M. Kamada

Electron gun for powerful large orbit gyrotron

In the article, results of numerical simulation of the gun with the cusp of magnetic field are presented. Short pulse version of the gun with explosion emission is investigated. Some preliminary analytical estimation of the beam and gun parameters are performed. Then, numerical optimization of the electrodes shape as well as magnetic field distribution is carried out. For preliminary separation of electrons and formation of the rectilinear beam, anode diaphragm is installed. After then, additional selection of electrons for decreasing the ripple is performed. For this purpose, channel walls are used for interception of some part of the electron beam. Reverse of the magnetic field in the diode part of the gun is formed. So, the formation of the rectilinear beam is combined with the region, where electrons obtain initial gyration energy. To prevent the disperse action of the own beam space charge forces, the system with big gradient of magnetic field (about 0.5-1 kGs/mm) is needed. According to results of the simulation, helical electron beam can be performed even at total compression ratio about 1000 and current density more than 50 kA/cm2. The designed electron gun provides acceptable performance for the large orbit gyrotron, such as operating current close to 300 A, the pitch-factor value about 1.5-1.7, deviation of the guiding centers from the axis (the ripple) λ/10 and λ/6 for operation on 3-rd and 5-th cyclotron harmonic correspondingly (wavelength λ=0.5 and 0.3 mm) and velocity spread within the range 10-15%.

Powerful large orbit gyrotron of submillimeter wavelength range

Preliminary design of cavity for high power submillimeter large orbit gyrotron has been described. Time-dependent, multi-mode calculation gives us the possibility to achieve output power of up to 1.5 MW with a frequency 420.8 GHz at third harmonic. The most important advantages of LOG are possibility of selective excitation of high harmonic number. However, due to low efficiency of this devices (typically a few percents), high output power needs powerful electron beams.

Development of high-frequency gyrotrons using high harmonics and a high magnetic field

Our gyrotrons developed at the Research Centre for the Development of the Far Infrared Region, Fukui University (FU), are high-frequency medium-power gyrotrons used as millimetre- to submillimetre-wave radiation sources for application to new far-infrared technologies. We have already developed a gyrotron FU series which consists of eight gyrotrons. The gyrotron series has achieved frequency tuneability in a wide range (from 38 to 889 GHz). The highest frequency corresponds to a wavelength of 337 μm. This is a current record for high-frequency operation of gyrotrons. Recently, we have developed a high-harmonic gyrotron with an axis-encircling electron beam and a terahertz gyrotron with a pulse magnet. In this paper the present status of the gyrotron FU series is described.

Development of an Ultra High Frequency Gyrotron with a Pulsed Magnet

An ultra‐high frequency gyrotron is being developed as a THz radiation source by using a pulsed magnet. We have achieved the highest field intensity of 20.2 T. High frequency operation at the second harmonic will achieve 1.01 THz; the corresponding cavity mode is TE6,11,1. On the other hand, an ultra‐high power gyrotron with a pulsed magnet is also being developed as a millimeter to submillimeter wave radiation source. The gyrotron is a large orbit gyrotron (LOG) using an intense relativistic electron beam (IREB). A pulsed power generator “ETIGO‐IV” is applied for generation of the IREB. A prototype relativistic LOG was constructed for fundamental operation. The output of the LOG will achieve 144 GHz and 9 MW; the corresponding cavity mode is TE1,4,1. Cavities for 2nd and 4th harmonic operations were designed by numerical simulation for achievement of higher frequency. The progress of development for prototype high frequency gyrotrons with pulsed magnets is presented.

Plasma scattering measurement using a submillimeter wave gyrotron as a radiation source

An FU II gyrotron was applied as a submillimeter wave radiation source for plasma scattering measurements of the compact helical system (CHS) in the National Institute for Fusion Science (NIFS) in Japan. The stabilization of the gyrotron output and the high quality probe beam are required to improve the performance of the measurements. The outputs (cw operation, f=301 GHz, P/spl sim/20 W) of the FU IV gyrotron are stabilized (/spl Delta/P/P/spl sim/1%, /spl Delta/f/spl sim/10 kHz) by decreasing the fluctuations of the potentials (/spl Delta/V/sub k//spl sim/0.6 V, /spl Delta/V/sub a//spl sim/0.2 V), by introducing smoothing circuits consisting of a resistor, an induction coil and a capacitor. Further stabilization of output frequency (/spl Delta/f<1 kHz) is attained by applying a phase lock loop. A high purity mode operation is a prerequisite for effective conversion of the output radiation into the probe beam for scattering measurements. We have constructed a new gyrotron, namely, the FU VA gyrotron, which has a carefully designed resonant cavity. The measurements of the radiation patterns have confirmed high-purity mode operation.

Development of a sub terahertz high power pulse gyrotron

A high power sub terahertz pulse gyrotron with a frequency around 400 GHz and the output power of tens of kW is under development in FIR-FU. Use of this gyrotron as a power source of collective Thomson scattering from a high density fusion plasma will reduce the plasma dispersion effect and the background ECE. As the first step of this development, a 200 GHz, TE03 mode pulse gyrotron operating at the fundamental resonance was constructed. The design of this gyrotron was optimized for an existing electron gun. The oscillation test was carried out and many modes other than the TE03 mode were observed. There was an indication of second harmonic oscillation with a frequency of about 400 GHz. In the second step, an electron gun working at the beam voltage of 70 kV will be used to realize the target gyrotron.

HIGH POWER SUBMILLIMETER WAVE RADIATION SOURCES – GYROTRON FU SERIES

Gyrotron FU series in the Research Center for Development of Far Infrared Region at Fukui University (FIR FU) has achieved highest frequency of gyrotrons, frequency tunability, high stabilization of frequency and amplitude and modulation of frequency (FM) and amplitude (AM). Recently, a new gyrotron, a large orbit gyrotron (LOG) was developed. This gyrotron has achieved high harmonic operations up to fifth. We have also designed the next ultra-high frequency gyrotron with a pulse magnet. The maximum field intensity is about 31 T. The gyrotron will achieve high frequency operations at 0.78 THz for the fundamental and at 1.44 THz for second harmonic. This is the first trial for the breakthrough of high frequency operation at 1 THz. We have already constructed a pulse magnet system and a gyrotron tube. The construction of the whole device will be completed and the operation test will be carried out soon. In this paper, the present status of Gyrotron FU Series is described.

Development of a terahertz gyrotron using a high field pulse magnet

Summary form only given. Development of a THz gyrotron is described. A pulse magnet using a condenser bunk (6 mF, 10 kV) generates a high magnetic field up to 25 T in the room temperature bore of a cryostat whose diameter is 85 mm. A demountable gyrotron tube is installed on the axis of the cryostat. The gyrotron achieve high frequency operations up to 680 GHz by fundamental resonance and up to 1.3 THz by its second harmonics. Detailed experimental results are presented.

Development of a short pulse, high power, large orbit gyrotron using a high voltage pulse supply ETIGO-IV

A high power gyrotron using pulsed power generator ETIGO-IV is being developed. The typical output waveform to a matched load of ETIGO-IV is 400 kV in peak voltage, 13 kA in peak current and 120 ns in pulse length. The combination of high energy electron beam with such a power supply and a high magnetic field opens the possibility of submillimeter wave generation with an output power above 1 MW to a large orbit gyrotron.