Beam Velocity Ratio Research Articles

Modeling, simulation, and fabrication of electron optic system for application on 105 GHz high‐power gyrotron

AbstractThis paper introduces the modeling, simulation, and fabrication of the electron optic system (EOS) for high‐power 105 GHz gyrotron, which includes single anode magnetron injection electron gun, magnetic system, and the collector. Based on the specific research project and the theory of electron optics, the structure with detailed parameters is modeled and simulated by using two different electromagnetic analysis softwares. The simulation results show that the electron beam velocity ratio is around 1.3, and the electron landing regions from the two softwares are almost agreed with each other. At last, the components of the EOS are fabricated for the next assembling step.

PROBABILISTIC MODEL OF BEAM–PLASMA INTERACTION IN RANDOMLY INHOMOGENEOUS PLASMA

We propose a new model that describes beam–plasma interaction in the presence of random density fluctuations with a known probability distribution. We use the property that, for the given frequency, the probability distribution of the density fluctuations uniquely determines the probability distribution of the phase velocity of waves. We present the system as discrete and consisting of small, equal spatial intervals with a linear density profile. This approach allows one to estimate variations in wave energy density and particle velocity, depending on the density gradient on any small spatial interval. Because the characteristic time for the evolution of the electron distribution function and the wave energy is much longer than the time required for a single wave–particle resonant interaction over a small interval, we determine the description for the relaxation process in terms of averaged quantities. We derive a system of equations, similar to the quasi-linear approximation, with the conventional velocity diffusion coefficient D and the wave growth rate γ replaced by the average in phase space, by making use of the probability distribution for phase velocities and by assuming that the interaction in each interval is independent of previous interactions. Functions D and γ are completely determined by the distribution function for the amplitudes of the fluctuations. For the Gaussian distribution of the density fluctuations, we show that the relaxation process is determined by the ratio of beam velocity to plasma thermal velocity, the dispersion of the fluctuations, and the width of the beam in the velocity space.

Design studies of the Ku-band, wide-band Gyro-TWT amplifier

This paper reports a Ku-band, wide band Gyrotron-Traveling-wave-tube(Gyro-TWT) that is currently being developed at Kwangwoon University. The Gyro-TWT has a two stage linear tapered interaction circuit to obtain a wide operating bandwidth. The linearly-tapered interaction circuit and nonlinearly-tapered magnetic field gives the Gyro-TWT a wide operating bandwidth. The Gyro-TWT bandwidth is 23%. The 2d-Particle-in-cell(PIC) and MAGIC2d code simulation results are 17.3 dB and 24.34 kW, respectively for the maximum saturated output power. A double anode MIG was simulated with E-Gun code. The results were 0.7 for the transvers to the axial beam velocity ratio (=alpha) and a 2.3% axial velocity spread at 50 kV and 4 A. A magnetic field profile simulation was performed by using the Poisson code to obtain the grazing magnetic field of the entire interaction circuit with Poisson code.

TIME-DEPENDENT NONLINEAR THEORY AND NUMERICAL SIMULATION OF 94 GHZ COMPLEX CAVITY GYROTRON

A time-dependent nonlinear theory for complex cavity gyrotron is presented in this paper. The theory includes generalized telegrapher's equations and electron motion equations, which are deduced in detail. A calculation code for the self-consistent nonlinear beam-wave interaction is developed based on the presented theory. Using the code, a 94GHz complex cavity gyrotron operating in TE021{ TE031 modes is thoroughly studied. Numerical results show that an output power of 180kW, about 36% e-ciency is achieved with a 50kV, 10A electron beam at a focused magnetic fleld of 1.78T and a beam velocity ratio of 1.65. The results from MAGIC simulation are also given and an output power of 192kW, 38.4% e-ciency is obtained This tells the agreement with these two simulation codes.

Study on Nonlinear Theory and Code of Beam-Wave Interaction for Gyroklystron

A nonlinear self-consistent theory of beam-wave interaction for gyroklystron with multiple cavities is analyzed in this paper. The electron motion equations and transient electromagnetic field equations in a complex form are deduced in detail. A calculation code including a time-dependent description of the electromagnetic fields and a self-consistent analysis of the electrons is designed and the corresponding software implementation is achieved using Fortran language. An example is presented for the operation of the code, namely a four-cavity, Ka-band gyroklystron operating in the TE011 mode at the fundamental of the cyclotron frequency. The numerical results show that a maximal saturated peak output power of 330 kW, corresponding to 39% efficiency and a saturated 3-dB bandwidth of 325 MHz is achieved with a 72.8 kv, 11.8 A electron beam at a focused magnetic field of 13 kG and a beam velocity ratio of 1.63 when the speed spread is 5%. By comparison, the numerical results agree with the experimental results.

