A New Beamline for High Energy-Density Test Beams at SLAC
P. Krejcik, February 2003.
A multipurpose beamline for test beams with all the properties of the current FFTB,
and more, could be built in the straight section of the instrument section of the SLC south
arc, shown in figure 1. The proposal is motivated by the growing need to provide a
facility for test beams once the existing FFTB is replaced by the LCLS. A test beam in
the south arc instrument section (SARC-IS) is capable of handling low-emittance beams
from the damping rings, energies up to 50 GeV and make use of the short bunches from
the linac bunch compressor chicane (LBCC) built for the SPPS. A test beamline in the
SARC-IS appears to have several advantages over our earlier study into an “FFTB-2”
built adjacent to the LCLS, both in terms of cost and operability.
Potential users for this beamline include experiments with plasma wakefield
acceleration, laboratory astrophysics experiments and numerous, small test beam
experiments and a continuation of the present SPPS program started in the FFTB. This
beamline would allow these users access to the unique properties of high-energy test
beams at SLAC, in particular with respect to the high energy density which comes from
low emittance and short bunch length.
SARC
Instrument Section
Figure 1. Location of the Instrument Section close to the beginning of the SARC
Figure 2. Beamline Twiss parameters starting at the SLC split in the BSY, through achromats
S01 and S02 to the instrument section S03.
General Description
The SARC-IS is an approximately 70 m long beamline inserted between achromats 2
and 3 of the SARC and is approximately 100 m downstream of the BSY. The length of
the beam line from the first BSY bend magnet to the end of the SARC-IS is shown in
figure 2. The SARC-IS could be operated as a test beam area by powering only a short
section of the SARC comprising 42 arc magnets. Rather than bend further in the 3rd
achromat the beam would be allowed to go straight ahead at the exit of the IS and be
stopped at a suitable beam dump.
Operation of the SARC-IS as a test beam area would conflict with any operation of
the SLC arcs in collider mode. However, the changes to be made to the SARC would be
relatively minor and easily reversible.
In its present configuration the SARC beam is split off from the main linac axis by a
DC bend magnet, 50B1, shown in figure 3. If it is desired to provide interleaved
operation of straight-ahead beams, such as the LCLS, together with test beams to the
SARC-IS it would be necessary to replace 50B1 with a pulsed magnet. In order to handle
50 GeV beams this would be in the form of a pulsed magnet and DC septum magnet
combination. A suitable pulsed magnet is already installed in the arc beamline where it
was used a single beam dumper with 120 Hz capability at 50 GeV for SLC operation, and
is shown in figure 3. This magnet would have to be moved upstream of the 50B1 magnet
location.
Figure 3. Start of the SARC from the SLC split at the 50B1 bend.
Figure 4. Instrument Section components in the straight section between achromats 2 and 3.
Figure 5 Dispersion function (left) and R56 term (right) up to the end of the SARC-IS.
Optics considerations
The SARC beamline was designed to transport low emittance beams at 50 GeV
without appreciable emittance growth. This is achieved by maintaining a large bending
radius in the arcs to avoid contributions from incoherent synchrotron radiation (ISR)
emittance growth. Operation with very short bunches also requires large bend radius to
minimize the contribution from coherent synchrotron radiation (CSR) emittance growth.
A consequence of generating very short bunches in the linac is the large, wakefieldinduced energy spread of the bunches. It is important that the energy aperture of the
beamline is large enough to accommodate the increase in energy spread above that of the
original design. The horizontal dispersion function through the arcs, shown in figure 4,
predicts that the linear beam size will increase from 200 m to approximately 500 m
when the energy spread is increased to 1.5%. This is well within the beamline aperture,
but particle tracking should still be done to confirm that the nonlinear terms in the arc
combined-function bend magnet does not adversely affect the tails in the bunch
distribution1.
The large wakefield-induced coherent energy spread in the bunch is an important part
of the final bunch compression process for achieving high energy density test beams. In
the SPPS a third bunch compression takes place in the FFTB where an R56 = + 1.2 mm is
generated in the final dog-leg bend in the beamline. As a result the chirped bunch
entering the FFTB is rotated in longitudinal phase space to produce the final 30
femtosecond rms bunch length at the experiments. The sign of the wakefield-induced
energy chirp is fixed so the final compression only works because the dog-leg bend has a
positive R56 term. This is not always the case because a chicane bunch compressor, for
example, has a negative R56 term and would produce the opposite effect in the FFTB.
Fortuitously, the arc bend also generates a positive R56 term and therefore will also act as
a third bunch compression stage for a SARC-IS test beam. The R56 term along the SARC
1
Thanks to Paul Emma for noting this.
is shown in figure 4 and reaches a value of +15 mm at the instrument section. This is
close to the 1.2 mm value in the FFTB so the dynamics of tuning for minimum bunch
length in the SARC-IS will be similar to the present SPPS.
