Relativistic Klystron Amplifier Research Articles

Design and 3D simulation of a two-cavity wide-gap relativistic klystron amplifier with high power injection

By using an electromagnetic particle-in-cell (PIC) code, an S-band two-cavity wide-gap klystron amplifier (WKA) loaded with washers/rods structure is designed and investigated for high power injection application. Influences of the washers/rods structure on the high frequency characteristics and the basic operation of the amplifier are presented. Generally, the rod structure has great impacts on the space-charge potential depression and the resonant frequency of the cavities. Nevertheless, if only the resonant frequency is tuned to the desired operation frequency, effects of the rod size on the basic operation of the amplifier are expected to be very weak. The 3-dimension (3-D) PIC simulation results show an output power of 0.98 GW corresponding to an efficiency of 33% for the WKA, with a 594 keV, 5 kA electron beam guided by an external magnetic field of 1.5 Tesla. Moreover, if a conductive plane is placed near the output gap, such as the electron collector, the beam potential energy can be further released, and the RF power can be increased to about 1.07 GW with the conversion efficiency of about 36%.

Study of nonlinear interaction between bunched beam and intermediate cavities in a relativistic klystron amplifier

In intermediate cavities of a relativistic klystron amplifier (RKA) driven by intense relativistic electron beam, the equivalent circuit model, which is widely adopted to investigate the interaction between bunched beam and the intermediate cavity in a conventional klystron design, is invalid due to the high gap voltage and the nonlinear beam loading in a RKA. According to Maxwell equations and Lorentz equation, the self-consistent equations for beam-wave interaction in the intermediate cavity are introduced to study the nonlinear interaction between bunched beam and the intermediate cavity in a RKA. Based on the equations, the effects of modulation depth and modulation frequency of the beam on the gap voltage amplitude and its phase are obtained. It is shown that the gap voltage is significantly lower than that estimated by the equivalent circuit model when the beam modulation is high. And the bandwidth becomes wider as the beam modulation depth increases. An S-band high gain relativistic klystron amplifier is designed based on the result. And the corresponding experiment is carried out on the linear transformer driver accelerator. The peak output power has achieved 1.2 GW with an efficiency of 28.6% and a gain of 46 dB in the corresponding experiment.

Brief Introduction and Recent Applications of a Large-Scale Parallel Three-Dimensional PIC Code Named NEPTUNE3D

This paper introduces a large-scale parallel 3-D fully electromagnetic particle-in-cell code named Nonlinear Evolution of Plasma-interaction Technology Under Nonneutral Environment-Three Dimensional (NEPTUNE3D) developed with the support of a parallel adaptive structured mesh application infrastructure JASMIN. NEPTUNE3D can be used to simulate beam-wave interactions in high-power microwave (HPM) generators. Some novel parallelization techniques are adopted in the NEPTUNE3D code for high parallel efficiency and good scalability. Some real-world HPM devices have been successfully simulated by NEPTUNE3D, such as the relativistic backward wave oscillator (RBWO), the magnetically insulated line oscillator (MILO), the virtual cathode oscillator, and the relativistic klystron amplifier. In this paper, two recent simulation examples are presented. The first is simulations of a knife-edge cathode RBWO, using 128 processors very efficiently. The second example is simulations of a MILO with a plate-inserted mode-transducing antenna, modeled using 27 million cells, 3 million particles, and 1024 processors with relatively high parallel efficiency.

Suppression of higher mode excitation in a high gain relativistic klystron amplifier

Suppressing higher mode excitation is very important in the high gain relativistic klystron amplifier because higher mode can seriously degrade klystron performance and cause pulse shortening. The mechanism of higher mode self-excitation is explored in the PIC simulation, and it is shown the coupling between cavities is the main cause of higher mode self-excitation. The coupling forms the positive feedback loop for higher mode to be excited just like that in the oscillator circuit. The formula for startup current of higher mode self-excitation is developed based on the coupling between cavities. And the corresponding methods are taken to avoid higher mode self-excitation. Finally, mode control is realized in the RKA with output power up to 1.02 GW when driven power is only few kilowatts.

Experimental study on an X-band coaxial multi-beam relativistic klystron amplifier

The advantages of the tri-axial relativistic klystron amplifier and the multi-beam klystron amplifier are analyzed, and then an X-band coaxial multi-beam relativistic klystron amplifier is designed in order to increase the output microwave power and frequency. Based on the results from partial-in-cell simulation, the experiment is performed on the Sinus accelerator. In experiment, the transmission and bunching of electron beam are analyzed, and then the amplifier is driven by a beam of 5.3 kA at 670 kV, and the maximum output power is 420 MW when input power is 30 kW, and frequency is 9.375 GHz. The experiment proves that a ten kW-level input power can drive the X-band coaxial multi-beam relativistic klystron amplifier to generate more than one hundred MW-level output power.

