C. Ankenbrandt

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Fermllab FERMILAB-TM-2187 October 2002 
A Linac Afterburner to Supercharge the Fermilab 
Booster 
C. Ankenbrandt, J. MacLachlan, M. Popovic, R. P. Johnson 
Fermi National Accelerator Laboratory 
Abstract. A Linac Afterburner is proposed to raise the energy of the beam injected into the Fermilab Booster from 400 
MeV to about 600 MeV, thereby alleviating the longitudinal and transverse space-charge effects at low energy that 
currently limit its performance. The primary motivation is to increase the integrated luminosity of the Tevatron Collider 
in Run II, but other future programs would also reap substantial benefits. The estimated cost is $23M. 
INTRODUCTION 
The Fermilab Linac Upgrade Project, completed in 
1993 at a total cost of $31M, increased the linac 
energy from 200 to 400 MeV. The Afterburner 
proposal emulates that successful project in several 
respects. The same kind of warm copper 805-MHz 
side-coupled cavities are used to accelerate the beam. 
Once again, existing enclosures and buildings are used 
to house the accelerator components and their 
supporting equipment. The installation and 
commissioning are planned to minimize their 
interference with Run II. Emulating the previous 
successful project allows a quick start and a rapid 
completion of the Afterburner Project. The luminosity 
of the Tevatron Collider increased by a factor of about 
2.5 after the Linac Upgrade Project; the Afterburner is 
expected to provide a corresponding luminosity 
enhancement in Run II. 
THE BOOSTER AND THE 
AFTERBURNER 
When the Booster runs at 5·1012 protons per cycle, 
as called for in the Main Injector era, several 
mechanisms of emittance growth and beam loss are 
evident. In the first few milliseconds after injection, 
transverse space-charge effects cause rapid emittance 
growth accompanied by substantial (-30%) beam 
losses. Meanwhile, longitudinal space-charge forces 
distort the potential well created by the rf focusing 
force, causing longitudinal emittance growth and 
failure to capture part (-10%) of the beam. Of course 
that uncaptured beam is swept radially inward out of 
the aperture as soon as the magnetic field ramps up 
significantly. These two low-energy space-charge 
effects are the dominant beam-loss mechanisms in 
normal operation, but they are not the whole story. 
Although aggressive pursuit of measures to control 
higher-energy losses is certainly necessary to the 
success of the near-future physics program, only 
modest intensity gains are likely to result, because the 
higher-energy losses amount to less than ten percent of 
the beam. 
The Booster is currently the bottleneck preventing 
higher-intensity operation of the whole facility, 
because both the upstream and downstream machines, 
the Linac [1] and the Main Injector/Tevatron complex, 
can handle higher intensities than the Booster. 
AFTERBURNER TECHNICAL DESIGN 
CONCEPTS 
To accelerate the beam between the Linac and the 
Booster, accelerating structures like those in the 
downstream end of the Linac will be installed along 
the outer wall of the Booster tunnel. (Putting a curved 
linac in the Booster tunnel was previously suggested in 
another context.[2]) The Afterburner is preceded by a 
longitudinal matching section and followed by a 
debunching section. The existing 400-MeV linac and 
beam transfer line to the Booster are left essentially 
untouched up to MH2, a horizontal bending magnet 
near the outer tunnel wall that currently bends the 
beam from the linac across the aisle toward the 
Booster injection system. 
A. The Afterburner 
The Afterburner is based on the 805 MHz warm 
copper side-coupled cavities installed at the 
downstream end of the existing linac during the Linac 
Upgrade Project [3]. There are several advantages to 
that choice. The technology is well understood and 
the costs well known, so no R&D is necessary prior to 
writing a definitive proposal. The transverse size of 
the side-coupled cavities is small enough to fit 
comfortably in the tunnel. Spare parts, including 
several 12 MW klystrons, exist and can be shared with 
the present system. Most importantly, the present 
system works very well, reliably producing high-
quality beam with low beam losses. The decision to 
employ that technology allows the quick start and 
rapid completion that are essential in order to benefit 
Run IL 
The Booster itself consists of 24 periods, with each 
period containing a long straight section, two 
combined-function magnets, a short straight section, 
and two more combined-function magnets. Figure 1 
shows the layout of the Booster in its tunnel on the 
south side, where the Afterburner is to be installed. 