The instrument section is long enough to house sufficient optics for a final focus
system to produce micron sized spots. If nanometer beams are desired then a scheme
similar to the NLC final focus optics needs to be considered. Pantaleo Raimondi devised
such a beamline layout for the FFTB2 proposal which would be short enough to also fit
in the SARC-IS tunnel. However, the chromatic correction of the beam requires control
of the dispersion function and this would have to be matched to the existing arc optics.
This aspect should be discussed in more detail with potential users before a detailed
optical design is made.
Beam Containment
A test beam in the SARC-IS can be safely dumped on a suitable stopper at the end of
the instrument section. The dump can be located even further down the tunnel since the
line of sight from the straight section extends more than 10 m before it intercepts the
tunnel wall. A permanent magnet bend could also deflect the dumped beam to the tunnel
floor or further into the arc tunnel wall so that any hot zone would be more remote from
experimental areas during controlled access. The design of the SLC final focus tune-up
dump TD23 installed at the end of the arc would be suitable for up to 10 Hz operation at
the end of the Instrument Section.
The arc magnets downstream of the instrument section would not be powered so that
no beam could be transported around the arc to the final focus area. The final focus area
could remain in access, if desired, during test beam operation.
The underground SARC tunnel has much better shielding than the FFTB which
would greatly alleviate concerns over maximum allowable beam power and beam loss
rate during operation. This enhances the flexibility of experiments in the test beam line.
Some experiments that could not be carried out in the FFTB because of beam losses due
to intentionally created energy tails (e.g. from plasma wakefield acceleration) or from
targets inserted in the beam, should now be possible in the SARC-IS.
Test beam experiments require frequent controlled access to the tunnel which should
not interrupt other beam programs. A PPS barrier separates the SARC-IS from the BSY
and it is possible to enter the SARC from the direction of the SFF and walk to the SARCIS while there is beam in the BSY. This is possible because there is no line of sight
between the SARC-IS and the BSY and no beam can enter the instrument section when
the arc main and the 50B1 power supplies are switched off and the beam stopper is in.
In its present configuration the closest access to the SARC-IS is through the BSY
main access tunnel, shown in figure 5, and is approximately 100 m to the SARC-IS
compared to approximately 1000 m from the south adit. BSY accesses must wait for an
opportunity when there is no beam scheduled in the BSY. However, an unused (and little
known!) access tunnel, labeled southwest adit in figure 5, intersects the instrument
section just downstream of the PPS gate that isolates the tunnel from the BSY2.
2
Thanks to Mike Saleski for pointing this out.
Figure 6. The SARC instrument section could be accessed and controlled independently from
the BSY by upgrading the southwest adit to a controlled PPS entrance
This tunnel emerges in the lower MCC parking lot where it is blocked with concrete
shielding blocks. If this entrance were to be upgraded with a PPS gate and a chicane it
would provide ideal access for experimenters to the SARC-IS test beam area without
interrupting beam delivery to any other user.
Engineering issues
The refurbishment of the SLC arcs has been frequently discussed in the context of
delivering test beams to the final focus and interaction point. Only a short section of the
SARC is needed to deliver beams to the SARC-IS and can be brought back into operation
at a much lower cost. The short section of arc tunnel is close to the BSY where the
integrity of the tunnel walls is still good and there are no serious moisture or corrosion
problems. The Appendix contains recent photos from which the general condition of the
systems can be judged.
Since only 2 of the 23 SARC achromats would be powered in this configuration
considerably less power would be required from the arcs main power supply. To power
both arcs requires 3 MW, but the supply can be operated into the smaller load of two
achromats, in which case only 160 kW is needed3. The reduced AC line power
requirements are also an important consideration in any discussion of recommissioning
3
Information courtesy of Martin Berndt.
this system. In addition to the main power supply some reconfiguration of the F-D
imbalance supply and the backleg supplies would be needed.
A summary of the engineering upgrades includes:
1. magnets for the test beam line itself and the associated power supplies
2. replacement of the BSY 50B1 bend with a pulsed magnet and septum
3. jumpering the arc bends at the end of achromat 2 and reconfiguring the arc power
supplies for low power operation.
4. installing a dump line and dump into achromat 3 downstream of the instrument
section
5. upgrading the southwest adit to a chicane entrance with a PPS gate.
Appendix
Downstream view taken near the start of the
SARC-IS
Upstream view taken near the end of the
SARC-IS
Start of the SARC-IS: up beam to the right is
the PPS chicane gate to the BSY; to the left
is the southwest adit.
View into the unilluminated southwest adit.
Junction between an arc bend magnet and
the instrument section.
Termination of the arc bend magnet where
jumper leads would be installed.
Upstream view taken from the approximate
location of a beam dump in achromat 3 of
the SARC that is in a line of sight with the
upstream instrument section, showing how
the (covered) arc magnets bend away and
down to the left.