LARP IR Cryogenics: Design Temperature Profile for an LHC Upgraded IR

Roger Rabehl

December 8, 2005

Version 2: March 20, 2006

# Introduction

The temperature profile resulting from increasing the heat load to the
1.9 K helium in the current LHC interaction regions (IR's) has been
documented \[1\]. A number of thermal limitations in the design were
identified. Based on these limitations and the expected IR heat load
increase accompanying a ten-fold increase in machine luminosity, a
design temperature profile for the upgraded IR's is presented. This
design temperature profile will provide guidance in designing the
upgraded IR cryogenics system.

**Design Temperature Profile**

Table 1 presents the design temperature profile for the cryogenics
system of an upgraded IR. This temperature profile is very similar to
that proposed by CERN for Next European Dipole (NED) and IR upgrades
\[2, 3\].

## Table 1 Temperatures in an upgraded IR.

  --------------------------- ---------------------- ---------------------
  Segment                     Temperature range      ∆T \[mK\]
                              \[K\]                  

  Magnet bore annular space   2.150-2.000            150
  to heat exchanger crossover                        
  pipe                                               

  Heat exchanger crossover    2.000-1.950            50
  pipe                                               

  Heat exchanger crossover    1.950-1.826            124
  pipe to heat exchanger                             
  (saturated side)                                   

  Pressure drop along the     1.826-1.816            10
  inner triplet heat                                 
  exchanger                                          

  Pressure drop through the   1.816-1.791            25
  J-T heat exchanger shell                           

  Pressure drop through       1.791-1.716            75
  piping to the first stage                          
  cold compressor                                    
  --------------------------- ---------------------- ---------------------

Pressure-induced, equivalent temperature drops in the subatmospheric He
system account for about 25% of the total available temperature budget.
These are not actual temperature drops and do not assist in heat
transfer; therefore they should be kept to a minimum. These equivalent
temperatures are shown in Figure 1.

![](media/image1.png)

**[Figure 1]{.underline}** Equivalent temperatures in the subatmospheric
He system.

A saturation temperature of 1.716 K assumes that the first stage cold
compressors are moved closer to the interaction regions and that they
are upgraded to reach 12 mbar instead of the current 15 mbar \[4\].
Moving the cold compressors means the associated piping could be
appropriately sized to keep the temperature drop due to pressure and
elevation changes at 75 mK.

Heat load scaling indicates that the existing J-T heat exchanger design
will have a significantly higher temperature drop than 25 mK at 1.2 kW
per inner triplet. However, moving cold compressors means that
modifications to some QRL (compound cryogenic distribution line) service
modules would be required to accommodate the piping changes. An upgraded
J-T heat exchanger could be integrated at the same time.

Three-quarters of the temperature budget is allocated to the pressurized
He II system and the magnet heat exchanger wall. The heat path through
the crossover pipe to the heat exchanger has the highest heat flux, and
the temperature gradient therefore grows quickly so a generous
temperature drop was specified. It is important to note that the piping
configuration of Figure 2 is different than that of the existing inner
triplet. An upgraded inner triplet will likely have additional crossover
pipes (one at each end of every cold mass), and these crossover pipes
will connect to the cold mass end domes rather than the interconnect
piping. These changes will minimize the heat flux and length per
crossover pipe. The distributed heat paths within the cold mass and the
heat exchanger annulus are somewhat easier to handle. Design
temperatures of the pressurized He II system are shown in Figures 2 and
3.

![](media/image2.png)

**[Figure 2]{.underline}** Pressurized He II design temperatures in the
Q1 and Q2a magnets.

![](media/image3.wmf)

**[Figure 3]{.underline}** Pressurized He II design temperature at a
representative magnet cross-section.

### Conclusions

A design temperature profile for an upgraded IR has been presented.
Three-quarters of the temperature budget has been allocated to the
pressurized He II system with the remaining one-quarter allocated to the
subatmospheric He system.

As the cryogenics system design evolves, it is expected that the
temperature profile will be adjusted to meet demands.

#### References

\[1\] R. Rabehl, "LARP IR Cryogenics: Scaling of LHC I IR Cryogenics
Model," LARP Document 15, October 2005.

\[2\] R. van Weelderen, "He II Heat Transfer in Superconducting
Magnets," Next European Dipole (NED) project, February 2005.

\[3\] R. van Weelderen, "Heat Transfer in Superconducting Magnets," AMT
01-Beam Generated Heat Deposition and Quench Levels for LHC Magnets,
March 2005.

\[4\] R. van Weelderen, private communication, June 2005.