The length of the outer tunnel wall is 20.5 meters per 
period. The shape of the Afterburner and its transfer 
lines is regular and symmetrical, like the Booster and 
most of its tunnel. There are 7.5 degree bend centers 
opposite the middle of the short and long straight 
sections, separated by 10.25 meters. The 10.25 meter 
distance between bend centers is sufficient to 
accommodate one accelerating module along with the 
required bending and focusing magnets. In the present 
design there are actually five small magnets at the 
bend centers: a quadrupole triplet with two 3.75 degree 
dipoles interspersed. The dipoles are 0.5 meters long 
to reduce field stripping to negligible proportions. 
FIGURE I. 
The accelerating structure has five modules. Each 
module consists of four sections joined by bridge 
couplers to accommodate quadrupole focusing 
magnets. Each section contains twelve accelerating 
cavities. The choice of twelve cavities per section, 
rather than sixteen as in the existing linac, makes the 
modules short enough so that the Afterburner can 
match the shape of the outer tunnel wall. The rest of 
the parameters are the same as in the existing linac. 
The maximal accelerating field is limited by the 
requirement that the electric field is less than 1.4 times 
the Kilpatrick value. The shunt impedance of the 
structure is taken to be 80% of the value calculated by 
Superfish; the same criterion was used in the design of 
the 400MeV energy upgrade. The transit-time factor, 
maximal field, and shunt impedance are extrapolated 
from the Superfish calculation for the Linac Upgrade 
Project for beta in the region from 0.57< ~< 0.71. 
Figure 2, taken from Trace3D output, shows the 
phase-space ellipses in the transverse and longitudinal 
dimensions. In the upper left comer are input beam 
ellipses with alphas and betas for three planes. The 
upper right comer shows the same beam parameters at 
the end of the Afterburner. The lower portion of the 
graph shows beam envelopes and accelerator elements. 
The displayed envelopes are the square root of five 
times their respective rms values. The two upper traces 
are the horizontal envelope and bunch length and the 
lower trace is the vertical envelope. The quad settings 
are adjusted to keep the beam envelope small along the 
linac. The full scale of 15 mm corresponds to the 
radius of the physical aperture. 
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FIGURE2. 
It is interesting to note the similarity of Figure 2 with 
the corresponding Figure 15 of the Fermilab Linac 
Upgrade Conceptual Design Report [Ref]. In 
particular the beam envelopes in both cases are 
contained within about half of the physical aperture. 
That shows that the dispersion associated with the 
required bends is not a problem. The momentum 
spread contributes only a few mm of beam size in 
quadrature with the transverse beam sizes. The 
transverse sizes shrink adiabatically from their values 
at 116 MeV while the physical apertures remain the 
same size. 
B. Longitudinal Matching 
The Afterburner requires a buncher to match into 
its side-coupled structure (SCS) at the upstream end 
and a debuncher at the downstream end to reduce the 
momentum spread for injection into the Booster. 
(Transverse matching is of course also required, but it 
is not discussed here because it is routine.) The 
bunching system makes use of the existing 400 MeV 
debuncher but requires an additional bunching tank 
closer to the Afterburner. 
The existing 805 MHz debuncher is located about 
30 m downstream of the end of the 400 MeV SCS 
linac where the beam bunches have sheared to span 
about 80 degrees of rf phase. This is a good spread for 
rebunching with good linearity using a modest rf 
system. However, it is much too far upstream to 
permit the sharp focus (small longitudinal beta 
function) required to match to the Afterburner. This 
design includes a new buncher structure located about 
25 m upstream of the Afterburner. To reduce the 
power required for the buncher, a twelve-cell SCS is 
used. It requires about 480 kW of rf power, which is 
not low enough to use the same kind of small TV 
klystron that powers the first buncher, so a shorter 
structure powered by one of the 12 MW klystrons 
might be preferred. The existing four-cell debuncher 
is excited to 2 MV/m to keep the bunch width below 
one hundred degrees. The new buncher is excited to 
an effective gradient of 1.7 MV/m so that the bunches 
entering the Afterburner have the same distribution in 
longitudinal phase space as the ones exiting the 400 
MeV linac: 12 degrees full width and 2.6 MeV full 
energy spread. 
