# The 8-GeV Superconducting Linac

Giorgio Apollinari, Dixon Bogert, Jean-Paul Carneiro, Brian Chase,
Arkadiy Klebaner, Sergei Nagaitsev, Peter Ostroumov, and Bob Webber

# Overview

The 8-GeV superconducting linac will provide 5.6E13 H- ions in 1 msec
pulses at a 5 Hz repetition rate for multi-turn charge exchange
injection into the Recycler Ring.

The linac design is based on ILC parameters and designs. It represents a
third generation of the 2005 Fermilab Proton Driver Linac design,
significantly modified to conform to key ILC parameters and to maximize
employment of ILC technical designs and components. Approximately 480
meters of the 670 meter linac would consist of ILC components. This
emphasizes the contribution to industrialization of ILC components in
the US and minimizes resources otherwise required for alternative
designs. The front-end of the linac relies on 325 MHz superconducting RF
technology starting at 10 MeV beam energy.

# Linac Parameter Table

Table 1 provides the basic operating parameters of the Linac beam and
technical systems. The beam current, pulse length, and repetition rate
are identical to corresponding ILC parameters. Transverse emittance,
energy spread, and bunch length values come from extensive simulations
of an intermediate linac design (described in the next section) that
incorporates eight standard ILC units. Although this is a precursor to
the present concept, which has yet to be completely simulated, the
parameters are believed to be compatible.

+---------------------------+------------------------+-----------------+
| **Parameter**             | **Quantity**           | **Unit**        |
+---------------------------+------------------------+-----------------+
| Particle Species          | H- ion                 |                 |
+---------------------------+------------------------+-----------------+
| Output Beam Energy        | 8.0                    | Gev (kinetic)   |
+---------------------------+------------------------+-----------------+
| 8-GeV Beam Power          | 360                    | kW              |
+---------------------------+------------------------+-----------------+
| Particles per Pulse       | 5.625                  | E13             |
+---------------------------+------------------------+-----------------+
| Pulse Repetition Rate     | 5                      | Hz              |
+---------------------------+------------------------+-----------------+
| Beam Pulse Length         | 1                      | msec            |
+---------------------------+------------------------+-----------------+
| Average Pulse Beam        | 9                      | ma              |
| Current                   |                        |                 |
+---------------------------+------------------------+-----------------+
| Beam Chopping at 2.5 MeV  | \- 6% at 89 kHz for    |                 |
|                           | 700 ns RR/MI           |                 |
|                           | abort/extraction gap   |                 |
|                           |                        |                 |
|                           | \- 33% at 53 MHz for   |                 |
|                           | 'pre-bunching' for     |                 |
|                           | transfer to RR         |                 |
+---------------------------+------------------------+-----------------+
| Particles per Linac bunch | 2.73                   | E8              |
+---------------------------+------------------------+-----------------+
| Nominal Bunch Spacing     | 3.1                    | nsec            |
+---------------------------+------------------------+-----------------+
| 8-Gev Transverse          | εH = εY = 0.4          | mm-mrad RMS     |
| Emittance                 |                        |                 |
+---------------------------+------------------------+-----------------+
| 8-GeV Longitudinal        | 2.5E-6                 | ev-sec/bunch    |
| Emittance                 |                        | RMS             |
+---------------------------+------------------------+-----------------+
| 8-GeV Energy Spread       | At Linac output -- 2.7 | MeV RMS         |
|                           |                        |                 |
|                           | At RR injection -- 0.3 |                 |
+---------------------------+------------------------+-----------------+
| 8-GeV Bunch Length        | At Linac output -- 1.0 | psec RMS        |
|                           |                        |                 |
|                           | At RR injection -- 8.6 |                 |
+---------------------------+------------------------+-----------------+

Table 1. Linac Parameters

# Linac Technical Design Description

![](media/image1.wmf){width="5.983333333333333in"
height="5.438194444444444in"}The front-end of the linac will consist of
a 50 keV H- ion source, a commercially fabricated 2.5 MeV RFQ, a Medium
Energy Beam Transport including a fast beam chopper, and a 16-cavity
room-temperature section to 10 MeV. From this point on, superconducting
cavities are employed. Two types of single spoke resonator structures
and one type of triple spoke structure complete the non-ILC section of
the linac. This section operates at 325 MHz, exactly ¼ of the ILC
frequency. Transverse beam focusing is provided by superconducting
solenoid magnets from the RFQ through the single-spoke sections;
superconducting quadrupoles are employed starting in the triple-spoke
section. Designs and pre-production of room temperature section
components, including the superconducting solenoid magnets, and the
single-spoke superconducting cavities are largely completed at this time
by the High Intensity Neutrino Source (HINS) R&D program. The charge of
the HINS program is to demonstrate performance of such a machine at
energies up to 60 MeV.

Tables 2 and 3 compare the superconducting sections of the intermediate
linac design that yields beam parameters given in Table 1 to the present
linac design concept. The special "squeezed ILC" cavity/cryomodule
design required to cover the β=0.8 energy section in the intermediate
design is eliminated, replaced with four additional triple-spoke
cryostats and two ILC RF units operating with the 8π/9 mode of the ILC
cavities tuned to the accelerating frequency. This mode offers higher
acceleration efficiency than the normal π-mode for the non-relativistic
particles in this energy range. Eleven additional ILC RF units complete
the linac. Transverse optics considerations require that the six
8π/9-mode cryomodules and nine "ILC-1" cryomodules each contain two
quadrupoles with seven ILC cavities. The "ILC-2" section consists solely
of modules of the standard ILC 'eight cavity - one quad' design type.
The final four RF units, 28 modules, are the standard ILC 9-8-9 cavity
configuration.

