The achievements resulting from the application of advanced pulsed power to the generation of high power microwaves (HPM) have included the generation of multi-gigawatt pulses of RF energy. The power achievable is orders of magnitude greater than conventional microwave sources can generate. However, the introduction of the HPM technology into logical applications has been limited to date due to the phenomenon of pulse shortening in which the RF pulse terminates before the pulse power source used to produce it. Conventional microwave tubes can generate a few to 10 MW of power with pulsewidths of many microseconds when required. High power microwave sources can produce gigawatts of power, but only for relatively short pulsewidths, typically tens to hundreds of nanoseconds. An international effort during the past few years has generated important new discoveries toward the elimination of pulse shortening. Some of the new techniques have the potential for helping the conventional tube industry as well as being practical for high power microwave sources. This paper reviews the pulse shortening problem, its causes, and the worldwide scope and direction of research conducted to date to resolve it. The paper also discusses the potential remedies for the problem and recommends a course of research to further progress on the issue.

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Evolution of pulse shortening research in narrow band, high power microwave sources

We have developed a new submillimeter wave pulsed molecular beam spectrometer with phase stabilized backward wave oscillators (BWOs). In the frequency ranges of 260–380 and 440–630 GHz, the BWOs output power varies between 3 and 60 mW. Part of the radiation was coupled to a novel designed harmonic mixer for submillimeter wavelength operation, which consists of an advanced whiskerless Schottky diode driven by a harmonic of the reference synthesizer and the BWO radiation. The resulting intermediate frequency of 350 MHz passed a low noise high electron mobility transistor amplifier, feeding the phase lock loop (PLL) circuit. The loop parameters of the PLL have been carefully adjusted for low phase noise. The half power bandwidth of the BWO radiation at 330 GHz was determined to be as small as 80 MHz, impressively demonstrating the low phase noise operation of a phase locked BWO. A double modulation technique was employed by combining an 80 Hz pulsed jet modulation and a 10–20 kHz source modulation of the BWO and reaching a minimum detectable fractional absorption of 2×10−7. For the first time, a number of pure rotational (Ka=3←2, Ka=4←3) and rovibrational transitions in the van der Waals bending and stretching bands of the Ar–CO complex were recorded.

Phase locked backward wave oscillator pulsed beam spectrometer in the submillimeter wave range

Systems and methods for generating a high voltage pulse. A series of voltage cells are connected such that charging capacitors can be charged in parallel and discharged in series. Each cell includes a main switch and a return switch. When the main switches are turned on, the capacitors in the cells are in series and discharge. When the main switches are turned off and the return switches are turned on, the capacitors charge in parallel. One or more of the cells can be inactive without preventing a pulse from being generated. The amplitude, duration, rise time, and fall time can be controlled with the voltage cells. Each voltage cell also includes a balance network to match the stray capacitance seen by each voltage cell.

High voltage pulsed power supply using solid state switches

The increase in efficiency of RF power generation for future large accelerators is considered a high priority issue. The vast majority of the existing commercial high-power RF klystrons operates in the electronic efficiency range between 40% and 55%. Only a few klystrons available on the market are capable of operating with 65% efficiency or above. In this paper, a new method to achieve 90% RF power conversion efficiency in a klystron amplifier is presented. The essential part of this method is a new bunching technique—bunching with bunch core oscillations. Computer simulations confirm that the RF production efficiency above 90% can be reached with this new bunching method. The results of a preliminary study of an $L$ -band, 20-MW peak RF power multibeam klystron for Compact Linear Collider with the efficiency above 85% are presented.

Toward High-Power Klystrons With RF Power Conversion Efficiency on the Order of 90%

We present the design and a proof of principle experimental results of an optically controlled high-power RP pulse-compression system. In principle, the design should handle a few hundreds of megawatts of power at X-band. The system is based on the switched resonant delay-line theory [1]. It employs resonant delay lines as a means of storing RF energy. The coupling to the lines is optimized for maximum energy storage during the charging phase. To discharge the lines, a high-power microwave switch increases the coupling to the lines just before the start of the output pulse. The high-power microwave switch required for this system is realized using optical excitation of an electron-hole plasma layer on the surface of a pure silicon wafer. The switch is designed to operate in the TE/sub 01/ mode in a circular waveguide to avoid the edge effects present at the interface between the silicon wafer and the supporting waveguide; thus, enhancing its power handling capability.

