# LER-LHC Injector Workshop Summary 

# and Super-Ferric Fast Cycling Injector in the SPS Tunnel

# 

Giorgio Ambrosio, Steven Hays, Yuenian Huang, John Johnstone, Vadim
Kashikhin, James MacLachlan, Nikolai Mokhov, Henryk Piekarz, Tanaji Sen,
and Vladimir Shiltsev, FNAL, Batavia, IL 60510, USA\
Gijsbert de Rijk and Lucio Rossi, CERN, Geneva, Switzerland

Abstract

A Workshop on Low Energy Ring (LER) in the LHC tunnel as main injector
was convened at CERN on October 11-12, 2006. We present the outline of
the LER based on the presentations, and respond to the raised questions
and discussions including the post-workshop studies. We also outline the
possibility of using the LER accelerator technologies for the fast
cycling injector accelerator in the SPS tunnel (SF-SPS).

## 1 Motivation

![](media/image1.wmf){width="3.4166666666666665in"
height="1.9618055555555556in"}A primary goal for the LER (Low Energy
Ring) injector accelerator is to inject 1.5 TeV proton beams into the
LHC, instead of the current injection scheme with 0.45 TeV beams from
the SPS. At this new energy, the field harmonics \[1\] of the LHC
magnets are sufficiently satisfactory to prevent the luminosity losses
expected to appear when applying the transfer of the 0.45 TeV SPS beams.
In addition, a feasibility study of batch slip stacking in the LER has
been undertaken with a goal of increasing in this way the LHC luminosity
by up to a factor of 4. A combined luminosity increase may, therefore,
be in the range of an order of magnitude. In the long term, the LER
injector accelerator would greatly facilitate the implementation of a
machine, which doubles the LHC energy (DLHC).

## 2 Layout of the accelerator and the SPS-LER-LHC beam transfer

The LER accelerator will be installed inside the LHC tunnel. This
accelerator would accept 0.45 TeV proton beams from the SPS through the
existing TI2 and TI8 transfer lines, and then accelerate these beams to
1.5 TeV, so as to better match the beam acceptance of the LHC magnets. A
preliminary outline of the LER was presented in \[2\]. The LER
accelerator would be based on super-ferric, combined function magnets.
These magnets were originally proposed for the VLHC Stage 1, a p-p
collider in the US \[3\]. A basic property is that they require only
minimal space in the accelerator tunnel. The magnet and its supporting
systems (conductors, power supply, current leads, etc.) were recently
successfully tested at Fermilab \[4-8\].

In the new LHC beam injection scheme, the proton bunch stacking and the
formation of the full intensity beam is performed in the LER ring. Once
the stacking of the clock-wise and the counter-clock 0.45 TeV beams in
LER is completed, the beams are accelerated to 1.5 TeV. At this top
energy, the beam is passed into the entire LHC ring using a single
transfer mode through one of the transfer lines. For this single
transfer, only one set of the LER transfer line magnets is to be ramped
down. The ramping down has to be done in a time period determined by the
time interval between the tail and the head of the beam train. The other
transfer lines (2 x 2 transfer lines in total) are switched off when the
hole in the beam train passes. This idea of LER to LHC beam transfer is
illustrated in Figure 1

Figure 1. Principle of the proposed LER-LHC beam transfer method

The minimum allowable vertical separation between the LER and LHC rings
is about 135 cm. In the IR regions, where the detectors reside, the
total length of the straight sections is about 528 m. The available
space, on each side of the detectors, at the LHC ring level that could
be used for inserting the LER-LHC transfer-line magnets totals only
about 80 m. This situation poses a great challenge for the transfer of
the 1.5 TeV beam between the two rings. A possible conceptual LER-LHC
transfer line design and arrangement of the transfer line magnets in the
IR1 and IR5 regions are discussed in Chapter 7. The proposed conceptual
LER-LHC injection scheme together with the LER ring arrangement is shown
in Figure 2.

A set of fast switching magnets placed in the front straight sections of
the IR2 (ALICE detector) and IR8 (LHCb detector) performs the beam
transfer into the LHC ring. We assume that in the high-luminosity LHC
operations these detectors will not be used, but the proposed scheme
will allow using these detectors any time as the LHC can be reverted to
its current-type operation.