One additional ILC klystron RF system is required in the transfer line
downstream of the linac to power four 17-cell copper debuncher cavities
to reduce momentum spread of the beam entering the Recycler. The
debuncher system is not superconducting due to its remote location with
respect to any cryogenics infrastructure.

Longitudinal dynamics of the non-relativistic H- beam require active
control of RF amplitude and phase on a cavity-by-cavity basis although
many cavities are to be powered by a single klystron. This is
accomplished by including high power fast ferrite vector modulators in
the RF distribution system throughout the linac with the exception of
the final four ILC 9-8-9 RF units. All cavities, fast modulators or not,
require that the static phasing of the RF distribution system be set as
required by the H- beam.

The machine design will include the possibility of electron beam
acceleration through eleven ILC RF units (\~8 GeV) with suitable
re-adjustment of cavity phasing and quadrupole currents. It is to be a
manual operation to switch between electrons and H-; pulse-to-pulse
switching is not supported in this design.

# Physics and Technical Issues

## Physics Issues

The HINS R&D program as presently defined will resolve some major
physics issues of the 8-GeV linac. This program is intended to
demonstrate:

-   Beam acceleration in spoke loaded SC cavities

-   H- beam acceleration in multiple cavities fed by single klystron

-   Phase and amplitude control of cavity fields by high-power fast
    ferrite vector modulators

-   Beam chopping at 53 MHz repetition frequency

-   High beam quality in the linac with solenoid focusing

-   Negligible effect of HOMs on beam quality in a proton/H- linac

Additional major physics issues that would remain to be resolved
include:

-   Demonstration of phase-locked operation of 9-cell ILC cavity on the
    8π/9 mode.

-   Transient beam dynamics analysis in the linac with realistic RF
    distribution and feedback system.

-   Definition of any potential project upgrades that are not to be
    excluded by the design of the 8-GeV linac.

## Cryogenics Considerations

Cryogenic system operating temperatures and pressure levels for the
8-Gev linac are largely determined by the choice of the ILC style
cryomodules and the design of the 325 MHz front end linac. The ILC
cavity cold mass structure operates at 2 K temperature. The 325 MHz
front end linac is cooled by 2-phase helium at 4.5K.

The scope of the 8-GeV linac cryogenic system includes cryogen
production and distribution. A helium refrigeration plant similar in
scale to that at the Spallation Neutron Source at ORNL will be required.
System components include the cryogenic plant, compressors, cooling
towers, auxiliary systems, distribution and interface boxes,
non-magnetic and non-RF cold tunnel components, and cryogenic transfer
lines.

Most major components of the 8-GeV cryogenic system have been used
successfully in similar systems before. Nevertheless, the 8-GeV linac
cryogenic distribution system presents challenges including:

-   Liquid level control in long strings of helium vessels filled by
    means of series flow under conditions of large dynamic heat loads
    from RF power and very long time constants. A conceptual design for
    electric heaters, to counteract the impact of sudden heat load
    changes, needs to be developed and the impact on total heat load
    evaluated. The flow pattern and cooling limitations of two-phase
    superfluid helium must be studied and validated with respect to
    slope and string length.

-   Heat load estimates of the ILC cryomodules as well as uncertainty
    and overcapacity factors all need refinement.

-   Optimization studies for capital and operating costs needs to
    consider tradeoffs of cryomodule complexity vs. heat loads.

-   Protection from over pressurization due to abnormal operating
    conditions, such as loss of beam tube or insulating vacuum, power
    outage, etc., must be designed. Vacuum isolation, lengths of vacuum
    units, and fast-acting vacuum valves all need further investigation.
    The 2 K cavity-cooling circuit is most critical due to the low
    pressure limits imposed by the niobium RF cavities, but the thermal
    shield flow circuits may also be difficult to protect.

-   Compliance with engineering standards and the associated component
    design pressures must be studied with respect to cost, operability
    and reliability.

These are all cryogenic system issues that ILC must also address.

## Civil Site Considerations

As described at the 8-GeV Linac Director's Review in 2005, the site for
an 8-GeV linac injecting into either the Main Injector or the Recycler
is subject to numerous rigid constraints. These include limited straight
sections in either machine lattice adaptable to the injection system,
site boundary considerations, existing facilities, retaining nearby
space for future possible experiments or facilities (e.g. a proton
accumulator ring for possible future muon production), and accommodation
for the large bending radius necessary for any 8-GeV H- transport line
to prevent magnetic field stripping. There is no new evidence to suggest
a more suitable site for an 8-GeV linac at Fermilab than inside the
south half of the Tevatron ring.

The entire southern half of the area inside the Tevatron Ring has been
professionally characterized for environmental considerations as well as
for soil and earth constructability. There will be a wetlands impact,
for example, but all possible sites in that general area meeting the
various criteria outlined above are similar. The Army Corps of Engineers
was consulted and it is likely that either the purchase of off-site
offset credits, or the construction of off-site mitigation, will be the
preferred mitigation option, so the on site location of this proposed
project may be set without reference to particular narrow
considerations. The total mitigated off-site area will probably be on
the order of 120 to 160 acres for the project now under consideration.