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Active high-power RF pulse compression using optically switched resonant delay lines

X-band klystrons capable of 75 MW and utilizing either solenoidal or Periodic Permanent Magnet (PPM) focusing are undergoing design, fabrication and testing at the Stanford Linear Accelerator Center (SLAC). The klystron development is part of an effort to realize components necessary for the construction of the Next Linear Collider (NLC). SLAC has completed a solenoidal-focused X-band klystron development effort to study the design and operation of tubes with beam microperveances of 1.2. As of early 2000, nine 1.2 (mu) K klystrons have been tested to 50 MW at 1.5 microsecond(s) . The first 50 MW PPM klystron, constructed in 1996, was designed with a 0.6 (mu) K beam at 465 kV and uses a 5-cell traveling-wave output structure. Recent testing of this tube at wider pulsewidths has reached 50 MW at 55% efficiency, 2.4 microsecond(s) and 60 Hz. A 75 MW PPM klystron prototype was constructed in 1998 and has reached the NLC design target of 75 MW at 1.5 microsecond(s) . A new 75 MW PPM klystron design, which is aimed at reducing the cost and increasing the reliability of multi- megawatt PPM klystrons, is under investigation. The tube is scheduled for testing during early 2001.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

X-band klystron development at the Stanford Linear Accelerator Center

The Stanford Linear Accelerator Center (SLAC) klystron group is currently designing, fabricating and testing 11.424 GHz klystrons with peak output powers from 50 to 75 MW at 1 to 2 μs rf pulsewidths as part of an effort to realize components necessary for the construction of the Next Linear Collider (NLC). In order to eliminate the projected operational-year energy bill for klystron solenoids, Periodic Permanent Magnet (PPM) focusing has been employed on our latest X-band klystron designs. A PPM beam tester has operated at the same repetition rate, voltage and average beam power required for a 75-MW NLC klystron. Prototype 50 and 75-MW PPM klystrons were built and tested during 1996 and 1997 which operate from 50 to 70 MW at efficiencies greater than 55%. Construction and testing of 75-MW research klystrons will continue while the design and reliability is perfected. This paper will discuss the design of these PPM klystrons and the results of testing to date along with future plans for the development of a low-cost Design for Manufacture (DFM) 75-MW klystron and invitation for industry participation.

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Periodic permanent magnet development for linear collider X-band klystrons

AJDISK is a useful design tool for sheet beams. The code is capable of simulating sheet beam klystrons on modern computers in under a minute. The code can accurately and quickly predict saturated gain, reduced plasma wavelength, cavity voltages, and beam loading. Two of the output plots from the code are shown in figure 3. The gain predicted from AJDISK is in good agreement with measured results and is shown in figure 4

Sheet Beam Klystron Simulations Using AJDISK

X-band klystrons capable of 50 MW and utilizing Periodic Permanent Magnet (PPM) focusing are undergoing design and fabrication at the Stanford Linear Accelerator Center (SLAC). The klystron development is part of an effort to realize components necessary for the construction of the Next Linear Collider (NLC). The first klystron to be tested this year has a 0.6 microK beam at 465 kV, a 5 cells traveling wave output structure and a predicted efficiency of 63%. A 465 kV, 190 A beam stick with 12 periods of PPM focusing has recently operated to verify the gun optics and transmission of the beam in the absence of rf bunching. Beam transmission greater than 99.8% has been measured. Design and simulation of the beam stick and klystron are discussed, along with performance of the beam stick under confined flow and shielded conditions.

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PPM Focused X-Band Klystron Development at the Stanford Linear Accelerator Center

Two S-Band klystrons operating at 150 MW have been designed, fabricated and tested at the Stanford Linear Accelerator Center (SLAC) during the past two years for use in an experimental accelerator at Deutsches Elektronen-Synchrotron (DESY) in Hamburg, Germany Both klystrons operate at the design power, 60 Hz repetition rate, 3 {micro}s pulsewidth, with an efficiency {gt} 40%, and agreement between the experimental results and simulations is excellent The 535 kV, 700 A electron gun was tested by constructing a solenoidal focused beam stick which identified a source of oscillation, subsequently engineered out of the klystron guns Design of the beam stick and the two klystrons is discussed, along with observation and suppression of spurious oscillations Differences in design and the resulting performance of the Klystrons is emphasized

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D. Sprehn

Papers

X-band klystron development at the Stanford Linear Accelerator Center

Periodic permanent magnet development for linear collider X-band klystrons

Sheet Beam Klystron Simulations Using AJDISK

PPM Focused X-Band Klystron Development at the Stanford Linear Accelerator Center

150-MW S-band klystron program at the Stanford Linear Accelerator Center