Comparative EGUN and TRAK simulation studies on the design of the magnetron injection gun of a 200kW, 42GHz gyrotron

A triode-type magnetron injection gun (MIG) for a 42 GHz, 200 kW gyrotron was designed by two commercial codes: EGUN and TRAK. An in-house code MIGANS was used as the p ost-processor of the EGUN. Some basic equations relevant to the problem available in literature were used while using the EGUN code for the purpose. The results on the design output parameters obtained by these two codes were found to be in close agreement. The gyrotron is a device to generate high powers at high frequencies. In this device, the source of electrons is a magnetron injection gun (MIG) which forms a beam of electrons in which the electrons execute the small cyclotron orbits at a frequency required for the cyclotron resonance interaction with RF waves in the device (1,2). The MIG is placed in an axially symmetrical magnetic field at the magnetic field strength determined by the magnetic compression ratio . The initial electron motion occurs in the crossed electric and magnetic fields so that the electron s follow the helical trajectories around the magnetic field lines. The gun anode accelerates the beam to th e final beam energy. Downstream from the gun, the magnetic field increases smoothly in the magneti c compression region to the required peak value at the interaction region. The magnetic field compress ion reduces the radius of the beam and results in an increase of the velocity component perpendicular to the magnetic field. Since the total energy is constant, the drift velocity is correspondingly reduced. In India, an activity related to the design and development of 42GHz, 200kW gyrotron has been started. The operating mode has been selected as TE03 with the second maximum of the transverse-plane field pattern of the cylindrical interaction cavity as the be am launching position for the fundamental beam-mode operation. For the better performances of the gyrotron and the gyro-devices, an electron beam of good quality is required (1-6). The triode-type MIG has been designed having the beam voltage V0 = 65kV, the beam current I0 = 10A and the transverse-to-axial beam velocity ratio � = 1.26. The

A submillimeter-wave gyroklystron: theory and design

New elements of the theory and a preliminary design for a pulse/continuous-wave 360-GHz gyroklystron are presented. Issues germane to very high-frequency operation of gyroklystron amplifiers, including drive power and limits on cavity Q, are discussed. A specific two cavity design is described. That device had a gain of 33 dB and a beam extraction power of 1 kW with an electronic efficiency of 8% and a power output of 700 W after taking into account Ohmic losses. To produce fast-rising pulses, the beam velocity ratio was varied via the modulating anode of a magnetron injection gun. A design for an electron gun through which reduction of the output signal by -50 dB was achieved by a 400-V modulation is given. The input coupler, cavities, electron gun, collector, output coupling optics and collector are described. In addition, issues concerning the fabrication of the high precision, /spl sim/2-mm diameter cavities are discussed.

Experimental studies of a four-cavity, 35 GHz gyroklystron amplifier

The experimental results from a four-cavity, Ka-band gyroklystron amplifier operating in the(TE)/sub 011/ cylindrical mode at the fundamental of the cyclotron frequency are presented. A peak output power of 208 kW at 31.90 GHz, with a 3-dB bandwidth of 178 MHz (0.5%), an electronic efficiency of 30%, and a saturated gain of 53 dB was obtained with a 72-kV, 9.6-A electron beam at a magnetic field of 12.95 kG and a measured beam velocity ratio of 1.36. The magnetic field was found to have a strong influence on the power-bandwidth tradeoff. At 13.35 kG, a peak output power of 174 kW at 31.90 GHz, with a 3-dB bandwidth of 240 MHz (0.7%), an electronic efficiency of 25%, and a saturated gain of 56 dB was obtained with the same beam parameters. The gyroklystron amplifier was unconditionally stable at these operating points and stability studies are reported. Experimental results were found to be in excellent agreement with large signal simulations.

