**Cryogenic design for a liquid hydrogen absorber system**

C. Darve^1^, D. Allspach^1^, E. Black^2^, M.A. Cummings^3^, C.
Johnstone^1^, D. Kaplan^2^, A. Klebaner^1^, A. Martinez^1^, B.
Norris^1^, M. Popovic^1^,

^1^Fermi National Accelerator Laboratory, USA

^2^Illinois Institute of Technology, USA

^3^Northern Illinois Univ., USA

> Muon ionization-cooling is under investigation at Fermilab and several
> other High Energy Physics Laboratories. A test area (MuCool Test Area)
> is being built at Fermilab to run components of a cooling cell
> together under a high power beam test. The first stage of the MuCool
> Test Area will ensure the feasibility of a liquid hydrogen absorber
> and cryogenic system. The main requirement of the system is to keep
> the density fluctuation in the liquid hydrogen absorber lower than
> 2.5%. Helium refrigeration can provide up to 500 W of cooling capacity
> at 14 K. This paper describes the cryogenic design of the system.

# Introduction

Future Muon Collider and Neutrino Factory will require a cooling system
to reduce the muon beam\'s transverse emittance. The current design uses
ionization cooling as the beam passes through number of liquid hydrogen
absorbers, alternating with accelerating radio-frequency cavities and
embedded within a focusing magnetic lattice \[1\].

A preliminary feasibility study will be conducted at Fermilab with one
liquid hydrogen (LH~2~) absorber at a new MuCool Test Area. The LH~2~
absorber is inserted in a hydrogen loop and housed in a containment
vessel to fit the bore of a 5 T solenoid magnet. The first stage of the
MuCool Linac Test Area operation under construction will validate the
mechanical and thermal design of the liquid hydrogen absorber system.

# 

# 

# Test facility description

The cryogenic test facility is composed of a hydrogen cryoloop and
helium refrigeration provided by an onsite cryoplant. The hydrogen
system is distributed between a manifold room and the experimental hall.
Figure 1 shows the flow schematic of the test facility. Hydrogen gas
bottles stored in a separate manifold room supply the hydrogen gas.
Pneumatically controlled valves regulate the cool down and the operation
condition. Transfer lines equipped with the appropriate regulation
valves, instrumentation and safety devices transfer H~2~ gas to the
containment vessel during the cool down. Gaseous hydrogen is then
liquefied via a He/H~2~ heat exchange process, where the helium is
supplied and regulated between14 K and 20 K. During normal operation
about 25 liters of liquid hydrogen flows in a close loop at 0.12 MPa.
The available helium refrigeration capacity is 500 W at 14 K.

The test facility of the liquid hydrogen absorber system under
development at Fermi National Accelerator Laboratory can be compared to
other experiments using LH~2~, like E158 hydrogen target system under
test at Stanford Linear Accelerator Center (SLAC) \[2-3\] and SAMPLE
experiment at Bates \[4\]. Although, these three experiments have
different physics goals, the test facilities house a similar cryo-system
and have to satisfy similar safety requirements. Operating parameters
like LH~2~ capacity, flow rate, pressure and temperature are the main
differences for such experiments.

![](media/image1.wmf)

Figure 1 Flow schematic of the MuCool Test Area

# 

# 

# liquid hydrogen absorber system 

## [Cryo-loop]{.underline}

The hydrogen loop consists of the liquid hydrogen absorber, He/H~2~ heat
exchanger, LH~2~ pump, transfer lines, safety devices and
instrumentation. Figure 2 shows the conceptual design of the LH~2~
system in the cryogenic solenoid magnet.

The LH~2~ absorber is composed of an aluminum manifold and two non
standard thin aluminum windows, with tapered thicknesses near the edge.
The future MuCool experiment will require windows, which are as thin as
possible in order to minimize the Coulomb multiple scattering in the
material. It has been empirically determined that the thickness of the
absorber and containment vessel windows should be less than 200 μm
\[5-6\].

