Paul J. Tallerico

Progress Report on the NBS/LOS Alamos RTM

The NBS-Los Alamos 200 MeV Racetrack Microtron (RTM) is being built under a program aimed at developing the technology needed for high-current intermediateenergy CW electron accelerators. In this report we give an overview of the present status of the project. Recent progress includes: (1) completion of testing of the 100 keV chopper-buncher system demonstrating a normalized emittance well under the design goal of 2.6 ¿ mm mrad at currents exceeding the design goal of 600 ¿A; (2) operation of the rf structures comprising the 5 MeV injector linac at power levels up to 50 kW/m, resulting in an accelerating gradient at ß=1 of 2 MV/m (compared to a design goal of 1.5 MV/m). The measured shunt impedance is 82.5 Mn/m; (3) construction and installation of the 30 ton end magnets of the RTM. Field mapping of one magnet has been completed and its uniformity exceeds the design goal of ±2 parts in 104; (4) performance tests (with beam) of prototype rf beam monitors which measure current, relative phase, and beam position in both transverse planes. (5) Installation and initial operation of the primary control system.

The gyrocon: A high-efficiency, high-power microwave amplifier

The gyrocon is a high-power, high-efficiency amplifier, which operates by deflection modulation of an electron beam. The bunching is better than that in a klystron, especially for very high powers and UHF frequencies, so the overall efficiency and the maximum output power can be higher than in a klystron. The present theory includes the effects of large signals, space charge, and finite beam size. The equations of motion are relativistically correct, but the space-charge fields are only correct to first order in v/c. The theory is derived and a computer code to solve these equations is discussed. Then the code is used to obtain radial gyrocon designs which have significant advantages over klystrons or gridded tubes in the 0.2-1.0- GHz frequency range. We briefly discuss other gyrocon types but, to date, we have not performed computational analyses for the spherical and planar gyrocons. The upper frequency limit may reach 3 GHz for these devices.

Modifications to harmonic current bunching of electron beams from RF cavities due to radial boundary conditions

Harmonic current bunching of an electron beam after it passes through a standing-wave RF cavity is well known, with analytic solutions in both the transversely infinite space-charge regime and the ballistic regime. The Wallander formula is often used between these regimes for finite-sized beams, with pretty good accuracy. These formulas describe simple space-charge modes that are launched by the RF cavity, and assume a single space-charge reduction factor. However, upon close examination these formulas do not provide the complete story. Additional space-charge modes are introduced by the radial boundary conditions, and the actual total harmonic current is modified somewhat from the simple formulas, and includes beating effects between the different modes, even in the small-signal, or linear, regime. This beating leads to mode mixing and eventual damping, which also affects the long-term propagation of longitudinal space-charge waves in accelerator structures.

The effects of linear-accelerator noise on the Los Alamos free-electron laser

The optical power produced by the Los Alamos free-electron laser has been flawed by a peristent instability. This instability appears as fluctuations in output power in a frequency band centered around 100 kHz. We have found that the fluctuations are dependent on the strength of the lasing and on variations in the electron micropulses arrival time. When the lasing is strong, the fluctuations are small; when the lasing is weak, the fluctuations are large. The variations in the electron micropulses arrival time primarily are due to fluctuations in the accelerator gradients. The primary noise sources in the accelerator are the electron gun and the klystron amplifier chain. In addition, this noise is uncontrolled because of a lack of bandwidth in the feedback controls for the RF system. Appropriate improvements are being made to eliminate these fluctuations in optical power.

Design considerations for the high-power multicavity klystron

A confined-flow large-signal formulation of the klystron interaction equations is presented and applied to the analysis of the multicavity klystron amplifier. The effects of cavity voltage, cavity phase, drift length, and beam parameters are studied. The two- and three-cavity amplifiers are studied in detail, and several numerical examples of four-cavity klystrons are also given. A significant second-harmonic component of velocity modulation is shown to improve the dc to RF conversion efficiency. Two methods of obtaining this modulation are discussed. The large-signal theory presented here has been used to design a five-cavity klystron which is 50 percent efficient and has a controllable power transfer curve. Theoretical and experimental power transfer curves are presented for several 1 1/4-MW klystrons; the calculated output power is between 4 and 10 percent greater than the experimental values.

