Double-gap Cavity Research Articles

Design, fabrication, and characterization of a drift tube linac type double gap re-buncher cavity.

The Low Energy High Intensity Proton Accelerator (LEHIPA) at Bhabha Atomic Research Centre, India, has recently been commissioned to the target energy of 20MeV by beam acceleration through a 352MHz radio frequency quadrupole (RFQ) and drift tube linac (DTL). The medium energy beam transport (MEBT) line of LEHIPA matches the 3MeV beam from RFQ to the DTL using four electromagnetic quadrupoles and a re-buncher cavity. In the beam transport lines of high frequency linacs, TM010 mode single gap buncher cavities are conventionally used, while a double gap re-buncher cavity was chosen for the short length LEHIPA MEBT based on beam dynamics simulations. The 352MHz double gap re-buncher cavity has been designed in a DTL type geometry with a cell length of βλ. This double gap cavity is found to be more power efficient with a higher shunt impedance and transit time factor than the conventional single gap buncher cavity. Electromagnetic design simulations, fabrication details, low power and high-power RF test results, and beam test results of the re-buncher cavity are presented in this paper.

Electron Flow Modulation in Double-Gap Cavity With a Multiple Ratio of the Two Modes Frequencies

The study presents the calculation and optimization of the double-gap cavity designs for a klystron-type multipath frequency multiplier. This construction allows to achieve a multiplied (equal to two) ratio of the frequency of the highest ( ${3}\pi $ ) mode to the frequency of the main ( ${2}\pi $ ) mode lying in the Ka -range. The design was optimized using a nondimensional quality parameter that combines the main electronic and electrodynamic characteristics of resonators. The features of the electron flow bunching when passing through a double-gap cavity in the two-frequency mode are studied using the 3-D numerical modeling methods. Analysis of the results showed that depending on the ratio of the voltages effective amplitudes generated by electric fields of the main and highest modes, two modulation modes are possible. They are nonsinusoidal modulation, which allows increasing the efficiency at the input signal frequency, and frequency multiplication regime, in which the frequency of the electron flow bunches at the output of the resonator doubles.

Optimization of multibeam klystron double gap cavities loaded by metal rods

AbstractIn this paper, the results of numerical investigation of a metal photonic band gap cavity are presented. The investigated resonator is dual‐gap and is considered as an intermediate for the K‐band multibeam klystron. A working mode is a 2n mode with a frequency of 24.15 GHz. Electrodynamic characteristics of the cavity (characteristic impendance, beam coupling factor, relative electronic conductivity, Q‐factor) were calculated and the conditions for achieving the maximum effciency are found. Based on the obtained data, the required number of gaps in the output cavity was estimated, as well as the power and efficiency of the multibeam klystron with this type of cavities, depending on the microperveance of the electron beam.

Investigation of the Electrodynamic Properties of a W-Band Double-Gap Cavity

The results of the calculation of the electrodynamic parameters and of the structural optimization of an in-phase-mode double-gap cavity for a W-band klystron with a strip electron flow are presented. The effect of the dimensions of the drift space on the characteristic impedance and interaction coefficient is investigated, and the conditions for obtaining a uniform distribution of the longitudinal component of the electric field over the width of the drift space are determined.

Design Guidelines of a Double-Gap Microwave Rebuncher Cavity for a 400 MHz, 2.5 MeV Energy Light Ion Accelerator with Lower Gap Voltage and Field

A detailed electromagnetic model of a double-gap microwave rebuncher cavity proposed for 400-MHz, 2.5 MeV energy of the front end section in 1 GeV energy H- ion linear accelerator, is presented and validated by extensive simulations. This design is intended to decrease both the utilized gap voltage and peak electric field of the rebunching cavity, henceforth may decrease X-ray radiation. A low cost 1/2 scaled aluminum cavity double gap model was built and tested for model validation. Mode frequencies and quality factors of the model were measured and compared to simulation. Additionally, bead perturbation method was used to measure the para-axial electric field. Simulation of the single and double gap cavities were compared to measurements and were in very good agreement. Subsequently, design guidelines have been developed to design the double gap rebuncher, optimize its gap size for lower electric fields and reduced X-ray radiation.

Estimation Method for Self-Impedance's Real Part of Multigap Output Cavity of Klystrons Using Group Delay

A new estimation method by group delay for the real part of the self-impedance of multigap (two or more gaps) output cavity of klystrons is presented. Although these electromagnetic data can be easily obtained by using electromagnetic simulation (e.g., MWS or HFSS), we still need a cold test to confirm and adjust the values because in actual manufacturing, there are several steps that can slightly change the sizes of cavities and the parameters of high frequency. The traditional impedance measurement method for output cavity needs three steps, i.e., open, short, and perturbation, and the problems caused by a multigap cavity in the measurement of middle gaps (excluding the gaps at two sides) will meet difficulties when setting perturbation and short states. Beginning with microwave network theory and aiming at three-gap output cavity, this paper finally establishes the relationship between the group delay and the real part of the self-impedance of the output cavity. Then, the new method is no longer limited by the number of gaps through eliminating the perturbation and the short states of the traditional method. Using the new and traditional methods, a double-gap output cavity of an S-band klystron has been separately measured. Moreover, the feasibility of the method has also been verified by comparing their results. Finally, the relevant results of the five-gap output cavity of an X-band sheet-beam klystron are given.

