W.B. Haynes

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 Line Design, Beam Alignment Procedure, and Initial Results for the

A gain experiment was performed at Los Alamos using a 120-keV 2-A cylindrical electron beam with a ridged waveguide slow-wave structure at 94 GHz, demonstrating 22 dB of amplification through a traveling-wave interaction. The structure was planar with a gap of 0.75 mm and a length of 5 cm. The 2-A electron beam was confined in a 3.2-kG axial magnetic field, with roughly a 0.5-mm diameter. The electron beam was aligned along the magnetic axis of the solenoid by scribing out its cyclotron motion on a novel optical diagnostic using a procedure that depends on varying the solenoidal field strength. The transport through the structure was verified by letting the beam drill holes in a series of thin metallic foils before insertion of the structure

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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 ( >

-Band Gain Experiment at Los Alamos

In this paper, a new numerical model for space-charge forces in electron beams based on a Greens function approach is described. In this model, the beam is simulated by a series of rings with nonzero thickness and length. The space-charge force on a ring is found by summing over all the interactions with all the other rings, where each interaction force is integrated over the entire volume of each source rings. With proper beam initialization of the simulation parameters, the rings can perfectly form the electron beam, leading to a very smooth and accurate calculation of the space-charge fields. The space-charge fields calculated this way can be more accurate than those found with particle-in-cell (PIC) calculations. The fields can also be distributed onto a mesh, as in a PIC calculation, leading to equivalent accuracy with greatly reduced simulation times. The accuracy of this type of model is demonstrated by comparing the harmonic-current evolution from an RF gap for a transversely cold confined beam to analytic theory and we show its utility with a large-signal annular beam klystron simulation using this technique

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

We theoretically and numerically investigate the operation and behavior of the discrete monotron oscillator, a novel high-power microwave source. The discrete monotron differs from conventional monotrons and transit time oscillators by shielding the electron beam from the monotron cavitys RF fields except at two distinct locations. This makes the discrete monotron act more like a klystron than a distributed traveling wave device. As a result, the oscillator has higher efficiency and can operate with higher beam powers than other single cavity oscillators and has more stable operation without requiring a seed input signal than mildly relativistic, intense-beam klystron oscillators.

Green's Function Simulation of Space–Charge Effects in Electron Beams

The sol–gel method was used to synthesize two different Ba0.75Sr0.25Ti0.95Zr0.05O3 powders: one of high purity and the other of low purity. These two sol–gel-synthesized powders show two distinct particle sizes and surface areas. The slip casting method was applied to these two sol–gel powders followed by a pressureless sintering, which shows large differences in sintered density and grain size for the pressureless sintered disks. Neutron powder diffraction shows a transition to the nonpolar cubic Pm–3m space group at higher temperatures for both materials. Pair distribution function analysis was used to examine the local displacements of the Ti4+ and Zr4+ cations. The dielectric constant, loss tangent, and bias were measured on these two materials.

Discrete monotron oscillator

The Accelerator Production of Tritium (APT) Project is investigating using a superconducting linac for the high-energy portion of the accelerator. As this accelerator would be used to accelerate a high-current (100-mA) CW proton beam up to 1700 MeV, it is important to determine the effects of stray-beam impingement on the superconducting properties of 700-MHz Nb cavities. To accomplish this, two 3000-MHz elliptical niobium cavities were placed in a cryostat, cooled to nominally 2 K in sub-atmospheric liquid helium, and irradiated with 798-MeV protons at up to 490-nA average current. The elliptically shaped beam passed through the equatorial regions of both cavities in order to maximize sensitivity to any changes in the superconducting surface resistance. Over the course of the experiment, 6/spl times/10/sup 16/ protons were passed through the cavities. After irradiation, the cavities were warmed to 250 K, then recooled to investigate the effects of a room-temperature annealing cycle on the superconducting properties of the irradiated cavities. A detailed description of the experiment and the results shall be presented. These results are important to employing superconducting RF technology to future high-intensity proton accelerators for use in research and transmutation technologies.

Slip casting of sol–gel-synthesized barium strontium zirconium titanate ceramics

Work is under way to develop a 17 GHz free electron maser (FEM) for producing a 500 MW output pulse with a phase stability appropriate for linear collider applications. We plan to use a 500 keV, 5 kV, 6 cm diameter annular electron beam to excite a TM{sub 02} mode Raman FEM amplifier in a corrugated cylindrical waveguide. The annular beam will run close to the interaction device walls to reduce the power density in the fields, and to greatly reduce the kinetic energy loss caused by beam potential depression associated with the space charge which is a significant advantage in comparison with conventional solid beam microwave tubes at the same beam current. A key advantage of the annular beam is that the reduced plasma wave number can be tuned to achieve phase stability for an arbitrary correlation on interaction strength with beam velocity. It should be noted that this technique for improving phase stability of an EM in not possible with a solid beam klystron. The annular beam FEM provides the opportunity to extend the output power of sources in the 17 GHz regime by well over an order of magnitude with enhanced phase stability. The design and experimental status are discussed.

In-situ proton irradiation and measurement of superconducting RF cavities under cryogenic conditions

The goal of this research effort is to develop a long-pulse relativistic klystron amplifier (RKA) by extending the pulse length of this gigawatt-class device by an order of magnitude beyond the current state-of-the-art (100 ns) to one microsecond. A research approach is described for obtaining kilojoule microwave pulses at 1.3 GHz. Achieving kilojoule microwave pulses requires extending the electron beam pulse duration beyond one microsecond without diode closure, and maximizing the microwave extraction efficiency at the fundamental frequency. Our earliest experiments have produced a modulated electron beam for one microsecond with a peak rf current of 0.9 kA and a voltage of 350 - 400 kV. In some cases we have observed beam modulation in excess of 2 microseconds. The component of beam power at the microwave drive frequency (1.3 GHz) was approximately 350 MW. Although only a small effort has been put forth to address the output coupling issues, approximately 50 - 70 MW was coupled into dominant mode rectangular waveguide. Recently, an electron beam diode has been tested that delivers peak currents in excess of 5 kA for a monotonically increasing current pulse exceeding durations of 1 microsecond(s) at beam kinetic energies above 500 keV. to achieve this result close attention was given to minimizing the current losses from the diode and maximizing the beam current transmission through the RKA.