### 5.2.11 Cryogenic System[^1]

#### 5.2.11.1 Cryogenic System Description

The magnet's superconducting transmission line is cooled by pressurized
supercritical helium at 4.5-6.0 K. The heat load of the magnet system is
removed by the sensible heat of the supercritical helium stream. The
fluid operates just outside the critical region expanding the helium
stream as it passes through the transmission line producing a large
effective heat capacity. Cryogens are fed from six (6) refrigerator
plants spaced at 38 km intervals around the ring.

![](media/image1.png)

Figure 1.

For efficiency reasons, refrigeration is provided at two (2) temperature
levels. Refrigeration at a higher temperature (40 K) provides cooling
for a heat shield that is used to intercept much of the heat that would
otherwise be absorbed by the lower temperature superconductor. Each
plant provides refrigeration for upstream and downstream magnet streams.
Each string is sub-divided into two (2) loops, referred to as "near" and
"far" loops.

![](media/image2.png){width="5.995833333333334in"
height="2.1694444444444443in"}

Figure 2.

##### 5.2.11.1.1 Helium Distribution System

The helium distribution system is designed to distribute refrigeration
to the magnet system with a minimal temperature variation. The
distribution system consists of cryogenic valve boxes, transfer lines,
and warm helium header.

###### (a) Cryogenic Valve Boxes

Cryogenic valve boxes are used to redistribute the helium between the
conductor loops for steady-state operation and perform various transient
state operations. These boxes are located every 9.5 km and house the
control valves and instrumentation required for each circuit. Boxes vary
in configuration dependent upon location in the magnet string. It is
believed these boxes are relatively simple devices mainly providing a
transition for flow redistribution.

###### (b) Transfer Lines

Transfer lines are a part of the magnet assembly. A line contains five
(5) circuits: 4.5 K superconductor supply line (near loop only), 6.0 K
superconductor return line, 40 K shield supply line, 70 K shield return
line, and a vacuum circuit. Each section of transfer line is equal in
length to a magnet (67.5 meters) and is attached to the magnet assembly
via a structural anchor at the midpoint of the magnet.

-   The 70 K return line is a 3 IPS stainless steel pipe connected to an
    extruded aluminum shield that surrounds all the inner lines. The
    shield is wrapped with a double-sided aluminized mylar (dimpled and
    perforated). The longitudinal contraction of the line is compensated
    with guided formed bellows in the inner connect. During this
    shrinkage, the G10 support structure is allowed to move within the
    vacuum space.

-   The 40 K shield supply line is a 2 1/2 IPS stainless steel pipe
    supported on the G10 spiders from the aluminum shield. Bellows are
    used at the end of each 67.5 m module to compensate for thermal
    contraction and expansion.

-   The 4.5 K supply -- near loop only -- and 6.5 K superconductor
    return line are Invar tubes which are also anchored at the midpoint
    of the module. These lines are a concentric design. The 6.5 K helium
    flows between two (2) concentric Invar tubes.

-   The 4.5 K supply line is suspended via G10 spiders from a 6.5 K
    inner line.

##### 5.2.11.1.2 Helium Pressure Relief and Venting System

Relief valves for each of the five (5) circuits are located every 540
meters. These valves will vent into the warm helium header that is
itself protected every 1 km at 3 MPa (30.4 bar) from over pressurizing.
Each relief is conventional in nature and set to the correct pressure
setting of its given circuit. The primary need for these reliefs are for
trapped volume and loss of insulating vacuum scenarios. The warm header
is further protected at the refrigerator (above ground) at a set
pressure of 0.2 MPa (2 bar), venting to atmosphere.

![](media/image3.png){width="5.991666666666666in"
height="2.5208333333333335in"}

Figure 3.

Quench protection of the transmission line is discussed in \[1,2\]. The
100 kA transmission line conductor contains enough copper that it can be
safely shut down with a 1 second time constant following a quench. A
number of detailed quench simulations have been performed \[3\]. The
peak temperature is \~250 K. The worst-case peak pressure comes from a
situation in which a mis-steered beam (kicker misfire) causes the beam
to oscillate side-to-side in the beam pipe and scrape heavily in a
series of locations separated by a half-betatron wavelength. In this
case the peak temperature is unaffected, but a worst-case pressure
occurs when the hydrodynamic shock waves from two adjacent quenches
collide at the midpoint between quenches. The peak pressure is below the
4.1 MPa (40 Bar) pressure rating of the transmission line. Thus no
helium needs be vented anywhere, even for a long period following the
quench. The pressure burst from the hydrodynamic shock wave, as
registered on pressure sensors along the transmission line, could
provide a convenient method of identifying where the quench occurred.

