Published Online: 14 December 2004 ... Whispering Gallery Pulse Compressor ... compressor, we propose to use a coupler based on a wave tunneling through ...
Whispering Gallery Pulse Compressor J.Hirshfield1, S.V.Kuzikov2, M.I.Petelin2, V.G.Pavelyev3 1
Omega-P, New Haven, 2Institute of Applied Physics, 3University of Nizhny Novgorod, Russia, 2Institute of Applied Physics, Nizhny Novgorod, Russia, 3University of Nizhny Novgorod, Russia Abstract. A barrel-like cavity resonant at a whispering gallery mode is known [1] as capable to provide a SLED-like [2] rf pulse compression. To enhance the power handling capacity of the compressor, we propose to use a coupler based on a wave tunneling through a continuous slot [3]. A modeling low power 11.4 GHz experiment proved to be consistent with theory. A preliminary technical design for an evacuated high-power compressor has also been developed. According to a theory, a twin-cavity version of the device can efficiently compress microwave pulses produced with sources of limited bandwidth, in particular frequency-chirped pulses.
INTRODUCTION Linear electron accelerators are usually fed with relatively long (narrow band) microwave pulses. So, relevant passive pulse compressors can be relatively compact only if based on use of resonant cavities. Such a cavity • should not reflect the power to the microwave source, and • should be over-coupled to input-output waveguides, so that the Ohmic loss could be neglected. In this case the wave transmission coefficient is of the standard form ω − ω s∗ (1) T= ω − ωs where (2) ω s = ω s′ 1 + i / 2Q s
(
)
is the loaded cavity complex eigen-frequency. Among numerous versions of such compressors, the most famous is SLED [2] operating at the S-band. However, if the classical SLED were scaled directly from the S-band to a higher frequency, its power handling capacity would be drastically reduced, because all dimensions of the compressor are commensurable with the wavelength. A promising method for overcoming this limitation at the X-band was proposed over 10 years ago by V. Balakin and I. Syrachev [1]. Their compressor operates at a whispering gallery mode of an open barrel-like cavity: such a cavity is rather selective, and the whispering gallery mode of E-type (TM-type) has a very high Ohmic Q. In their design, the cavity mode is coupled to the input-output waveguide by a perforation which, obviously, is the weakest point relative to the rf breakdown.
CP737, Advanced Accelerator Concepts: Eleventh Workshop, edited by Vitaly Yakimenko © 2004 American Institute of Physics 0-7354-0220-5/04/$22.00
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PROPOSED UPGRADE To mitigate the latter limitation, it seems expedient to replace the perforation with a continuous slot [3] (Fig. 1), so that the aperture coupling of the cavity with the inputoutput waveguide would be due to a wave tunneling effect: • within the axis-symmetrical barrel-like cavity, the operating E-mode is weakly coupled to the H-mode of the same azimuthal index; • this H-mode, in its turn, is coupled, through a below-cut-off neck, to the fundamental H-mode of the input-output waveguide.
FIGURE 1. Tunnel-feed whispering-gallery pulse compressor.
The system parameters should be adjusted so that the operating mode would not be radiated in the axial direction opposite to the waveguide. However, the broad opening in this direction is necessary for free radiation and, so, suppression of parasitic modes. The system as a whole (Fig. 1) is devoid of axial symmetry. To simplify calculations, it is convenient to use a relation 2 (3) ω+ − ω− = Q s Qm between the loaded Q of the compressor cavity (Fig. 1), Qs , and parameters of a subsidiary axis-symmetrical system produced of that shown in Fig. 1 by circling of the input-output waveguide. In the relation (3), Qm = 2πm , m is the azimuthal index of the operating mode, ω + and ω − are normal frequencies of counter-phased and synphased coupled oscillations of the axis-symmetrical conservative system composed of the main and the subsidiary axis-symmetrical “feed” cavity; it is assumed that Qs >> Qm . Once the beat frequency ω + − ω − of the subsidiary coupled cavities is calculated, the equation (3) gives the complex eigen-frequency (2) of the pulse compressing cavity (Fig. 1) and the standard wave transmission coefficient (1) can be used to describe the pulse compression performance.