Studies of electronic noise in gyroklystrons

Direct measurements of phase noise in a four-cavity, 35 GHz gyroklystron producing 50 μs pulses of 175–210 kW output power with 50–53 dB saturated gain are presented. The measurements were performed at a 10.7 MHz frequency offset from the carrier, where the noise is expected to be dominated by shot noise and where the extrinsic noise from the electron gun’s pulsed power supply is manageable. At an operating point with 70 kV beam voltage, 9 A beam current, and a beam velocity ratio of 1.3, a phase noise of −149±1 dBc/Hz was measured during the production of 180 kW output power at 50 dB gain. At a higher beam current of 10 A, the measured phase noise was −146±1 dBc/Hz during production of a 200 kW output power carrier with 53 dB gain. The directly measured phase noise levels were generally within 2–3 dB of the values expected on the basis of carrier-free noise temperature measurements. Overall, the measured gyroklystron noise levels are similar to those of conventional klystrons. Also presented are analytic calculations of the growth rates expected for electrostatic cyclotron instabilities in the region between the electron gun and the input cavity in a previous three-cavity gyroklystron. Very modest perpendicular velocity spreads (rms greater than 2%) are found to completely suppress such noise growth. This lack of significant noise growth above that of bare shot noise is in agreement with experimental results.

Measurements of intrinsic shot noise in a 35 GHz gyroklystron

Experimental measurements of electron beam shot noise in a 35 GHz, 225 kW, three-cavity gyroklystron have been obtained from both the input and output cavities. This intrinsic noise was studied in the absence of an applied carrier (i.e., at zero drive power). The spectrum of the noise emitted by the input cavity is found to have a Lorentzian shape, with peak noise power densities from the input cavity typically reaching 6.3×10−15 W/Hz (−112 dBm/Hz), and typical 3 dB bandwidths of 160 MHz. The output cavity noise spectrum is found to be equal to the input cavity noise spectrum multiplied by the measured linear frequency response of the gyroklystron. The measured noise levels at the input cavity are 0–5 dB lower than theoretical predictions for shot noise unaltered by collective effects. Furthermore, the input cavity noise power exhibits complex variations as a function of beam current, beam velocity ratio, and circuit magnetic field that are not predicted by present theory. Noise-to-carrier ratios expected in the input cavity during full power amplifier operation are inferred from the noise measurements and known values of drive power required to saturate the gyroklystron. The noise-to-carrier ratio, with typical values of −90 to −80 dBc, is found to be a strong function of the operating parameters.

Broadband operation of a Ka-band tapered gyro-traveling wave amplifier

A wideband low-voltage millimeter-wave gyro-traveling wave tube (gyro-TWT) amplifier operating in the TE/sub 10/ rectangular waveguide mode at the fundamental cyclotron frequency is under investigation, The device incorporates precise axial tapering of both the magnetic field and the interaction circuit for broadband operation. Experimental results of a wide (33%) instantaneous bandwidth with a small signal gain in excess of 20 dB and saturated efficiency of /spl sim/10% were achieved and shown to be in good agreement with the theory. Reflective instability due to multi-pass effects by mismatches was observed and characterized. Gain and efficiency have been limited by this reflective instability rather than by absolute instabilities which limit the performance of gyro-TWT's with uniform cross-section. The start-oscillation current in terms of the relevant experimental parameters such as the beam velocity ratio (/spl alpha/), magnetic field detuning and reflection coefficient has been measured and compared with theory. Measurements of the phase variation in terms of the RF frequency have shown that the phase varies /spl plusmn/30/spl deg/ from fitted linear phase line. >

Experimental study of a high-frequency megawatt gyrotron oscillator

A detailed experimental study of the efficiency and output power of a pulsed gyrotron operating in the TE16,2,1 mode at 148 GHz has been conducted. A peak efficiency of 30% was achieved at 80 kV and 20 A for an output power of 480 kW. The highest output power of 925 kW, corresponding to an efficiency of 19%, was measured at 120 kV and 40 A. Two cavities with different interaction lengths (6.0λ and 4.2λ) were investigated. In both cases, agreement was found between the theoretical and experimental efficiency for beam currents up to 15–20 A. At higher currents, the experimental efficiency saturated between 20% and 25%, well below the 35%– 40% predicted by theory. No increase was obtained for modest positive or negative linear tapering of the cavity magnetic field. Measurements indicate that the beam velocity ratio decreases as beam current increases, partially explaining the reduced efficiency at higher currents. Operation in different azimuthal rotations of the cavity modes was also observed. The measured rotation was found to be consistent with the theoretical coupling between the beam and rf field.