Investigation of Double-Gap Cavity Generating-Amplifying Klystrons

Based upon the discrete model of large particle flux investigated is the possibility of operation of a double-gap cavity resonator in the mode where the oscillation generating function is aligned with the function of velocity-modulating electrons in the phase ensuring further sufficient electron bunching. It is established that the required distribution of velocities is gained on the edge of the generation area with longest distance between gaps where both phase and amplitude conditions of self-excitement are met. Shown is the possibility of obtaining the highest efficiency at the expense of using a double-gap cavity resonator with wide gaps. Performed is the investigation of generating–amplifying klystrons of different configurations. Found are conditions of obtaining a maximum value of amplitude of the convection current first harmonic and a maximum value of the efficiency factor.

Design formulae of unsymmetrical double-gap output cavities of klystrons

A new perspective is proposed that unsymmetrical double-gap output cavities of klystrons will have greater energy conversion efficiency of beam to microwave than the conventional symmetrical double-gap output cavities. The equivalent circuit method is used to get the relationships of the two gap electric fields in π-mode and 2π-mode unsymmetrical double-gap cavities in the case that the two gaps of the unsymmetrical double-gap cavities are approximately regarded as parallel grid gaps. The energy conversion efficiency in the process of an ideal bunching beam passing through the π-mode or 2π-mode unsymmetrical double-gap cavity is calculated by simulating the ideal cluster beam with a succession of periodically repeated electrons; then, the formulae of the optimal gap width, the optimal length of the drift tube between two gaps and the optimal gap voltage amplitude corresponding to the maximal conversion efficiency are obtained, which can be used to design π-mode and 2π-mode unsymmetrical double-gap output cavities of klystrons.

The efficiency of a pitron

A technique is proposed for the calculation of the electron efficiency of a pitron, a self-oscillator that is based on a double-gap cavity and operates on the π-mode oscillations. For six generation zones, the parameters of grid gaps are optimized to ensure the maximum efficiency. Recommendations on the recalculation of the grid-gap parameters to the parameters of gridless gaps are formulated. It is shown that the total electron efficiency grows with the zone number from 14% for the zero zone to 65% for the fifth one. For practical application, it is recommended to employ the first and second zones with an electron efficiency of about 50% and 60%, respectively.

Large Signal Calculation and Simulation of the Admittance of an Electron Beam in Double-Gap Coupling Cavity

A model is constructed to evaluate the electronic conductance of an electron beam in double-gap coupling cavity resonator. The electronic conductance is studied both analytically and numerically. The analytical expression for the electronic conductance is derived based on the nonlinear space-charge wave theory, and it is also presented as a function of modulation coefficient in detail. It is shown that the analytic results of electronic conductance are in reasonable agreement with the numerical simulations. In addition, the simulated results of the beam susceptance are also given, and the results show that the change of susceptance with modulation coefficient is very small.

Theoretical and experimental researches on C-band three-cavity transit-time effect oscillator

The C-band three-cavity transit-time effect oscillator (3C TTTO) is a novel high power microwave device based on the transit-time effect of the three-cavity (3C) resonator. The operational principle of this device is briefly expounded in this paper, and the theoretical and experimental researches on the radial insulation diode, the 3C resonator, the double-gap output cavity and the circular waveguide bevel cut radiation antenna are presented in detail. By using the analytic method, the eigen modes and their field distributions of the 3C resonator are developed, and some basic laws of the transit-time effect are obtained in non-π mode field of the 3C resonator. At last, the experimental results are given. This device generated RF peak power in excess of 400MW with 15ns FWHM, and its beam-wave power conversion efficiency is about 17%.

An impedance measurement method for double-gap klystron cavity

A new method has been developed for measuring the impedance of a two-gap cavity used in high-power klystrons. The principle is based on network analysis. The cavity under test is considered as a microwave network. The two gaps of the cavity and its output terminal are referred to as three ports. We then can use an impedance matrix to characterize this system, and the six independent impedance parameters can be found by measuring the input impedance seen from the output waveguide when the gaps are in different conditions; viz., either open, shorted, or perturbed. The gap impedance then can be deduced therefrom. It is shown that there are three impedances for a double-gap cavity instead of one for a single-gap cavity. Another problem dealt with here is how to evaluate the capacitance introduced by the perturbation. A few typical experimental results are presented.

Large-signal behavior of distributed klystrons

A theoretical large- and small-signal analysis of the behavior of standing-wave-type distributed circuit klystrons, based on a confined flow model, is described. A two-cavity klystron with a π-mode double-gap distributed output circuit and a conventional single-gap input cavity was selected as a specific example for study. The field distribution for the experimental cavities was measured and a conversion efficiency of 52 percent was predicted. The effects of the output cavity position relative to the input cavity, the output cavity gap angle, and the space charge were analyzed theoretically. The theoretical results were compared with measurements made on a precision demountable klystron with identical beam and circuit parameters, but with a Brillouin-focused beam rather than a confined-flow beam. A load efficiency of 35 percent, a conversion efficiency of 42 percent, and small-signal gains up to 15 dB were measured. The corresponding figures for the conventional single-gap output circuit were 24 percent, 28.6 percent, and 9 dB. The measured effects of changes in the tube parameters were in general agreement with the theoretical predicted results. The study indicates that the π-mode double-gap cavity is a very practical output structure yielding high efficiency with a very short interaction length. It should prove useful in medium- and high-power klystrons.