Summary Report on Working Group 3:
Millimeter-Wave Sources
J. L. Hirshfield
Omega-P, Inc., 199 Whitney Ave., New Haven, CT06511 USA
Beam Physics Laboratory, Yale University, 272 Whitney Ave., New Haven, CT 06511
Abstract. A summary is presented of talks and deliberations carried out at AAC2002 during
sessions of Working Group 3—Millimeter Wave Sources.
INTRODUCTION
The Working Group on millimeter-wave (mm-wave) sources at AAC2002 was
guided by the following "vision statement" that was circulated prior to the meeting.
The motivation for development of mm-wave rf sources for a future normal
conducting collider arises from the anticipated scaling (in rough proportion to
frequency) of the dark current limit for the maximum accelerating gradient that can
be sustained by a copper accelerating structure. Thus NLC at 11.4 GHz is expected
to operate with a gradient roughly four times that of SLC, while another factor-ofthree or higher might be possible by operating at 30 GHz or above. The maximum
gradient will be further constrained by rf breakdown and surface fatigue due to
pulsed heating. In any case, experimental tests of accelerating gradient under a
variety of conditions must be carried out before the absolute limits will be known.
In this working group, new millimeter-wave rf sources will be described and
examined for their potential long-term suitability as rf drivers for a future collider;
and for their short-term suitability in high-power testing of rf components,
accelerator structures, and rf pulse compressors. Parameters to be considered for a
candidate mm-wave rf amplifier include peak output power, pulse width, average
power, gain, frequency stability, efficiency, bandwidth, and ability to operate into a
highly-reflecting resonant load without excitation of spurious modes. Few suitable
high-power mm-wave rf components exist, so their development must go hand-inhand with that of the sources, to allow evaluation of the sources and of their utility
in driving accelerator structures. Likewise, rf pulse compressors must also be
developed to generate the high-peak-power short rf pulses needed to excite the
accelerator structures. Efforts to develop mm-wave sources should be carried out
in light of recent experimental results for rf breakdown and accelerating gradient
limits.
Talks were presented and discussions held on a variety of topics that generally
followed this vision statement. Brief summaries follow.
CP647, Advanced Accelerator Concepts: Tenth Workshop, edited by C. E. Clayton and P. Muggli
© 2002 American Institute of Physics 0-7354-0102-0/02/$19.00
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RF BREAKDOWN AND PULSED SURFACE HEATING
Two talks on rf breakdown were presented, the first by S. G. Tantawi (SLAC) entitled
"RF breakdown in high vacuum x-band waveguides," and the second by W. Wuensch
(CERN) entitled "Investigations of rf breakdown and pulsed surface heating in 30 GHz
CLIC accelerating structures." Written versions of these talks are provided elsewhere in
this volume. The contents of these presentations, and the discussions stimulated by the
presentations, showed that considerable experimental and theoretical work is needed
before the complex phenomena of rf breakdown and pulsed surface heating can be
understood—at least insofar as these phenomena limit the achievable accelerator gradient
and structure lifetime for parameters appropriate for a future high-gradient linear collider.
There was general agreement that proposed experimental tests of breakdown and surface
heating at mm-wavelengths were needed for advancing this basic understanding, whether
for determining the limits to achievable accelerating gradient at mm-wavelengths, or for
adding to the data base for validating models that guide NLC accelerator structure design
at X-band.
MILLIMETER-WAVE RF SOURCES
The only mm-wave sources with parameters suitable for accelerator applications that
were discussed at AAC2002 were gyroklystrons and magnicons. A plenary session
review talk that focussed mainly on these two categories of devices was delivered by the
author; its text is found elsewhere in this volume. Review talks were presented by V. P.
Yakovlev (Omega-P) and W. Lawson (U.Md.) on magnicons and gyroklystrons,
respectively; texts of these talks are also found elsewhere in this volume. A review talk
was presented by S. H. Gold (NRL) describing use of the Omega-P/NRL 11.424 GHz, 60
MW magnicon installed at NRL and operated as a user's facility for accelerator physics
applications. To date, two types of structures have been so tested: active rf pulse
compressors employing plasma switches (A. L. Vikharev et al), and dielectric-loaded
smooth-bore waveguides as accelerator structures (W. Gai et al). The text of Gold's talk
is also found elsewhere in this volume. Contributed papers were presented by G.
Nusinovich et al (U.Md.) and L. Ives et al (Calabazas Creek Research) on various aspects
of gyroklystron development: the former on the inverted MIG concept for a relativistic
gyroklystron and on the clustered-cavity gyroklystron, the latter on design and
construction of a 10 MW gyroklystron at 91 GHz. Texts of these papers are found
herein. W. Carlsten (LANL) described developments of W-band sources, but mainly for
applications other than accelerators.
MILLIMETER-WAVE COMPONENTS
A program for development of high-power mm-wave components at 34 GHz for
accelerator applications has recently been initiated by G. G. Denisov et al (Omega-P,
IAP). A number of designs and experimental low-power cold tests of some components
have been completed, and fabrication and tests at high power are planned for the near
future, using the Omega-P 34 GHz magnicon. A talk summarizing this work was given
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on Denisov's behalf by A. L. Vikharev (Omega-P, IAP). The text of this talk is included
elsewhere in this volume. Design concepts and experimental moderate-power tests were
reported by J. Haimson on a 4x peak power booster for flat-top rf testing of high gradient
linac structures at 17 GHz; the text of this talk is included elsewhere in this volume.
MILLIMETER-WAVE RF PULSE COMPRESSORS
Preliminary results were presented by A. L. Vikharev (Omega-P, IAP) on designs for
34-GHz quasi-optical passive and active rf pulse compressors. Low-power cold tests of
a passive quasi-optical version were also described. The text of his talk is included
elsewhere in this volume.
MILLIMETER-WAVE ACCELERATOR STRUCTURES
A concept for a phased-array quasi-optically fed high-gradient mm-wave accelerator
structure was presented by M. Petelin (Omega-P, IAP). The text of his talk is included
elsewhere in this volume.
CONCLUSIONS
Participants in this Working Group endorsed the need for a vigorous R&D program
on millimeter-wave rf sources and components as necessary to extend the data base on rf
breakdown, to determine limits to accelerator structure lifetime from surface fatigue
caused by pulsed heating, and to possibly establish a basis for an upgrade scenario for
NLC (from 0.5 to 2 TeV) or for a >4 TeV future collider. Such an R&D program must
include coordinated activity on mm-wave amplifiers, components, and pulse compressors
(or power enhancers). Gyroklystrons and magnicons are the most mature mm-wave
amplifiers that might be suitable for accelerator applications, but tests at mm-wavelengths
are needed to confirm their suitability. An unanswered question is whether successful
development of mm-wave accelerator technology can result in high-gradient accelerator
structures capable of handling beams with the high enough luminosity; the optimists in
attendance opined that only after the underlying mm-wave technology is developed can
this issue be subjected to experimental test. But healthy skepticism was also expressed.
ACKNOWLEDGMENTS
The author is grateful to the many individuals, too numerous to mention, who
provided input and valuable food-for-thought in the organization and meetings of the
Working Group on RF Sources at AAC2002. Successes are due to the efforts of the
participants, but shortcomings in the organization and deliberations are the responsibility
of the author. Support for this work was provided by the US Department of Energy,
Division of High Energy Physics.
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