##### 5.2.11.1.3 Heat Shield Cooling System

Pressurized 40 K helium is used to intercept the radiation and
conduction heat leak to the transmission line. Helium is supplied from
the refrigerator at 1.72 MPa (17 bar) and returns at \< 70 K and 1.62
MPa (16 bar). Flow is supplied via a 3.5" O.D. tube and distributed via
a pair of 0.25-inch lines every 270 meters carrying 2.0 g/sec. This
scheme provides shield cooling for every two magnets. Control valves and
temperature sensors are included every 270 meters for heat shield flow
control based on temperature requirements.

##### 5.2.11.1.4 Transmission Line Heat Load Estimates

Transmission line heat load estimates are divided into two (2)
components: the primary heat load on the 4.5 K system and the heat load
to the 40 K shield system. The calculated heat loads are presented in
Table 1.

![](media/image4.wmf)

##### 5.2.11.1.5 Power Lead Cooling

Due to its low inductance (3 μH/m) and low stored energy (15 kJ/m), the
transmission line magnets require only a single 50 V power supply and a
single set of 100 kA current leads for the entire machine. If these are
conventional (gas-cooled metal) leads, the helium consumption will be 5
g/s for each lead. If desired, this power requirement can be reduced by
a factor of 4\~5 with the use of High Temperature Superconductor (HTS)
current leads.

#### 5.2.11.2 Refrigeration System 

The refrigeration system is composed of six (6), 4.5 kW @ 4.5 K capacity
facilities equally spaced around the ring. Each facility will also
provide 80 kW of 40 K refrigeration used in the shielding of the magnet
system. This provides approximately a 50% margin above expected steady
state requirements. This margin is required to ensure continued
operation when the heat load increases either due to events such as a
magnet quench or vacuum leak, or because refrigeration capacity falls
below its intended design value (i.e. decreasing machine efficiency or
inefficient heat transfer due to contamination). With this scheme, all
six refrigerators must be operational for the accelerator to function.
No plant redundancy is built into this design.

![](media/image5.png){width="2.8847222222222224in"
height="3.251388888888889in"}

Figure 4.

The process helium is compressed from suction pressure of 0.111 MPa
(1.01 bar) up to an intermediate pressure of 0.41 MPa (4.0 bar) by means
of first stage compressor discharging into a second stage, which
compress the helium to 1.88 MPa (18.5 bar). Turbo machinery is used for
the refrigeration plants while reciprocating machines comprise the
compressor system. There are two cold boxes: the upper and lower cold
boxes. A cold box contains five (5) plate-and-fin type heat exchangers
and three expanders. To avoid return helium gas instabilities due to the
elevation change from the tunnel to the surface, the lower cold box will
be located at the tunnel level. This requires a dedicated space of
16,000 cubic feet (20\' along the beam by 20\' transverse by 40\' high)
located near the accelerator tunnel.

These plants are well within the existing commercial vendor
capabilities. These plants contain no nitrogen pre-cooling. The cold
configuration is a horizontal carbon steel shell enclosure housing all
heat exchangers, piping, valves, cold end of turbo expanders and vessels
operating at cryogenic temperatures. The equipment within the cold box
and the inner face of the vacuum jacket are covered by multilayer
aluminized Mylar. The cold boxes are designed to withstand the outside
atmospheric pressure while they are evacuated and are also protected
against overpressure by safety valves. All internal valves, filters,
turbo expanders, bayonets and major pipe connections are fixed at the
top plate and are accessible from a structural steel platform along the
length of the cold box. The turbo expanders are self activated gas
bearing, superior in their performance to oil bearing turbines. Each
turbo expander is a small single-stage centrifugal turbine braked by a
direct-coupled, single stage centrifugal compressor. The turbine housing
is vertically mounted to a plate at the cold box top, projecting into
its vacuum space. Within the cold box are brazed aluminum heat
exchangers, pressure vessels containing and adsorbent to remove
impurities and the cold end of gas bearing turbo expanders. Part of the
outer shell cover is flanged and has an O ring seal, and can be moved
away on wheels, for complete access to all cold box internal components
and piping. Two cold box panels will be mounted on the structural steel
platform next to the cold box. One containing all instrument
terminations and transmitters, and the other an assembly of purge and
defrost valves. Provisions for purification of the process helium will
be provided at each plant as will oil removal systems. A commercial
control system package will permit the unattended operation of these
plants.

##### 5.2.11.2.1 Helium Compressor System

The compression package will consist of three oil flooded rotating screw
compressors. One compressor will compress from 0.111 MPa (1.1 bar) to
0.4 MPa (4.0 bar), and the two other will compress in parallel from 0.4
MPa (4.0 bar) to 1.88 MPa (18.5 bar). The first stage compressor is
smaller than the two (2) second stage machines. Each compressor skid
contains the compressor, main drive motor, oil reservoir/separator
vessel, and lubrication system mounted on a common rigid skid base. The
base plate provided is suitable for installation on a flat pad without
grouting. The helium enters the compressor and is compressed through the
rotating screw compressor, also flooded with oil. The oil seals the
rotor and absorbs some heat of compression. The compressed warm helium
passes through a water-cooled heat exchanger and then into a bulk oil
separator. The oil is also passed through a separate water-cooled heat
exchanger. Each compressor is direct driven by an electric induction
electric motor. The compressor assemblies are equipped with automatic
inlet and outlet block valves and automatic recycle valves.

