Experimental Investigation of 94 GHz Second-Harmonic Gyrotrons

Niu Xin-jian Wang Li Li Hong-fu Vacuum Electronics National Laboratory, Institute of High Energy Electronics, UESTC, Chengdu 610054, China

Abstract: A second-harmonic gyrotron intended as a compact lightweight source has been designed. 94-GHz gyrotron is driven by a 51-kV 11.8-A

( %3/,32.1/ =Δ=⊥ zzz vvvv ) electron beam from a magnetron injection gun and employs a low-loss TE021/TE031 complex cavity for mode control. A pulse output power of 156kW is obtained with cryogen-free superconducting magnet for the gyrotron, magnetic field 1.67T with operating frequency 94GHz, corresponding to an efficiency of 26%.

Keywords: complex cavity, gyrotron, cryogen-free magnet, magnetron injection gun, second-harmonic.

Introduction Gyrotrons are the primary high-power source of millimeter waves. Although the application of electron cyclotron heating of fusion plasmas has been primarily responsible for the gyrotron’s development, many other applications exist, including millimeter-wave ceramic sintering, high-density communications, and high-resolution radar within the atmospheric windows of 35, 94, and 140 GHz. One problem with high-frequency gyrotrons is the strong magnetic fields required to satisfy the fundamental electron cyclotron resonance condition. For example, a 35-kG magnet would ordinarily be needed to generate 94 GHz at the fundamental cyclotron frequency. However, by operating the gyrotron at the sth cyclotron harmonic, the required magnetic field can be decreased by a factor of s. To develop 94-GHz oscillator that is both compact and efficient is the objective of this work. The approach is to use the efficient gyrotron interaction at the second harmonic so that the required magnetic field of 17.2kG can be supplied by a cryogen-free low-Tc superconducting magnet.

Complex cavity gyrotron The complex cavity with gradual transition has the advantage of overcoming mode competition and enhancing interaction efficiency. The complex-cavity gyrotrons[1,2], the interaction cavity was comprised of a set of coupled back-to-back cavities that supported TEmn modes with different n. They were designed so that the disparate modes of interest resonate in each cavity at the same frequency. The resulting complex mode extends over both cavities and therefore possesses a lower starting current than the other modes of the system, which are localized to only one cavity. It will employ a diffraction-coupled complex cavity, operating in the TE02 and TE03 circular modes. The low

ohmic loss of the TE0n modes is important for gradual transition complex cavity of the design.

Mode converter Adopting the periodic perturbation of radius, the TE03-TE02-TE01 circular waveguide mode converter is optimized with input mode TE03 and frequency of 94GHz, radius of 9mm. Variable uptaper of sinusoidal curvature is used in waveguides outputs.

Experiments To avoid mode competition in the relatively long second-harmonic gyrotron, a complex cavity is employed. The complex cavity is comprised of two cavities that support the low loss TE02 and TE03 modes at 94 GHz. This device has been designed to minimize its size and weight by using a cryogen-free magnet and operating. Gyrotron design includes TE03-TE02, TE02-TE01 inner mode converter and collectors of second-harmonic complex cavity Gyrotron. A pulse output power of 156kW is obtained at a beam voltage of 51kV with beam current of 11.8A, operating mode TE02-TE03, average output power 3.05kW, magnetic field 1.67T with operating frequency 94GHz, corresponding to an efficiency of 26%, and the burned patterns measured at the ends of converter are produced by the gyrotron as shown in table 1 and fig 1-6.

Conclusions 94GHz second-harmonic complex cavity gyrotron inner TE03-TE02-TE01 mode converter and collectors are designed with conversion efficiency 98.7%, A pulse output power of 156kW is obtained with cryogen-free superconducting magnet for the gyrotron, magnetic field 1.67T with operating frequency 94GHz, corresponding to an efficiency of 26%.

Table 1 Test Results of 94Ghz Gyrotron

Beam voltage 51kV Beam current 11.8A Magnetic field 1.56T

Mode TE02-TE03 Puls width 100μs

Freuquency 94GHz Peak power 156kW

Average power 3kW Efficiency 26%

References 1. Li Hong-fu. A 35-GHz Low-Voltage 3rd-Harmonic

Gyrotron with a Permanent-Magnet System[J]. IEEE Trans on plasma science, 2003,31(2):264-271.

2. Niu Xinjian, Yu Sheng, Li Hongfu. Corrugated Waveguide Mode Conversion for 94GHz Second-Harmonic Gyrotron [A]. 2006 31# International conference on Infrared and Millimeter Waves and THz Conference[C]. Shanghai, China, 2006.

Figure 1: Track of electron beam in the MIG gun

1.76 1.78 1.80 1.820

2

4

6

8

10

I s(A)

B0(KG)

H2031.5~2.0

Figure 2: Dependence of the start-oscillation current on

the magnetic field.

Figure 4: Interaction efficiency distribution with cavity length z

0 5 10 15 20-0.05

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

I=10A,

kB=0.966,a = 1.3

Effic

ienc

yZ

Figure 3: Interaction efficiency distribution with cavity

length z Figure 5: TE01 mode measured at the ends of converter are produced by the gyrotron.

Figure 6: Photograph of 94GHz second-harmonic gyrotron with complex cavity