Analysis and design of X-band coaxial multi-beam relativistic klystron amplifier

The space charge limiting current in cylinder drifting tube and coaxial drifting tube and the energy that could be obtained and also the restrict magnetic field are analyzed, and the results show that it is propitious to change the energy of electron beam into microwave power in the coaxial conduit. The advantages of the tri-axial relativistic klystron amplifier and the multi-beam klystron amplifier are analyzed, then an x-band coaxial multi-beam relativistic klystron amplifier is designed by using a three-dimensional particle simulation software, and the transmition and the bunching of electron beam are analyzed. A 1.23 GW averaged microwave power over the oscillator period is generated by simulation with 600 kV electron beam voltage, 5 kA current and 0.8 T leading magnetic field density, the frequency is 9.37 GHz and the efficiency is 41%.

Investigation of suppression of non-working mode oscillation in a high gain relativistic klystron amplifier

Because non-working modes are very easy to excit in the high gain relativistic klystron amplifier, and seriously affect the beam-wave interactive action, the suppressing of non-working modes is very important. These modes can seriously degrade klystron performance and cause the pulse shortening, the normal operation of HGRKA will be affected greatly. This paper deals with the mechanism of non-working mode self-excitation by the PIC simulation, and it is obvious that the coupling between cavities is the main cause of non-working mode self-excitation. The coupling can form the positive feedback loop. The formula for starting current of non-working mode self-excitation is developed according to the coupling between cavities, and the corresponding measures are taken to avoid non-working mode self-excitation. Then the corresponding simulations and optimization are conducted. Finally non-working mode control is realized in the HGRKA experimentally when driven power is only few kilowatts. The RF output has a power of 0.98 GW and pulse width of 100 ns.

RF phase jitter in a klystron amplifier

RF phase jitter is a very important parameter for a relativistic klystron amplifier. This parameter is closely linked with the physics processes in the klystron. RF phase jitter is theoretically studied together with Particle in Cell (PIC) simulations in the paper. The main factor is deduced and verified in the PIC simulation. RF phase jitter is significantly affected by the fluctuation of the beam voltage. The relation between the phase jitter and the voltage fluctuation is linear in certain ranges.

Phase investigation on relativistic klystron amplifier

The relevant facts to phase of relativistic klystron amplifier (RKA) are analyzed in this paper. The output microwave phase of RKA relevant to voltage, current, size, rising time and delay time of electron beams is simulated by PIC. It is found that the phase shift of RKA is changed with the voltage, current and size of electron beams. On the contrast, the leading magnetic field, rising time and delay time of electron beams do not change the phase shift of RKA obviously. The phase of RKA is investigated in experiment too. The phase sensitivity of RKA is about 2.6 degrees; the phase shaking of RKA is less than 20 degrees. The experimental results are content well with the simulations.

线性变压器源驱动相对论速调管放大器的初步实验

线性变压器源驱动相对论速调管放大器的初步实验

双耦合口相对论速调管放大器输入腔特性的等效电路法研究

双耦合口相对论速调管放大器输入腔特性的等效电路法研究

Design of X-band tri-axial relativistic klystron amplifier

In this paper, we analyze the interaction between electron beam and microwave in the coaxial cavity, and obtain the coupling coefficient of the coaxial cavity and the electronic load conductance. The X-band tri-axial relativistic klystron amplifier is designed and simulated. Simulation results show that coaxial cavity can increase the efficiency. With an input microwave power of 70 kW, electron beam voltage of 600 kV, and electron beam of 5 kA, we obtain a microwave with a power of 1.1 GW, frequency of 9.37 GHz, and an efficiency of 37%.

Mode control in a high-gain relativistic klystron amplifier

Middle cavities between the input and output cavity can be used to decrease the required input RF power for the relativistic klystron amplifier. Meanwhile higher modes, which affect the working mode, are also easy to excite in a device with more middle cavities. In order for the positive feedback process for higher modes to be excited, a special measure is taken to increase the threshold current for such modes. Higher modes' excitation will be avoided when the threshold current is significantly larger than the beam current. So a high-gain S-band relativistic klystron amplifier is designed for the beam of current 5 kA and beam voltage 600 kV. Particle in cell simulations show that the gain is 1.6 × 105 with the input RF power of 6.8 kW, and that the output RF power reaches 1.1 GW.

