Test facilities for future linear colliders Page: 4 of 7
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factors to be considered. On the cost side the achievable
gradients need to be raised from the 5 MeV/m typical of
existing storage ring cavities to 20 or 25 MeV/m. The
cost per unit length of the accelerating cavity plus cryostat
needs to be lowered a factor of five to bring the price to
about 50k$/m to give a competitive cost per MeV figure.
Another source of concern is the relative delicacy of the
superconducting state. How reliable would such a system
be in the face of the realities of vacuum and cryo system
failures? Will it be possible to recover from such a failure
by in situ processing? At the long rf wavelength, which
is a strength of the SC approach, electrons present in the
accelerator vacuum can be more easily captured from rest
to make parasitic beams. These parasite beams could per-
haps sap energy from the main beam and perhaps cause un-
wanted cryo losses and beam background in addition to in-
stabilities of the wanted beam. Because of the very high Q
of the cavities, small changes in dimension can make large
changes in amplitude and phase of the accelerating wave.
The pressure caused by the growing stored energy during
the pulse can make such changes, the so-called Lorentz de-
tuning. Can an economical cavity stiffening with active
control scheme be found? Finally, the large beam power
which is on the one hand a virtue implies the need for the
most powerful positron source of all the approaches and
gives the challenge of safe disposal of the high beam power
in addition. Progress in all of these areas has been made
and a plan to demonstrate solutions to many of them put
A 500 MeV TESLA Test Facility Linac (TTFL) is now
under construction at DESY. It will consist of four cry-
omodules of a type that could be used in a linear collider.
Each module contains eight 1 m, nine-cell cavity units, plus
a focusing doublet assembly with beam monitors. Each
cavity subunit has its own couplers. Two cryomodules are
driven by one klystron/modulator set. An injector sec-
tion brings the initial beam up to 15 MeV nominal before
introduction into the TTFL proper. A beam analysis sta-
tion will be installed both after the injector and at the
high energy end of the TTFL. The linac will be installed
in Halle 3 at DESY adjacent to the recently completed
cavity chemical processing area, with cleanrooms, verti-
cal test cryostats, and rf power supply for cavity testing
and high power pulse processing (HPP). The first cavities
and cryostats have for the first cryomodule have arrived
at DESY and initial cavity tests have begun. In a proto-
type processing and test setup at Cornell, several multi-
cell cavities have been chemically processed and subjected
to HPP. Initial tests with HPP at DESY have also been
started. HPP has been quite effective in raising the achiev-
able gradient. After exposure to air, HPP was successful in
recovering the gradient. How this will apply in a linac en-
vironment must await the completion and operation of the
TTFL now expected in the 1997-1998 time frame. Expe-
rience with the facility will yield information about many
of the critical issues cited above; for example, dark cur-
rent, Lorentz detuning, cost, robustness against vacuum or
cryogenic failures, and so on.
NLCTA rf System Parameters.
Parameter Design Upgrade
energy gain 540 MeV 1080 MeV
Linac active length 10.8 m 10.8 m
gradient 50 MV/m 85 MV/m
Injection energy 90 MeV 90 MeV
rf frequency 11.424 GHz 11.424 GHz
Number of klystrons 3 6
Klystron peakpower 50 MW 75 MW
Klystron pulselength 1.5 ps 1.5 ps
rf pulse compression
power gain 4.0 4.0
Phase advance/cell 2 -r/3 2 -r/3
VI. The NLC: The Next Linear Collider Test
The NLC uses positron and electron sources similar to
those in the SLC. Much of the early acceleration is done
with S-Band as in the SLAC linac. The Damping Ring is
similar to the ATF Damping Ring described earlier. The
FFTB is addressing the Final Focus system issues. The
acceleration in the NLC is accomplished with an 11.4 GHz
RF system (X-Band) designed to accelerate the beam with
gradients in the range of 50-85 MV/m. The purpose of the
NLC Test Accelerator is to bring together all the separate
developments on X-band in a model of a section of the NLC
The NLCTA is a high-gradient X-band linac consisting
of six 1.8 m-long accelerator sections. These sections are
fed by three 50 MW klystrons, which make use of SLED-II
pulse compression to increase the peak power by a factor of
four. This yields an acceleration gradient of 50 MV/m, so
that the total unloaded energy gain of the beam in the X-
band linac is 540 MeV. The NLCTA parameters are listed
in Table 1. The right-hand column of Table 1 lists the pa-
rameters for an upgrade of the X-band linac to 100 MV/m
by the use of six 100 MW klystrons.
The NLCTA injector will consist of a 150 kV gridded
thermionic cathode gun, an X-band prebuncher, a capture
section with solenoid focusing, and a rectangular chicane
magnetic bunch compressor.
The high-gradient accelerator will be fed with rf power
through overmoded circular waveguides which penetrate
the shielding blocks above the accelerator. Four 50-MW
klystrons will be positioned along the accelerator, outside
the shielded enclosure. Each klystron is powered by an
independent modulator, allowing the flexibility needed for
multibunch energy control and adequate power for an up-
grade to a 85-MV/m accelerating gradient with six 75-MW
klystrons, as indicated in Table 1. Each klystron feeds
a SLED-II pulse compressor. The pairs of delay lines of
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Ruth, R.D. Test facilities for future linear colliders, article, December 1, 1995; Menlo Park, California. (digital.library.unt.edu/ark:/67531/metadc619423/m1/4/: accessed October 23, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.