Dynamics of Retrograde Electrons Returning From the Output Cavity in Klystrons

Loss of efficiency and tube oscillations have been attributed to electrons returning from the output cavity in klystrons due to excessive output cavity voltages. It is generally believed that the retrograde electrons lead to a relatively large harmonic current component in the input cavity, which overwhelms the input drive. Here, for the first time, detailed simulations describing the dynamics of retrograde particles for a nominal klystron design and cases with the shunt impedance of the output cavity increased, which show persistent harmonic bunching induced by the penultimate cavity, are presented. The maximum retrograde harmonic current is comparable to the current induced in the input cavity without the retrograde particles and can significantly influence the overall klystron behavior

Progress on the NBS-LANL CW Microtron

The NBS-LANL racetrack microtron (RTM) currently under construction at the National Bureau of Standards is a demonstration accelerator to determine the feasibility of, and to develop the technology necessary for building high-energy, high-current, continuous beam (CW) electron accelerators using beam recirculation through room temperature rf accelerating structures. Parameters of the RTM are: injection energy - 5 MeV; energy gain per pass - 12 MeV; number of passes - 15 or 16; final beam energy - 185-197 MeV; maximum current - 550 ¿A; rf frequency - 2380 MHz. At present, the electron gun and 100 keV beam transport line are operational, and most other major subsystems are in the construction or installation phase. Exceptions are the rf structure (under development), the 5 MeV beam transport line (in engineering design), and the extraction beam line (in conceptual design). Our studies of the original candidate accelerating structure, the disk-and-washer, have led to the discovery of beam steering modes which render this structure unsuitable for the RTM without at least substantial further development beyond the scope of the project. The most promising alternate for meeting the design goal of CW operation at 1.5 MeV/m is the side-coupled structure. A shunt impedance of 80 M¿/m has been measured in a test section of side-coupled structure at 2380 MHz, adequate cooling has been designed, and a 2.7 m long section of this design is under construction. The electron optics of the RTM have been studied in detail.

Relativistic effects in the traveling-wave amplifier

A relativistically correct large-signal theory is developed for the analysis of high-power, axially symmetric traveling-wave amplifiers in order to investigate the physical phenomena involved in the interaction process. The nonlinear integro-differential system equations are developed from the Lorentz force equation, the one-dimensional equivalent circuit equation, the wave equation, and the continuity of charge relation. These equations are applied to two electron stream models: a ring model which permits the effects of nonlaminar flow and space-charge forces to be evaluated, and a disk-electron model in which these effects are ignored. The ring model space-charge fields are obtained from the appropriate Greens function for Poissons equation in a moving frame of reference. Numerical solutions are presented and discussed with major emphasis on the disk-model solutions. The principal results are that the gain per unit length decreases with increasing beam velocity, the circuit phase velocity for optimum power output approaches the dc beam velocity u 0 , as u_{0}/c approaches unity, and the conversion efficiency is almost independent of u 0 for the synchronous case. The linearized one-dimensional theory of the traveling-wave tube is also discussed. Several of the large-signal results are predicted from the small-signal theory.

The SNS linac high power RF system design, status, and results

The Spallation Neutron Source being built at the Oak Ridge National Lab in Tennessee requires a 1 GeV proton linac. Los Alamos has responsibility for the RF systems for the entire linac. The linac requires 3 distinct types of RF systems: 2.5-MW peak, 402.5 MHz, RF systems for the RFQ and DTL (7 systems total); 5-MW peak, 805 MHz systems for the CCL and the two energy corrector cavities (6 systems total); and 550-kW peak, 805 MHz systems for the superconducting sections (81 systems total). The design of the SNS Linac RF system was presented at the 2001 Particle Accelerator Conference in Chicago. Vendors have been selected for the klystrons (3 different vendors), circulators (1 vendor), transmitter (1 vendor), and high power RF loads (3 different vendors). This paper presents the results and status of vendor procurements, test results of the major components of the Linac RF system and our installation progress.

A 150-kW, 450-MHz Gyrocon RF Generator

The output-power and conversion efficiency of the Los Alamos gyrocon were increased by a factor of 150 in the past year. Major improvements in the phase- and amplitude-control system and in understanding the electron multipactor and surface-conditioning processes increased the output power. The highest measured efficiency on this gyrocon is 23%, which was obtained with several combinations of parameters. Both the output power and the efficiency are a factor of 3 below the design values, and several possibilities are being examined to remove the discrepancy between theory and experiment.