Experimental study of a low radio frequency power driven relativistic klystron amplifier

Using particle in cell simulation codes, a low radio frequency (rf) power driven relativistic klystron amplifier is designed according to the beam with current of 7.5 kA and voltage of 750 kV with special measures to avoid the mode competition. Simulated power reaches 1.7 GW when the driven rf power is 7.0 kW, the corresponding gain is 53.9 dB. Also the experiment is carried out on a telsa-typed accelerator, whose beam is with current of 8 kA and voltage of 800 kV. The measured rf output power reaches 2.04 GW in the experiment when the input rf power is 62 kW and frequency 2.850 GHz, the corresponding gain is 45.1 dB and efficiency is 32%. The maximum gain reaches 46.7 dB when the input decreases to 39 kW, the corresponding output rf power is 1.84 GW.

Virtual cathode emission of an annular cold cathode

Recent measurement of voltage $V$ and current $I$ of the electron gun of a relativistic klystron amplifier revealed that the resulting current-voltage relationship appeared to differ from the usual Child-Langmuir law ($I\ensuremath{\propto}{V}^{3/2}$) especially during the initial period of voltage increase. This paper attempts to explain this deviation by examining the emission mechanism using particle-in-cell simulation. The emission area in the cathode increased stepwise as the applied voltage increased and within each step the current and voltage followed the Child-Langmuir law. The electron emission began when the voltage reached a threshold, and the perveance increased with the emission area. Furthermore, an apparent virtual cathode was formed which was larger than the cathode tip. This occurs because, above a certain voltage, the emission from the edge and the side of the cathode surface dominates the emission from the front-end surface.

Investigation of RF phase jitter in relativistic klystron amplifier

RF phase Jitter is a very important parameter for a relativistic klystron amplifier, and it is closely related with the physical processes in the klystron. In view of the physical process in the klystron, the RF phase jitter is theoretically studied together with particle in cell （PIC） simulation. The main factor which affects the RF phase jitter is deduced and verified in the PIC simulation. The RF phase jitter is significantly affected by the fluctuation of the beam voltage at the steady state when the cavity is in resonanec. The relation is linear in a certain range.

Simulation Studies of a Relativistic Klystron With Strong Input Power

As one of the potential high-power microwave devices on the level of gigawatts, relativistic klystron amplifier (RKA) can be used for power combination to further improve the radiation microwave power. Because of self-excited oscillation of the intense relativistic electron beam, the frequency and phase characteristics of general relativistic klystron devices are independent with the driven microwave pulse. In order to obtain frequency and phase locking, a method of improving the input power is put forward to inhibit these parasitic oscillations. This paper reports a particle-in-cell simulation study of the RKA's characteristics under the condition of strong power feeding. By making use of the 500-keV 6-kA electron beam, the simulation results show that strong input power can inhibit the parasitical oscillations in cavity and modulate the beam very well with only one cavity. About 5.4-kA modulation current and a microwave with power of 1.4 GW, bandwidth of 5%, and efficiency of 50% are obtained. The new amplifier driven by the strong input power proves to be a potential device to attain high amplitude stability, high efficiency, high spectral purity, wide bandwidth, and low level of phase and amplitude noise.

Theoretical investigation of an electron beam propagating through a wide gap cavity

This paper presents a self-consistent nonlinear theory of the current and energy modulations when an electron beam propagates through an inductively-loaded wide gap cavity. The integro-differential equations are obtained to describe the modulation of the beam current and kinetic energy. A relativistic klystron amplifier (RKA) model is introduced, which uses an inductively-loaded wide gap cavity as an input cavity. And a numerical code is developed for the extended model based on the equations, from which some relations about the modulated current and modulated energy are numerically given.

Repetitive Operation of an $S$-Band 1-GW Relativistic Klystron Amplifier

This paper reports on the operation of a repetitively pulsed relativistic klystron amplifier at 100 pps. The electron beam was reproducibly modulated to > 74%. Using a 700-kV, 6.5-kA, 25-ns electron beam and a 600-kW Magnetron as inputs, a 1.2-GW radiated power in 20-ns full-width at half-maximum pulses at 2.95 GHz is extracted. The average power is 2.4 kW. The efficiency is 27%, and the gain is about 33 dB

Analysis and design of triaxial output cavity in a relativistic klystron amplifier

This paper discusses the space-charge-limited current and its energy distribution in the coaxial drifting pipe. In order to bypass the potential depression problem, the triaxial output cavity of relativistic keystron amplifier was implemented. The triaxial output cavity was designed and simulated. A hollow electron beam of 590kV, 5kA generates 1GW rf power in S band klystron amplifier using the triaxial output cavity. The efficiency is about 35%, which is 9% bigger than that of the coaxial output cavity. The results agree well with the experiment.