Compression of frequency modulated pulses using a high order helically corrugated waveguide
Lead Research Organisation:
University of Strathclyde
Department Name: Physics
Abstract
A waveguide can be used to compress frequency-modulated pulses. Consider compression of a quasi-monochromatic pulse with frequency monatonically varying in time from one frequency (w1) to another frequency (w2). This pulse is then propagated down a dispersive medium in which the group velocity of the wave is a function of frequency only. If the wave group velocity in the dispersive medium is an increasing function of frequency, vgr(w2) > vgr(w1), then the tail of the pulse will overtake its leading edge, resulting in pulse shortening and a corresponding growth in the amplitude if the losses are sufficiently low. Compression experiments were first carried out using a smooth metal waveguide and were found attractive because of its capability of handling high power. However a serious drawback of a smooth waveguide as a powerful compressor is its operation very close to the cut-off. In optimum cases, the frequency at the beginning of an input pulse should be only 0.5-1% above the cut-off frequency. If one uses such a compressor at the output of a powerful amplifier, then the low-frequency part of the amplification band will be reflected back to the amplifier resulting in its possible parasitic self-oscillation (RF isolation using unidirectional elements is impossible at very high power). We propose to design and construct a waveguide with a helical corrugation of its inner surface as the frequency dispersive medium, to couple a pair of circularly polarized partial modes of the smooth waveguide having significantly different group velocities. Pulse compression using a 3-fold helically corrugated waveguide using an optimised frequency swept pulse generated by a state-of-the-art programmable Arbitary waveform generator and microwave sweeper will be studied, with the predictions of theory compared to experiment. We propose also to design, build and investigate the use of a larger diameter 5-fold helically corrugated waveguide as the frequency dispersive medium to compress MW frequency swept pulses generated by a gyrotron travelling wave amplifier. The advantage of using a 5-fold helical structure as compared to a 3-fold helical waveguide is that it requires the use of a higher order mode and hence the diameter of the compressor is increased which means that higher peak power radiation can be propagated down the compressor before the electric fields becomes excessively large resulting in RF breakdown. A favourable wave dispersion can be synthesized for the higher-order modes (near cut-off TE2,2 mode which couples to a counter-rotating TE3,1 mode) excited in a 5-fold helical compressor resulting in an increase of the helical waveguide diameter by a factor of 1.5-2 (without significant overlapping of the coupling bands) and correspondingly in an enhancement of its RF breakdown strength. Unique high power (multi-MW), short pulse (~1ns) radiation required in a number of applications will be generated.
Publications
Alan Phelps (Author)
(2008)
Free-electron maser based on two-dimensional distributed feedback
Bratman V
(2007)
Method for achievement of a multigigawatt peak power by compressing microwave pulses of a relativistic backward-wave oscillator in a helical waveguide
in Radiophysics and Quantum Electronics
Bratman V
(2007)
High-efficiency wideband gyro-TWTs and gyro-BWOs with helically corrugated waveguides
in Radiophysics and Quantum Electronics
Bratman V
(2010)
Generation of 3 GW microwave pulses in X-band from a combination of a relativistic backward-wave oscillator and a helical-waveguide compressor
in Physics of Plasmas
Constable D
(2009)
A co-harmonic gyro-oscillator with a novel interaction cavity
Constable D
(2012)
Numerical simulations of a co-harmonic gyrotron
in Journal of Physics D: Applied Physics
Constable DA
(2010)
A novel cylindrical TE(2,1) mode converter.
in The Review of scientific instruments
Cross A
(2007)
Helically corrugated waveguide gyrotron traveling wave amplifier using a thermionic cathode electron gun
in Applied Physics Letters
Cross A
(2007)
Generation and application of pseudospark-sourced electron beams
in Journal of Physics D: Applied Physics
Donaldson C
(2010)
A cusp electron gun for millimeter wave gyrodevices
in Applied Physics Letters
Description | Experiments performed at 5.8kW, demonstrated that X-band microwave pulses of 80-ns duration with a 5% frequency sweep (9.6GHz to 9.0GHz) can be compressed into 145kW, 1.5-ns pulses having 25 times higher peak power. |
Exploitation Route | In the experiments the input radiation was launched from an X-band (8.2-12.5 GHz) waveguide and proceeded through a rectangular-to-circular adapter then through an elliptical cross-sectional polarizer which converted the linearly polarized TE1,1 mode to a circularly polarized mode. This mode was adiabatically transformed into the operating mode [Fig. 1(b)] via a TE1,1 to TE3,1 mode converter. At the output, the opposite transition occurred. The compression experiments were carried out by programming an Agilent N6030A arbitrary waveform generator (AWG) to generate an optimized input signal, in the form of I and Q data, to the mixers of an Agilent E8267D vector signal generator (VSG) [Fig 1(d)]. An PTC6321 Traveling Wave Tube Amplifier (9.6GHz to 9.0GHz) supplied by TMD Ltd was used to amplify the radiation generated by the solid-state oscillators and mixers, up to a power of 5.8 kW. Experiments performed at 5.8kW, demonstrated that X-band microwave pulses of 80-ns duration with a 5% frequency sweep (9.6GHz to 9.0GHz) can be compressed into 145kW, 1.5-ns pulses having 25 times higher peak power. Approximately 50% of the input energy was compressed to the main body of the compressed pulse. Good agreement was achieved between theory and experiment providing improved understanding of this method of compressing frequency swept microwave radiation which was applied to the generation of high-power (GW) microwave pulses. An X-band relativistic BWO, designed to drive a similar compressor, was then built and tested at the IAP, with the accelerating potential provided by a SINUS-6 high-current accelerator. The experimental RBWO operated close to predicted powers (600 - 800MW) with its oscillation frequency varied from 10 - 9.6GHz via the falling edge of the voltage pulse. It was demonstrated that the ~15ns duration frequency-swept part of the RBWO pulse was effectively compressed resulting in about a 4.5-fold peak power increase with a maximum power of 3.2 GW generated |
Sectors | Aerospace Defence and Marine Manufacturing including Industrial Biotechology |
Description | The findings have been used to generate extreme high peak powers of coherent radiation in X-band to be presented by Prof Adrian cross at the Pulsed Power Conference 2017 Brighton, UK from the 18th to the 22nd June 2017. |
First Year Of Impact | 2010 |
Sector | Aerospace, Defence and Marine |
Impact Types | Societal Economic |
Description | DSTL |
Amount | £100,000 (GBP) |
Funding ID | Dstl |
Organisation | Defence Science & Technology Laboratory (DSTL) |
Sector | Public |
Country | United Kingdom |
Start | 03/2011 |
End | 03/2012 |
Description | Dstl |
Amount | £192,000 (GBP) |
Funding ID | Dstl |
Organisation | Defence Science & Technology Laboratory (DSTL) |
Sector | Public |
Country | United Kingdom |
Start | 05/2004 |
End | 05/2007 |
Description | Dstl |
Organisation | Defence Science & Technology Laboratory (DSTL) |
Country | United Kingdom |
Sector | Public |
PI Contribution | Dstl supplied 1/2 the funds to EPSRc via the MoD joing grant scheme |
Collaborator Contribution | David Gamble Dstl supplied his time to act as the the project manager for this compression project |
Impact | Dstl have the capability to generat multi-GWs of power at X-band if required |
Start Year | 2007 |