VOLUME 84, NUMBER 12 PHYSICAL REVIEW LETTERS 20 MARCH 2000 High-Gain Wide-Band Gyrotron Traveling Wave Amplifier with a Helically Corrugated Waveguide V. L. Bratman, 1 A. W. Cross, 2 G. G. Denisov, 1 W. He, 2 A. D. R. Phelps, 2 K. Ronald, 2 S. V. Samsonov, 1 C. G. Whyte, 2 and A.R. Young 2 1 Institute of Applied Physics, Russian Academy of Sciences, Nizhny Novgorod, 603600, Russia 2 Department of Physics and Applied Physics, University of Strathclyde, Glasgow, G4 0NG, Scotland, United Kingdom (Received 4 October 1999) First bandwidth measurements of a novel gyrotron amplifier are presented. The coupling between the second harmonic cyclotron mode of a gyrating electron beam and the radiation field occurred in the region of near infinite phase velocity over a broad bandwidth by using a cylindrical waveguide with a helical corrugation on its internal surface. With a beam energy of 185 keV, the amplifier achieved a maximum output power of 1.1 MW, saturated gain of 37 dB, linear gain of 47 dB, saturated bandwidth of 8.4 to 10.4 GHz (21% relative bandwidth), and an efficiency of 29%, in good agreement with theory. PACS numbers: 84.40.Ik, 41.60.Cr, 52.75.Ms, 52.75.Va The cyclotron resonance maser (CRM) instability between a gyrating electron beam (in a guide magnetic field) and a mode of a cylindrical waveguide has been investigated due to its potential for efficient, broadband amplification of radiation [1,2]. The relative dispersion relations of the beam and the radiation have conspired to seriously limit this potential. In the regime of low axial wave numbers, the bandwidth is limited by the curvature of the hyperbolic wave dispersion (as compared to the linear beam dispersion), and the fact that in this regime the wave group velocity is close to zero means that the interaction is prone to undesirable oscillations. On the other hand, if one operates with high axial wave numbers, where the group velocity becomes close to c, then Doppler broadening of the resonance spoils the efficiency of the interaction and limits the bandwidth [3]. Nonetheless, broadband amplifiers have a rather large range of applications, and the ability to increase the radia- tion power capability above that possible in conventional slow-wave amplifiers is desirable. Gyroklystrons have thus far been developed to offer the high power and frequency capabilities of the CRM instability to amplifier applications, but these experiments use resonant cavities as their interaction spaces, inherently limiting the relative bandwidth 1% [4]. A number of variations to increase the bandwidth of gyrotron amplifiers have been investigated [5,6]. For ex- ample, a series of gyroamplifiers have obtained 16% effi- ciency at a frequency of 5 GHz with a uniform magnetic field, increasing to 26% efficiency with 7% bandwidth us- ing a tapered field [7]. Theoretical investigation of a gy- rotron traveling wave amplifier (gyro-TWT) interaction in a rectangular waveguide having a variable cross section in the form of a two stage tapered device having an inter- mediate server has predicted an efficiency of 37%, with bandwidth of 1.8% for an axial velocity spread of 10% [8]. In a similar two stage configuration [9], 20% band- width over 32–39 GHz with 16% efficiency was experi- mentally obtained, using bidirectional tapering of both the microwave circuit and magnetic field. Recently, impres- sive experimental results on a gyro-TWT were achieved by Chu et al., who studied the amplifier at the fundamen- tal cyclotron harmonic. By stabilizing oscillations with the use of an interaction structure with distributed wall losses, this 35 GHz Ka-band amplifier produced 93 kW of power at 26.5% efficiency and 70 dB gain with a 3-dB bandwidth of 8.6% [10]. Second and higher harmonic interactions have attracted attention because they reduce the magnetic field required for any frequency of opera- tion in addition to the potential for frequency multiplica- tion (the beam may be perturbed in a low harmonic input interaction and the energy extracted in a higher frequency, higher harmonic output stage) [11–14]. The experimen- tal results presented in this paper also used a second har- monic interaction, halving the required magnetic field, but primarily to enable coupling between a special waveguide eigenmode having a strong quadrupole component and an axis encircling electron beam. All the gyrotron travel- ing wave amplifier experiments referenced above identified spurious oscillations and insufficient electron-beam qual- ity as major limitations. A new idea has recently been reported [15,16] which has reopened the potential for a broadband gyrotron traveling wave amplifier where a helical corrugation on the wall of the interaction waveguide couples together a near to cutoff mode and a far from cutoff mode to produce a wave dis- persion with a finite and nearly constant group velocity in the region of near infinite phase velocity. This dispersion may be readily matched to an electron beam’s linear dis- persion characteristic over rather a broad bandwidth. Be- cause the group velocity is large over the resonance, the problem of oscillations is considerably reduced while the small axial wave numbers limit the impact of electron ve- locity spread. An experiment based on this approach has recently been reported [17] at two rather close frequencies, but the potential of the system for high gain and efficiency 2746 0031-9007 00  84(12)  2746(4)$15.00 © 2000 The American Physical Society