The He/H~2~ heat exchanger regulates the temperature of the hydrogen
system so that the LH~2~ density fluctuation in the liquid hydrogen
absorber remains less than 2.5%. We chose to regulate the LH~2~
temperature between 17 K and 18 K. A 500 W heater mounted on the outer
shell of the heat exchanger is used to balance the heat transfer of the
hydrogen system, while keeping a constant helium refrigeration capacity
of 30 g/s.

During operation the hydrogen is circulated by a mechanical pump at a
flow rate up to 550 g/s. This 2 HP LH~2~ pump was designed and built by
Caltech as a spare pump for the SAMPLE experiment \[4\]. The MuCool
Linac Test Area LH~2~ pump is loaned by the SAMPLE collaboration.

The transfer lines connecting the absorber, heat exchanger and pump are
equipped with safety devices and relief valves venting outside the
experimental hall.

## [Containment vessel]{.underline}

The hydrogen cryo-loop is housed in a cryostat and is inserted in the 44
cm diameter bore of a cryogenic solenoid magnet. The LH~2~ loop is
thermally insulated in the vacuum vessel by a thermal shield and G10
supporting system. The thermal shield is actively cooled at liquid
nitrogen (LN~2~) temperature and wrapped with 30 layers of multilayers
insulation. The containment vessel windows are shaped like the LH~2~
absorber windows to minimize multiple scattering. The vacuum vessel
volume is 52 times larger than the hydrogen cryo-loop capacity in order
to withstand the expansion of the saturated liquid hydrogen. Therefore
the vacuum vessel volume, being 1300 liters is distributed around the
cryo-loop and through pipe venting outside the experimental hall. A
pumping system composed of a roughing pump and a turbo-molecular pump
will provide an insulation vacuum of 10^-4^ Pa.

![](media/image2.wmf)

Figure 2 Conceptual design of the MuCool Test Area cryostat

# Cryogenic design

Design of the liquid hydrogen absorber system is completed with respect
to the thermo hydraulic behavior of hydrogen flow, thermal calculations,
heat exchanger and safety relief valve calculations. The design is based
on the American Society of Mechanical Engineers code (ASME) and the
Fermi National Accelerator Laboratory safety recommended requirements
for hydrogen. The associated controls and instrumentation are based on
the US National Electrical Code (NEC) safety requirements for hydrogen.
The use of hydrogen is challenging and implies stringent safety
controls. A safety Programmable Logic Controller (PLC) is considered.
More than 150 temperature sensors, pressure elements, valves within the
cryostat are chosen and installed regarding the safety requirements.
Additional requirements are radiation hardness for the instrumentation
as well as structures to withstand forces during potential quenches of
the magnet. Oxygen deficiency detectors and flammable gas detectors are
installed in both the experimental hall and the hydrogen manifold.

To satisfy safety requirements for the hydrogen system, the relief valve
system is redundant. Relief system is composed of fast acting valves,
AGCO type valve and parallel plates. The operating pressure of the
hydrogen cryo-loop is 1.2 atm. The hydrogen cryo-loop is set to open at
10 psig (0.17 MPa). The containment vessel (vacuum vessel) is sized for
a Maximum Allowable Working Pressure (MAWP) of 25 psig (0.27 MPa).
Parallel plates designed at Fermilab will be used.

The helium refrigeration system will ensure the cooling of the heat
deposited by the beam (up to 150 W) and by the static heat load from the
cryo-system. Insulation vacuum of 10^-4^ Pa, low thermal conductivity
supporting system and multilayer superinsulation minimize the heat load
to the liquid hydrogen system. The heat load to the 17 K LH~2~ absorber
is about 6 W due to radiation and conduction through multilayer
insulation, 17 W due to radiation from the two containment vessel
windows and 0.2 W due to conduction though the G10 supporting system.
The heat load due to solid conduction from the motor shaft of the LH~2~
pump is about 50 W. A safety coefficient of 2 was used for the
refrigeration system sizing. Therefore, the total static heat load from
the cryo-system to be extracted by the helium refrigeration system at 17
K is less than 150 W. Hence, the helium refrigeration system will ensure
less than 300 W. The cryostat thermal heat load to the 77 K temperature
level is calculated to be 69 W. Hence, nitrogen cooling ensures a
cooling up to 140 W. Table 1 summarizes the heat load calculated for the
current conceptual design of the cryostat.

