Amplification of Picosecond Pulses in a 140-GHz Gyrotron-Traveling Wave Tube

Abstract

An experimental study of picosecond pulse amplification in a gyrotron-traveling wave tube (gyro-TWT) has been carried out. The gyro-TWT operates with 30 dB of small signal gain near 140 GHz in the HE 06 mode of a confocal waveguide. Picosecond pulses show broadening and transit time delay due to two distinct effects: the frequency dependence of the group velocity near cutoff and gain narrowing by the finite gain bandwidth of 1.2 GHz. Experimental results taken over a wide range of parameters show good agreement with a theoretical model in the small signal gain regime. These results show that in order to limit the pulse broadening effect in gyrotron amplifiers, it is crucial to both choose an operating frequency at least several percent above the cutoff of the waveguide circuit and operate at the center of the gain spectrum with sufficient gain bandwidth.Gyrotrons are a form of electron cyclotron maser capable of producing kilowatts to megawatts of output power in the microwave, millimeter wave, and terahertz bands [1][2][3][4]. In recent years gyrotron amplifiers have demonstrated high output power levels with significant gain bandwidths [5][6][7]. One important application of millimeter waves is in spectroscopy, where coherent pulses on a subnanosecond or picosecond time scale are needed for optical pumping of molecular states. The pulses must be shorter than the relaxation time, typically requiring subnanosecond (or picosecond) pulse lengths [8].Picosecond microwave pulses have been demonstrated in a vacuum electron device by superradiance [9], but such pulses cannot be used for spectroscopy. Subnanosecond pulses must contain a spectral bandwidth exceeding the transform limit, that is, exceeding 1 GHz. In the conventional microwave bands, at frequencies of one to several GHz, the required gain bandwidth to amplify such picosecond pulses is generally not available, since the GHz bandwidth is a large fraction of the carrier frequency. In recent years, high power, wideband amplifiers in the millimeter wave band have been developed that are suitable for amplifying picosecond pulses. For example, a form of klystron called an extended interaction klystron has been developed at 95 GHz with a gain bandwidth of about 1 GHz. This amplifier has been used to successfully amplify 1 kW output pulses as short as 800 picoseconds [10]. A gyrotron-traveling wave tube (gyro-TWT) at 95 GHz has been demonstrated with a gain bandwidth of 6.5 GHz at an output power level of 2 kW [7]. This gyro-TWT could, in principle, be used to amplify a 150 ps pulse. To our knowledge, amplification of picosecond pulses has not been tested with these or other powerful wideband gyro-amplifiers. Because of the possibility of distortion in amplification of picosecond pulses, detailed studies are needed of the amplification process in such devices. In this paper, we report a detailed study of amplification of pulses as short as 400 ps in a 1 kW, 140 GHz gyro-TWT with a gain bandwidth exceeding 1 GHz. To our knowledge, this is the first report o...

Photonic-Band-Gap Traveling-Wave Gyrotron Amplifier

“…The large bandwidths observed, as much as ~2% of its operational frequency, will allow for the amplification of very short pulses. Previously, pulses as short as 400 ps with pulse broadening and 1 ns without broadening were investigated using a 140 GHz gyrotron amplifier [3]. With the achieved bandwidths reported in this Letter, we would expect that pulses as short as 250 ps could be amplified without broadening.…”

“…As oscillators they are capable of producing megawatts of output power from microwave to THz bands [6,11-14]. In recent years, gyrotron amplifiers have demonstrated high output power levels with significant gain bandwidths [3,15-22]. A gyrotron amplifier works on the same fundamental principles as a gyrotron oscillator for the extraction of energy from an electron beam.…”

Time domain DNP with the NOVEL sequence

“…We predict that at magnetic fields of 0.5 T and above, NOVEL DNP would surpass the SE. With the recent advances in the gyroamplifier technology, [54][55][56] we anticipate that pulsed DNP will become available at high magnetic fields in the near future. Furthermore, current applications of CW DNP in NMR require operation at cryogenic temperatures, which imposes limitations in many cases.…”

Self-Induced Transparency and Electromagnetic Pulse Compression in a Plasma or an Electron Beam under Cyclotron Resonance Conditions

“…The dielectric-loaded waveguide used in [12] will be replaced by the regular metallic waveguide and the magnetic field strength will correspond to the cyclotron resonance value. It should be noted that in difference with [13] where cyclotron mechanism of picosecond microwave pulses amplification have been studied on a beam of gyrating electrons in linear (small signal) regime our case includes nonlinear processes when initial pulse amplitude is rather high and nontrivial pulse transformation can occur even for initially rectilinear electron beam. We should emphasize that similar to gyrotrons [5,6], transverse wave propagation considered in this Letter is the most feasible for experimental testing of the discussed nonlinear effects due to reduction of sensitivity to the spread of electron beam parameters.…”