Experimental investigations for micro-fabricated folded-waveguide traveling-wave tube oscillators

Abstract

Vacuum tube (folded-waveguide traveling-wave tube oscillator) fabricated by LIGA (deep X-ray lithography: Lithographic, Galvanoformung, Abformung; German acronym) process was successfully developed for the first time.

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Experimental Investigations for Micro-fabricated Folded-Waveguide
Traveling-Wave Tube Oscillators
S. T. Han, J. K. So, K. H. Jang, Y. M. Shin, S. G. Jeon (a), J. H. Kim, S. S. Chang(b), N. M.
Ryskin(c), and G. S. Park (a)
a School of Physics, Seoul National University, Seoul, 151-742, Korea
Te1:+82-2-880-7749, Fax:+82-2-882-9374, E-mail: gunsik@,ulaza.snu.ac.kr
b Pohang Accelerator Laboratory, POSTECH, Pohang, 790-784, Korea
c Saratov State University, Saratov, Russia
ABSTRACT
Vacuum tube (folded-waveguide traveling-wave tube oscillator) fabricated by LIGA (Deep X-
Ray Lithography: Lithographie, Galvanoformung, Abformung; German acronym) process was
successfully developed for the first time.
INTRODUCTION
Recent developments in micro-fabrication technologies draw a great attention in the vacuum
electronic community to demonstrate high frequency devices such as millimeter-wave and
submillimeter-wave radiation sources [l]. Precise micro-fabrication enables the construction of
miniature high-frequency vacuum tube circuits with the dimensions less than few hundreds
micrometer. Integration of field emitter cathode with this kind of circuit would be challenging
goal. Therefore, many researches on miniature high-frequency and high-power vacuum devices
using micro-fabrication technologies have been conducted. Among them, a folded waveguide
traveling-wave tube (TWT) is one of the strong candidates because it is compatible with
lithographic technologies and its all-metal structure enables high power-handling at higher
frequencies [2-31.
EXPERIMENTAL RESULTS
For a proof-of-principle experiment, Ka-band folded waveguide oscillators were studied.
Relatively low values of the electron beam voltage (12 kV) and beam current (150 mA) were
used in this experiment. A pulsed electron beam with 10 Hz repetition rate and 10 us duration
were used. In the folded-waveguide backward wave oscillator (BWO) experiment, about 17-watt
output power at 35GHz was obtained with 6% linear tunability, choosing the second backward
space harmonic. The efficiency enhancement of folded-waveguide BWO was observed adopting
an energy recovery scheme using external feedback. The experimental observation agreed well
with a particle-in-cell (MAGIC3D) simulation. Measured efficiency improved in accordance
with the strength of external feedback at the optimal phase, which is attributed universally to the
increase of the normalized current defined by the ratio of the beam current to the start-oscillation
current, regardless of each assigned feedback power level. Also, the external feedback scheme
was employed in a folded-waveguide TWT amplifier (TWTA). To study the delayed feedback
oscillator using the fundamental forward space harmonic of a folded waveguide, a Ka-band
amplifier with a linear gain of 25 dB, and bandwidth of 10% was developed. Threshold feedback
strength for the onset of oscillation and that of self-modulation were measured to be -30 dB and -
16 dB, respectively. The optimum value of feedback strength for the single frequency oscillation
at 32.5GHz was about -18dB with a net electronic efficiency of 6%, which shows reasonable
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agreement with the prediction of one-dimensional non-stationary simulation. These results imply
that a micro-fabricated high-frequency folded-waveguide oscillator with high efficiency can be
realized using a re-circulating loop with the optimum level of feedback.
The exploratory experiment of LIGA-fabricated folded-waveguide TWT oscillator was carried
at Ka-band. The negatively patterned polymethylmethacrylate(PMMA)-mold on the copper
substrate for the folded waveguide circuit was fabricated by a lithographic process using
synchrotron X-ray source and the final metal structure was filled out by electro-deposition [FIG.
11. The LIGA-fabricated Ka-band amplifier shows a linear gain of 15 dB, and bandwidth of
1.7% with 12.4-kV, 47-mA electron beam. The threshold for the onset of oscillation and the
optimum value of feedback strength for the single frequency oscillation at 35GHz with a net
electronic efficiency of 3.5%, were measured to be about -1 1 dB and about -8dB, respectively
[FIG. 21.
CONCLUSION
This kind of new approach using modem micro-fabrication technology might open a new era for
vacuum tube at millimeter and sub-millimeter wave band. Its mass production capability, micro-
sized vacuum device and ultra-high frequency applications are the hints of its effect when
realized.
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ACKNOWLEDGEMENT
This work was supported by the Ministry of Science and Technology (MOST) of the Republic of
Korea under the National Research Laboratory Program. Experiments at Pohang Light Source
(PLS) were supported in part MOST and POSCO.
REFERENCES
[I] J. H. Booske, Proceedings of 3rdZnt. Vacuum Electronics Conference, 11 (2002).
[2] S. T. Han, J. I. Kim, and G. S. Park, Microwave and Optical Tech. Lett., vol. 32 (2), 161
(2003).
[3] S. T. Han, K. H. Jang, J. K. So, J. I. Kim, Y. M. Shin, N. M. Ryskin, S. S. Chang, and G. S.
Park, IEEE Trans. Plasma Sci., vol. 32 (l), 60 (2004).
Feedback level (de)
FIG. 1 LIGA-fabricated folded waveguide CKT
FIG. 2 LIGA-fabricate oscillator performance
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