A motor drives the pump via a long metallic shaft coupled to the
rotating pump shaft, with one cold and one warm bearing to keep the heat
load from the motor at a minimum. Being located outside the vacuum
vessel, the motor is sealed in a container in order to prevent air
leaking into the cryostat. This container is cooled by a flow of
nitrogen gas. The shaft inside the cryostat is contained in a vessel,
which is continuously pumped to limit the hydrogen and nitrogen leaks,
which limits the heat load to the cryo-loop and the risk of nitrogen
freezing in the cryostat. The motor runs between 10 and 60 Hz.

Table 1 Static heat loads calculated for the current conceptual design

![](media/image3.wmf)

The main limiting factor for the thermo hydraulic hydrogen flow is the
density change for the LH~2~ inside the absorber. For subcooled
hydrogen, the allowable density change in the LH~2~ absorber should be
less than +/-2.5 %, which corresponds to a temperature gradient less
than 4 K. In order to keep some margin with the hydrogen boiling point,
hence large density change, the cryo-loop cryogen will be subcooled. The
heat exchanger is designed for 500 W, for a nominal temperature gradient
of 1 K at 17 K and for the maximum helium flow available by the
cryo-plant being 30 g/s. The pressure drop in the helium side will be
accommodated by the refrigeration cryo-plant. An extrapolation to 1 kW
would be possible with a larger LH~2~ temperature gradient and an
average temperature of 20 K. The He/H~2~ heat exchanger is composed of a
copper coil housed in a 500 mm x 150 mm diameter stainless steel outer
shell. The copper tube is wrapped around a solid aluminum core in order
to reduce the total volume of liquid hydrogen.

The design of thermo hydraulic hydrogen flow schematic inside the
containment window is also dictated by the LH~2~ pump characteristics.
The pressure drop admissible by the pump has been measured \[4\], being
0.5 psig (0.1 MPa) for a 550 g/s flow of liquid hydrogen. Therefore the
cryo-loop including the absorber, heat exchanger and transfer lines is
designed to fulfill this allowable pressure drop. In order to reduce the
pressure drop in the system, two 25 mm diameter pipes are used to supply
liquid hydrogen to the absorber volume and three pipes are used for the
return to the LH~2~ pump. The connecting transfer lines to the heat
exchanger pump and absorbers are 50 mm diameters. Less than 0.4 psi (2.8
10^-3^ Pa) of pressure drop is estimated for a 550 g/s liquid hydrogen
absorber system flow at 0.12 MPa and 17 K.

# 

# 

# Conclusion

A cryogenic design for a liquid hydrogen absorber system has been
developed at Fermilab. The requirements are based on ASME code and
Fermilab safety requirements and US NEC standards. The cryogenic design
is based on its ability to maintain a temperature gradient of 1 K at 17
K and 0.12 MPa within a structurally safe containment vessel. The
high-powered beam test at the MuCool Test Area using protons extracted
from the Fermilab Linac is scheduled to run in 2005 with cryogenic
operations beginning in 2003.

# 

# 

# References

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3.  J.G. Weisend II et al., Safety Aspects Of The E158 Liquid Hydrogen
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4.  E.J. Beise et al., A high power liquid hydrogen target for parity
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5.  D. Kaplan et al., Progress in Energy Absorber R&D 2: windows,
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6.  J. Greenwood et al., Failure Metrology using Projected target
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