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EXPERIMENT The above theory was used to design an 11.4 GHz pulse compressor (Fig. 2)
FIGURE 2. X-band pulse compressor with input and output waveguides (at the side) and adjustable plunger (at the top).
operating at E 20.1.1 mode, the intermediate coupling mode being H 20.1 . The compressor was fed with a low power SLED-like modulated pulse (Fig. 3): of a
a b FIGURE 3. SLED-like performance of compressor (compression ratio s=4):envelopes of input (thin lines) and output (thick lines) pulses; a) no plunger, efficiency 55%, b) with plunger, efficiency 80%.
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rectangular envelope and a 180 0 phase reversal inside the pulse. The curves (Fig. 3) proved to be in a reasonable agreement with the theory and similar to those typical for the SLED: at the compression ratio near to 4, the efficiency varied between 55% and 80%, depending on the compressor configuration. Some peculiarities of the output pulse (a low pre-pulse and small oscillations of the compressed pulse) can be attributed to presence of near-resonant modes additional to the main one. After further optimization and experiments at the low power level, the compressor is planned to be converted into an evacuated version and tested at a high microwave power: at first with a magnicon in the NRL and then with a klystron in the SLAC.
COMPRESSION OF CHIRPED PULSE WITH BI-RESONANT SYSTEM The frequency band of high power microwave amplifiers at high frequencies being limited, the rf power envelope cannot be strictly rectangular and the quick phase flip is not available. However the same efficiency as of the classical SLED can be realized if a smooth-enveloped gradual-frequency-modulated pulse is compressed with a multiresonant non-reflection system, for instance, if a Gaussian-enveloped chirped pulse is compressed in a twin-cavity whispering gallery pulse compressor (Figs. 4-6).
FIGURE 4. Twin-cavity whispering gallery pulse compressor.
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5.5
P
5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5
1/s
1.0 0.5 0.0 -0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
t0
FIGURE 5. Transmission of Gaussian-enveloped chirped pulse through bi-resonant compressor: powers P of the input (dashed bell) and output (solid curve) pulses as functions of a dimensionless time t 0
=t T
(dashed rectangle is convoluted to dashed bell).
η, % 90
2
3
1
P
g
6
A
80 5 70 4 60 3
50
2
40 4
1
5
3 6
2 7
8
s FIGURE 6. Efficiency η (solid curves) and power gain
s
Pg
(dashed curves) vs. compression ratio
for : 0
1 – single-cavity SLED fed with 180 - phase-step-modulated pulse, 2 – single-cavity compressor fed with chirped pulse, 3 - bi-resonant compressor fed with chirped pulse; point A corresponds to Fig. 5.
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CONCLUSION Thus •
the whispering gallery pulse compressor is a relatively compact and robust device capable to operate at high carrier frequencies and powers, • a SLED-like performance of a whispering gallery pulse compressor has been demonstrated at X-band, • a twin-cavity version is capable to compress chirped pulses.Such compressors might be used for testing components of novel electron-positron colliders and for feeding relatively small medical electron accelerators.
ACKNOWLEDGMENTS This work was supported by DoE, Office of High Energy Physics. Authors are grateful to Oleg Nezhevenko and Sami Tantawi for helpful discussions and advises.
REFERENCES 1. Farkas, Z.D. et al, SLED: “A Method of Doubling SLAC's Energy, Proc. 9th Conf. On High Energy Accelerator”, SLAC-PUB-1453, SLAC, Stanford, CA, USA, May 2-7, 1974, p. 576, SLAC-PUB1453. 2. Balakin V.E., Syrachev I.V. VLEPP RF Power Multiplier. Proc. III-rd Int. Workshop on Next Generation Linear Collider, Branch INP, Protvino, Russia, 1991. P. 145-156. 3. Petelin, M. I., Hirshfield, J. L., Kuzikov, S.V., Vikharev, A. L., “High power microwave pulse compressors: passive, active, and combined”, SPIE's 14th Annual Symposium on Aerosense, Orlando, April 2000, Intense Microwave Pulses, pp. 224-231.
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