The 600 MeV debuncher serves exactly the same 
purpose as the existing debuncher in the present 
transfer line between the 400 MeV linac and the 
Booster, but substantially more power is needed. It 
can be located 40 m downstream of the Afterburner 
and run at exactly the same gradient as the buncher. 
The energy spread of the 600 MeV beam is reduced 
from 2.8 MeV full width to 0.54 MeV at the Booster, 
corresponding to a mom en tum spread of 0. 03 % . 
C Injection into the Booster 
A new injection system to be located at Long 
Straight Section 11 is a part of this proposal. The 
existing injection system will be left where it is at 
Long Straight Section 1 so that it can continue to be 
used for injection of 400-MeV beam until the 
commissioning of the Afterburner is complete. 
Temporarily having dual injection systems will reduce 
the interruption to Run II, and a new injection system 
can incorporate the latest design ideas. Besides, the 
existing injection system probably would not work at 
600 MeV anyway. 
Figure 3 shows the layout of the new ff injection 
system. The design borrows ideas from that of the 
injection system for the Proton Driver[4], but unlike 
that system it is local, that is, confined to a single 
straight section with no lattice elements interspersed 
among injection components. 
Injecting a high-energy beam in a limited space is a 
design challenge because the field strength B of the 
magnets that transport the incoming ion beam must be 
limited; they must satisfy the constraint B<0.7 Tesla/p 
(GeV/c) in order to reduce the probability of field-
stripping to about 10-3. The parameter choices used in 
Figure FFF-INJ satisfy that constraint and comfortably 
accommodate 600 MeV beam in the available space. 
If necessary considerably higher incoming kinetic 
energies could be injected using this kind of design. 
FIGURE3. 
SUMMARY AND CONCLUSIONS 
A Linac Afterburner is proposed to supercharge the 
Fermilab Booster by raising its injection energy, 
thereby alleviating the transverse and longitudinal 
space charge effects that currently limit its 
performance. The Booster is the bottleneck in the 
whole Fermilab accelerator complex because all the 
other machines can handle higher intensity beams. 
Considerable leverage is involved because a modest 
improvement upstream of the Booster would benefit 
all components of the Fermilab experimental high 
energy physics program. Increasing the brightness and 
intensity of the Booster and Main Injector beams 
would be quite valuable to the US HEP program. 
However, the primary motivation for this proposal is 
to increase the integrated luminosity for Run II of the 
Tevatron Collider. CERN's remarkable effort to 
discover the Higgs in the last LEP run was impressive; 
now Fermilab must pick up the torch. A Linac 
Afterburner is the logical Fermilab response to the 
mandate to maximize the discovery potential of Run 
II. This document has demonstrated the feasibility of 
the Afterburner Project. The nature and scope of the 
project would be much like that of the highly 
successful Linac Upgrade Project. A quick start is 
possible because no R&D is necessary; we already 
know everything we need to know to write a 'credible 
proposal to the Department of Energy. A rapid 
completion is possible because the cost and scope are 
modest and because existing structures can be used to 
house the accelerating components and supporting 
equipment. Can Fermilab afford not to pursue these 
opportunities vigorously? 
REFERENCES 
1. LINAC Staff (M. Popovic for the collaboration), 
PRESENT, NEAR FUTURE AND FUTURE 
PERFORMANCE OF THE FERMILAB LINAC, 
FERMILAB-PUB-96-046, Mar 1996 
2. M. Popovic, A. Moretti, R. Noble, and C.W. 
Schmidt, 1-GeV Linac Upgrade Study at Fermilab, 
1998 Linac Conf 
3. http://wwwlib.fnal.gov/archive/linac/FERMILAB-
LU-Conceptual Desi gn.pdf 
4. W. Chou, C. Ankenbrandt, and E. Malamud, Eds., 
The Proton Driver Design Study, FERMILAB-
TM-2136, December 2000. 


A Linac Afterburner to Supercharge the Fermilab Booster

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A Linac Afterburner to Supercharge the Fermilab Booster

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