Bernhard Piosczyk

A 2 MW, 170 GHz coaxial cavity gyrotron

The feasibility of manufacturing a 2-MW CW coaxial cavity gyrotron at 170 GHz has been demonstrated and data required for fabrication of an industrial tube have been obtained. An engineering design of a prototype started recently with the goal to provide gyrotrons with 2-MW microwave output power for International Thermonuclear Experimental Reactor (ITER). The design of critical components of the prototype tube as electron gun, cavity and RF output system will be verified under realistic conditions at short pulses using the experimental coaxial gyrotron at Forschungszentrum Karlsruhe.

2.2-MW Record Power of the 170-GHz European Preprototype Coaxial-Cavity Gyrotron for ITER

A 2-MW continuous-wave (CW) 170-GHz coaxial-cavity gyrotron for electron cyclotron heating and current drive in the International Thermonuclear Experimental Reactor (ITER) is under development within the European Gyrotron Consortium (EGYC1), a cooperation between European research institutions. To support the development of the industrial prototype of a CW gyrotron, a short-pulse tube (preprototype) is used at KIT Karlsruhe (former FZK) for experimental verification of the design of critical components, like the electron gun, beam tunnel, cavity, and quasi-optical RF output coupler. Significant progress has been achieved recently. In particular, RF output power of up to 2.2 MW with 30% output efficiency has been obtained in single-mode operation at 170 GHz. Furthermore, a new RF output system has been designed, with an efficient conversion of the generated RF power into a Gaussian RF output beam. The results have been successful, yielding a Gaussian mode content ~96%.

Coaxial cavity gyrotron- recent experimental results

The feasibility of fabrication of coaxial cavity gyrotrons with an output power up to 2 MW, continuous wave (CW) has been demonstrated and information necessary for a technical design has been obtained. Experiments with a gyrotron equipped with newly designed components-electron gun, cavity, RF output system-have been performed. In short pulses, a maximum radio frequency (RF) output power of 2.2 MW has been reached in stable operation. At the nominal output power of 1.5 MW an efficiency of 30% has been achieved. This has been enhanced to 48% in operation with a single-stage depressed collector. The stability of the coaxial insert has been measured to be within /spl plusmn/0.03 mm under operating conditions. The losses at the coaxial insert have been found to be about 0.1% of the RF output power. Investigations of the microwave stray radiation captured inside the tube have been performed with the following results: (1) the captured stray radiation due to diffraction losses is approximately uniformly distributed inside the mirror box; (2) about 8% of the captured microwave power is radiated through a relief window with 100-mm diameter in the used setup; and (3) the total amount of stray radiation has been found to be about 11% of the RF output power. Parasitic low-frequency oscillations have been successfully suppressed and stable operation has been achieved over a wide parameter range. Fast (/spl sim/0.1-ms) frequency tuning has been demonstrated by applying a rapid variable bias voltage at the coaxial insert. In particular, step frequency tuning by /spl plusmn/2.2 GHz due to switching from the nominal mode at 165 GHz to its azimuthal neighbors has been done and continuous tuning by up to 70 MHz within the bandwidth of the TE/sub 31,17/ mode has been performed.

Experimental Investigations and Analysis of Parasitic RF Oscillations in High-Power Gyrotrons

Megawatt gyrotrons are found to suffer from various parasitic oscillations, in particular, RF oscillations in the beam tunnel prior to the desired interaction zone (the cavity). This paper describes the experimental results from a gyrotron experiment which was dedicated to investigate parasitic oscillations in the beam tunnel and to verify improved beam-tunnel structures. A system for improved spectral measurements and a new analysis method are presented. The results verify theoretical predictions on the parasitic oscillations, and in effect validate the corresponding improved beam-tunnel structure. In addition, other types of parasitic oscillations were observed and explained.

Novel Numerical Method for the Analysis and Synthesis of the Fields in Highly Oversized Waveguide Mode Converters

A numerical method for the analysis of the fields in highly oversized waveguides is proposed in this paper. This method allows the simulation of the fields on waveguide walls with arbitrary surface deformations in the case that the waveguide is highly oversized, and the wall deformations are shallow and smooth. Combined with the analysis method, an algorithm has been developed for synthesizing the waveguide wall to provide a desired field distribution. As an example, a 309.6-mm-long waveguide launcher has been designed for a 170-GHz coaxial-cavity gyrotron to transform the TE34,19 cavity mode to a fundamental Gaussian distribution. An efficiency of transformation to the desired fundamental Gaussian mode of 96.3% has been obtained at the launcher aperture, whereas the transformation efficiency is just 86% using a conventional dimpled-wall launcher with a length of 660 mm.

