented. It relies on the concept of interaction impedance matrix which is a generalization of the scalar interaction impedance in periodic structure. The efficiency is directly related to the largest eigen‐value of this matrix. We conclude with a brief discussion of the design of a new electron beam source for rf generation studies at low repetition rates.

High efficiency X-band TWT amplifiers

Summary form only given. We report on a research program to increase the efficiency of relativistic traveling wave amplifiers to >50%. The two stage amplifier consists of a bunching periodic structure with phase velocity and a decelerating section with phase velocity significantly lower than the beam velocity. The position of the decelerating stage with respect to the bunching stage is chosen such that the narrowest bunches are sustained in the decelerating field for the longest possible time before significant debunching occurs. Two schemes are under investigation. In the first scheme, a resistive sever is placed between the two stages to suppress temporal phenomena. In the second scheme, the bunching and decelerating stages merge into each other by a gradual change in the iris radius over a wavelength. An absorbing section in this case is placed before the start of the bunching stage.

High efficiency TWT amplifiers

We report on a research program to increase the efficiency of relativistic traveling wave amplifiers to >50%. The two stage amplifier consists of a bunching periodic structure with phase velocity significantly higher than the beam velocity and a decelerating section with phase velocity significantly lower than the beam velocity. The position of the decelerating stage with respect to the bunching stage is chosen such that the narrowest bunches are sustained in the decelerating field for the longest possible time before significant debunching occurs. Two schemes are under investigation. In the first scheme, a resistive server is placed between the two stages to suppress temporal phenomena. In the second scheme, the bunching and deceleration stages merge into each other by a gradual change in the iris radius over a wavelength.

High power X-band TWT amplifiers

Our recent research into multi-stage X-Band TWTs producing output powers of 100-200 MW has shown that it is essential to minimize the reflections in each stage of the amplifier in order to avoid sideband development. These reflections also cause fluctuations in the RF output power envelope. Following extensive MAGIC code simulations we have designed tapers that adiabatically increase the iris diameter in the output sections of the amplifier to provide a smooth, broad-band transition from the slow-wave structure to cylindrical waveguide. We report results, extracting in the TM/sub 01/ mode, showing smooth output pulses in the range 30-50 MW, with no evidence of sidebands. A second approach seeks to isolate the first amplifier stage with a drift tube beyond cutoff. The second stage and output section are quasi-periodic structures designed to minimize reflections, and allow the radial or longitudinal RF power extraction to be distributed over an extended region. The first stage of this system has been developed and initial operation results using an 0.8-1.0 MV, 0.5-1.0 kA, 50 ns cylindrical beam will be reported.

Studies of the high power traveling wave interaction

We summarize the results from our continuing study on the high power interaction of electrons and electromagnetic waves in periodic structures. Experiments indicate that power levels in excess of 150 MW are achievable in a low group velocity periodic structure for a pulse duration of less than 100 nsec. The momentum distribution of electrons was measured and energies ranging from 250 kV to 2 MV were detected; this energy spread may have a substantial effect on the design of traveling wave output structures. Another limitation is the large gradients (≳200 MV/m) associated with the power levels mentioned above. The high gradient is a direct result of the low group velocity design which in turn was dictated by the need to make the feedback time of the order of the pulse duration. We have developed a method also to analyze quasi‐periodic structures which helps us to design output traveling wave structures and optimize these contradictory constraints.