Ray M. Stringfield

Experimental confirmation of the reditron concept

A description is given of experiments demonstrating a method for producing high-power microwave emission. The unstable oscillations of a virtual cathode, which forms when a magnetized relativistic electron beam is injected into a circular waveguide, generates the microwave radiation. In contrast to other virtual-cathode microwave-generation techniques, electrons in the waveguide are prevented from reflexing back into the diode region by use of a slotted range-thick anode. Electrons injected into the waveguide are guided through the slot by an applied magnetic field, while reflected electrons, under the proper conditions, are intercepted by the anode. Several advantages of this approach are described, and experimental confirmation of this mode of high-power microwave generation is demonstrated. Data showing frequency scaling with beam parameters and magnetic field are also presented. Using this technique, 1.4 GW was produced at 3.9 GHz with several hundred megawatts radiated in harmonic radiation. >

Intense space-charge beam physics relevant to relativistic klystron amplifiers

In this paper, we examine intense space-charge beam physics that is relevant to beam bunching and extraction in a mildly relativistic klystron amplifier, and give numerical examples for a 5 kA, 500 keV electron beam in a 1.3 GHz structure. Much of the peculiar beam physics in these types of devices results from the partitioning of beam energy into kinetic and potential parts. Both tenuous-nonrelativistic and intense-relativistic beams produce effects different in nature from those produced by intense, mildly relativistic beams because the potential energy requirements are either negligible or fixed. In particular, we demonstrate anomalous beam bunching aided by the nonlinear potential requirements and we discuss maximum power extraction as a function of beam bunching. We show that although the space-charge effects can produce quite high harmonic current content, the maximum power extraction from the beam into RF typically occurs at relatively modest bunching. >

A 500 MW, 1 /spl mu/s pulse length, high current relativistic klystron

This paper describes the experimental development of a long pulse high current relativistic klystron amplifier (RKA). The desired performance parameters are 1 GW output power and 1 /spl mu/s pulse length with an operating frequency of 1.3 GHz. Peak powers approaching 500 MW have been achieved in pulses of 1 /spl mu/s nominal baseline-to-baseline duration. The half power pulse width is 0.5 /spl mu/s. These pulses contain an energy of about 160 J. RF output rises linearly in concert with the beam current pulse, and terminates abruptly just before the highest part of the pulsed voltage curve is reached. A possible explanation, not yet experimentally confirmed, for the premature termination of the RF pulse is an output cavity gap voltage that is too high, causing electron reflection at the gap and RF breakdown across the gap. A new output cavity has been designed with a much lower shunt impedance and a loaded Q of 4. >

Beam-cavity interaction physics for mildly relativistic, intense-beam klystron amplifiers

We examine beam-cavity interaction physics relevant to mildly relativistic, intense-beam klystron amplifiers. This is an interesting but difficult regime of operation, because of the combination of high beam current and low voltage. The advantage of this regime is that it is relatively easy to access high beam powers (and potentially high microwave output powers) at relatively low beam energy. We calculate the effect of the extremely high beam loading in the input and idler cavities. The output cavitys shunt impedance must match the low beam impedance in order to prevent high output gap voltages that will reflect electrons back upstream. This leads to very low cavity Q factors ( >

Development of a long-pulse 1.3 GHz relativistic klystron amplifier

A research approach for obtaining kilojoule microwave pulses of microsecond duration at 1.3 GHz from the relativistic klystron amplifier is described. Achieving kilojoule microwave pulses requires extending electron beam pulse durations and maximizing the microwave extraction efficiency at the fundamental frequency. An electron beam diode has been constructed that delivers peak currents in excess of 5 kA with a monotonically increasing current pulse exceeding durations of 1 mu s at beam kinetic energies above 400 keV. Close attention has been given to minimizing the current losses from the diode. Maximum microwave extraction efficiency at the fundamental frequency has been related to the beam bunching amplitude and output cavity shunt impedance in terms of a simple circuit theory. The circuit theory predictions have been tested by particle-in-cell code calculations of the electron beam interactions with the proposed cavity structures. The successful cavity structures have been constructed and are awaiting testing. >

A microsecond-pulse-length, frequency-stabilized virtual-cathode oscillator using a resonant cavity

Experiments at the microsecond pulse length have demonstrated that the virtual-cathode oscillator generates narrowband (0.3% bandwidth) microwave pulses when the virtual cathode is surrounded by a resonant cavity and is driven by an appropriate electron beam. This result is a significant departure from the behavior of a free-running virtual-cathode oscillator where the frequency depends on the (current density)/sup 1/2/. The long-pulse experimental results are described. >

Continuing Studies of Plasma Erosion Switches for Power Conditioning on Multiterawatt Pulsed Power Accelerators

Recent PITHON experiments with plasma erosion switches (PES) have extended the range of operation of the switches by about 50 percent, in terms of closed time and charge passing through the switch. The quantity of charge passed through the switch has been increased to as much as 35 mC. Currents as large as 1 MA and voltages as great as 1.8 MV have been switched off to be diverted to a downstream load. The impedance of the erosion switch can be described as having three stages: 1) essentially zero impedance, 2) a transitional opening phase, and 3) an impedance which is very large (greater than 5 ¿) in comparson with the subohm downstream load. Current diagnostics, consisting of Rogowski coils and segmented shunts, have been successfully developed to monitor the current which propagates to the load region. These monitors have measured rise times as short as 38 ns and slew rates as great as 1014 A/s at the load. With wire array loads, the pulse conditioning of the switch has been observed to reduce the magnitude of the current losses in the feed which are present when no switch is used. Correlations have been made between the switch closed time, voltage, current, and power with the feed inductance and the generator power injected into the magnetic insulated transmission line (MITL).

Temperature distribution in a flowing fluid heated in a microwave resonant cavity

This paper presents results of an analytical study of microwave heating of a fluid flowing through a tube situated along the axis of a cylindrical microwave applicator. The interaction of the microwave field pattern and the fluid velocity profiles is illustrated for both laminar and turbulent flow. Resulting temperature profiles are compared with those generated by conventional heating through a surface heat flux. It is found that microwave heating offers several advantages over conventional heating.

Experimental results from a J-band relativistic klystron amplifier developmental effort

Experimental results to-date will be presented from a developmental effort to a produce a J- band (5.85 - 8.2 GHz) relativistic klystron amplifier (RKA) of the high current Naval Research Laboratory (NRL) genealogy. The nominal experimental parameters of this RKA are: V0 approximately equals 600 kV; I0 approximately equals 2 - 4 kA; Bz approximately equals 1.5 T; (tau) beam approximately equals 300 ns; vin approximately equals 6.6 GHz; Pin <EQ 500 kW. Because of the smaller component sizes which accompany this frequency ((lambda) approximately equals 4.5 cm as compared with (lambda) approximately equals 30 cm for the bulk of other RKA research efforts), much of the discussion will concentrate on the physical principles, fabrication issues, and experimental pitfalls associated with scaling the RKA design.

Theory and modeling of a relativistic klystron amplifier with high space charge for microsecond applications

We discuss basic Relativistic Klystron Amplifier physics. We show that in the intense space- charge regime the maximum power extraction does not coincide with the maximum harmonic bunching. In addition, we show that as the beam is bunched, the additional power stored in the Coulomb fields does not add significantly to the overall power extraction. Because of these effects, the power extraction at 1.3 GHz for a 500 kV, 5 kA beam with reasonable beam-to- wall spacing is limited to around 35%.