165-GHz coaxial cavity gyrotron

The basic investigations on a coaxial cavity gyrotron operating at 165 GHz have been accomplished and the feasibility of manufacturing a 2-MW continuous-wave coaxial cavity gyrotron at 170 GHz has been demonstrated. In the last measurements, the data required for fabrication of an industrial tube have been completed. The influence of misalignment of the insert on gyrotron operation has been investigated. The limitation of the high-voltage performance of the electron gun due to buildup of a Penning discharge has been suppressed and the leakage current to the insert has been measured as a function of the retarding collector voltage.

Single-stage depressed collectors for gyrotrons

Two 140 GHz gyrotrons with a single-step depressed collector have been operated. The different position of the isolating collector gap in the stray magnetic field causes the electron motion in the retarding region to be in one case adiabatic and in the other case nonadiabatic. The kind of motion within the retarding field influences strongly the behavior of the gyrotron with a depressed collector. In the case of nonadiabatic motion a significant amount of transverse momentum is given to the electrons reflected at the collector potential. This causes the reflected electrons to be trapped between the magnetic mirror and the collector. The electrons escape from the trap by diffusion across the magnetic field to the body of the tube thus contributing to the body current. Despite the high body current there is no observable influence of the collector voltage on the RF output power. In the case of adiabatic motion the reflected electrons do not gain a sufficient amount of transverse momentum to be trapped by the magnetic mirror. They pass the cavity toward the gun and they are trapped between the negative gun potential and the collector. The interaction with the RF field by electrons traveling through the cavity enhances the diffusion in the velocity space thus enabling the trapped electrons to overcome the potential barrier and escape toward the collector. Therefore the body current stays at low values since in this case the reflected electrons do not contribute to it. However, at higher collector voltages a reduction of RF power occurred and some noise in the electron beam was observed.

165 GHz, 1.5 MW-coaxial cavity gyrotron with depressed collector

A further step in the development of a coaxial-cavity gyrotron operated in the transverse electric TE/sub -31,17/ mode at 165 GHz is presented. The gyrotron has been equipped with a quasi-optical output system consisting of a Vlasov launcher with a single cut and two mirrors, one with a quasi-elliptic and the other with a nonquadratic phase correcting surface. The radio frequency (RF) power is transmitted through a single output window. A maximum output power of 1.7 MW has been achieved. At the nominal operational parameters an RF power of 1.3 MW with an efficiency of 27.3% has been measured. The efficiency increases to 41% in operation with a single-stage depressed collector.

A new approach for a multistage depressed collector for gyrotrons

A feasibility study for a two-stage depressed gyrotron collector has been performed. A new approach for an adiabatic magnetic decompression of the hollow electron beam has been used. It permits control of the radius of the constant magnetic flux surface, which determines the radial extension of the electron beam. Independent of the value of the magnetic field around the beam. For this purpose, either solenoidal coils or a ferromagnetic insert can be placed inside the hollow electron beam. Thus, the radial dimensions of a multistage depressed collector of a high-power high-frequency gyrotron can be kept within limits given by technological constraints. The energy sorting of the electron beam is improved by using electrodes inside the hollow electron beam for controlling the potential distribution. The additional control electrodes make it possible to eliminate almost all of the effect of secondary electrons on the operation of the collector. In order to demonstrate the proposed approach, a compact two-stage depressed collector has been designed for a 1.5-MW coaxial cavity gyrotron operating at 165 GHz in the transverse electric (TE)/sub 31,17/ mode, which is under development at FZK, Karlsruhe, Germany. Including the effect due to secondary electrons, a collector efficiency of 73% has been calculated with an average and peak heat dissipation density of about 240 W/cm/sup 2/ and 500 W/cm/sup 2/, respectively. This results in an increase of the output gyrotron efficiency from 36.5% to 62.6% when internal radio frequency (RF)-losses inside the gyrotron tube of 15% are taken into account.

A novel 4.5-MW electron gun for a coaxial cavity gyrotron

A novel 4.5-MW (90 kV, 50 A) electron gun for a 165-GHz coaxial cavity gyrotron operated in the transverse electric mode TE/sub 31, 3.7/ has been designed, fabricated, and operated in a gyrotron. The electrons are extracted toward the anode as in a conventional magnetron injection gun (MIG) and not toward the coaxial insert as in the previously used so-called inverse gun. The main advantage of this arrangement is the reduced overall radial size which becomes comparable to the size of a conventional gun with the same diameter of the emitter. The design and the technology of the electron gun fulfil the requirements for a high-power gyrotron operated at long pulses up to continuous wave. The coaxial insert is fully cooled and can be adjusted when the tube is completely assembled. The amplitude of mechanical vibrations has been measured to be less than 0.03 mm under operating conditions. This is sufficiently small for stable long-pulse operation. In operation at short pulses (ms) a microwave RF output power of 2.2 MW at 165 GHz has been obtained with a beam current of 84 A. The best operating conditions have been observed with an intermediate type of electron flow.