The high-pressure helium from the compressors passes through a skid
mounted oil removal system. The oil removal system consists of three
stages of coalescing filter housings followed by a vessel containing
activated carbon. All entrained oil is filtered out of the gas stream by
the three stages of filters. The small amount of oil in the vapor phase
is removed by the activated carbon. The total amount of oil contained in
the helium leaving the oil removal assembly is less than 10 ppb (w/w).

##### 5.2.11.2.2 Helium Liquid and Gas Storage and Inventory Management

##### 5.2.11.2.3 Cryogenic Control System

#### 5.2.11.3 Design Operating Conditions

##### 5.2.11.3.1 Steady State

Helium is supplied from a refrigerator at 4.5 K, 0.4 MPa (4 bar) and
returned at 5.8 K, 0.19 MPa (1.9 bar). Supercritical supply helium is
fed from the helium plant to four parallel loops. Each loop is comprised
of 9.5 km of magnet and transmission line. There are cryogenic
connecting boxes at the beginning and end of each 9.5 km section. At the
exit of the refrigerator the helium process is split into four (4)
paths. There are upstream and downstream magnet strings that are mirror
images of each other. The typical flow schematic of a magnet string is
shown in Figure [?]{.mark}. As helium enters the upstream or downstream
magnet strings the flow is split into two parts: half enters the 4.5 K
transmission line circuit (for the near loop) and half enters the 4.5 K
current return circuit contained in the magnet (for the far loop). There
is expected to be an approximately 1 K gradient across each loop. These
loop flows are then re-distributed at valve boxes and become the return
6.0 K shield flow.

The cooling scheme relies on a balance between heat leak and pressure
drop. Any scenario causing an upset in the heat load (i.e. vacuum
degradation) can be compensated by increasing the helium flow rate and
supply pressure for the proper loop. The refrigerator has been oversized
by 50% to compensate for these types of upset conditions.

![](media/image6.wmf){width="2.917361111111111in"
height="2.0694444444444446in"}![](media/image7.wmf){width="2.792361111111111in"
height="2.0104166666666665in"}

Figure 5. Figure 6.

##### 5.2.11.3.2 System-Wide Purification

Purification of this machine will be done in two (2) steps. First each
sector will be pumped and backfilled five (5) times to 25 inches of
mercury via pump outs on every helium circuit at each valve box located
every 10 km. This method has been proven to work in the Tevatron. Levels
of contamination less than 100 ppmv are achieved with this first step by
using roughing pumps. The second step is dilution purification. A
purifier with a nitrogen cooled charcoal bed will be located at each
refrigeration plant. The discharge gas of the compressor system will be
circulated through this purifier and then will be distributed to the
magnet/transmission line system. Contamination instrumentation, such as
arc cells used on the Tevatron, will be used to monitor the ppmv levels
in each sector. The system will be ready for cool down when
contamination levels are \< 5 ppm. It is estimated that the purification
for a sector in the VLHC will be 72 hours.

##### 5.2.11.3.3 System-Wide Cool down

Cool down of the VLHC will be done in three (3) stages. The first stage
involves the cool down of the helium plants. The second stage is the
cool down of the transmission line to 40 K. Finally the transmission
line will be cooled done to operating temperatures.

In the first stage, cooling is provided by turbines and should not take
more than 24 hours per plant. In the second stage, the plant is setup in
the liquefaction mode and 40 K gas at 0.4 MPa (4 bar) is sent via cool
down valves located at each valve box back to the compressor suction
warm gas header located in the tunnel. Gas flows from the refrigerator
into the warm header through the near loop transmission line, the 6.5 K
return, and the current return circuits. When the temperature at the end
of the "near" loop (measured at the valve box) is 70 K the flow is then
re-directed to the "far" loop and into the return header. As the "far"
loop temperature reaches 70 K the second cooldown stage can be started.

During the second cool down stage the plant is re-tuned for an outlet
temperature of 4.5 K. The flow pattern for the second stage is the same
as the first stage. As the temperature for each loop is at 7 K the cool
down valves can be closed and the flow will be looped back in
refrigerator mode.

Total cool down time is expected to be on the order of 10 days.

#### References {#references .Reference-heading}

\[1\] Koepke et al.

\[2\] DR doc

\[3\] N. Shatil final report

[^1]: current file:
    /media/pszemraj/PeterTrove/project-files/DATA/ms-word-files/big-DOC/362495.doc