Figure 2. Conceptual arrangement of the LER-LHC injection scheme

The beam from the TI2 transfer line enters the LHC ring at the short
straight section just in front of IR2. It travels through IR2, passes
the IR3 and IR4 areas, and is transferred into the LHC beam pipe only in
IR5 where it passes through the CMS detector. It is again bumped into
LER passing through the IR6, IR7 and IR8, and it re-enters the LHC beam
pipe at IR1 (ATLAS). Immediately after IR1 it is bumped back into the
LER ring, and will stay there until reaching IR5. The beam from TI8 has
the same path in LHC/LER as the TI2 one, except that it enters the LER
ring right after IR8 and it travels in counter-clock direction. The LER
has its own momentum and betatron clearing at IR3 and IR7, respectively
as well as its own RF system at IR4 and the beam dump system at IR6.

## 3 The VLHC Stage 1 main arc magnet and its application to LER

### 3.1 Main arc magnet design considerations

![](media/image2.wmf){width="3.5430555555555556in"
height="2.8048611111111112in"}A conceptual view of the VLHC Stage 1 main
arc dipole magnet is shown in Figure 3. The magnet features two pole
gaps between the top and the bottom half-cores. The magnetic field is
induced by a current of up to 100 kA from a single transmission line
conductor located in the centre of the half-core assembly. The field in
the pole gaps is entirely shaped by the iron. The magnet is a combined
function gradient dipole with two half-cell versions, focusing and
de-focusing, which are placed alternating along the accelerator ring.
The magnet pole gap is 20 mm high, and the beam pipe is elliptical with
an effective vacuum space of 18 mm (v) and 28 mm (h).

![](media/image3.wmf){width="3.0965277777777778in"
height="2.3333333333333335in"}

Figure 3. The VLHC Stage 1 magnet conceptual view

The characteristic measured dipole strength versus current is shown in
Figure 4.

![](media/image4.wmf){width="3.3194444444444446in"
height="1.8611111111111112in"}

Figure 4. Measured dipole field, and projected B(I) response for a 30 mm
LER magnet gap

![](media/image5.wmf){width="3.5430555555555556in"
height="2.214583333333333in"}Figure 5. Proposed new lamination design
for the LER main arc magnet

As demonstrated in Chapter 6, the required dipole field for the LER main
arc magnet is 1.6 T for a 1.5 TeV beam. This leads to a strong reduction
of magnet current (45 kA), and very importantly the magnet operations
are moved nearly out of the saturation zone. We take advantage of this
situation and propose to widen the magnet gap to 30 mm. This new gap
will help to reduce beam impedance effects (Chapter 7.1) as well as it
will facilitate implementation of bunch coalescing techniques (Chapter
7.2) with LER. The lower field also allows reducing the vertical size of
the magnet core lamination to just contain the 1.6 T magnetic flux. A
new possible lamination design is shown in Figure 5. The beam pipes are
enlarged to 28 mm (v) and 40 mm (h). New simulations are needed,
however, to determine the definitive dimensions of the lamination.

### 3.2 The LER main arc magnet conductor 

The LER main-arc dipole magnets will be powered with a single
transmission line conductor \[3\] using a single power supply \[5\] and
a single set of current leads \[6\] as shown in Figure 6.

![](media/image6.wmf){width="3.3465277777777778in"
height="2.4520833333333334in"}

Figure 6. Arrangement of the LER transmission line conductor

The drive conductor loops through half of the accelerator circle, turns
180º, runs as return conductor the full circle, turns again 180º and
then excites the other half of the accelerator magnets. This makes the
continuing length of the conductor \~ 53.3 km. As the currents in the
overlapping conductors run in opposite directions, the magnetic fringe
field of the LER magnet string is strongly suppressed at far distances,
and the magnetic field in the space between the two neighboring magnets
is totally suppressed.

Both the drive and the return conductors must bypass the detector areas.
In this area the drive and the return conductors are separated
vertically by 28 cm, and the cryostat pipe enclosing both of them is
only 36 cm in diameter. This indicates the space, which is needed in the
area behind the detectors for the installation of the bypass conductor
lines.

The conductor lines must be equipped with quench detection and
protection circuits. A conceptual design of the quench detection and
protection system for the VLHC is presented in \[9\]. With a magnet
inductance of 3 H/m, the total inductance per LER ring is \~ 80 mH, and
the stored magnetic energy per ring at 45 kA current is \~ 53.3 MJ. In
the VLHC design report \[3\] it was shown that only one quench
detection/protection circuit per 20 km of the conductor length is
required. Scaling from that study we project that with one quench
detection and protection system for the LER ring, the helium peak
temperature and pressure will rise up to \~ 65 K and \~ 9 bar.

### 3.3 The LER main arc magnet power supply

The original concept for the VLHC transmission line magnet system was to
be a p-p collider that required extended operation at 100 kA. The magnet
system would be loaded to the 3 kA level and ramp to the 100 kA level in
100 s. To provide the ramping voltage, a 400 V pulsed duty supply was to
be constructed and a power supply of lower voltage would be used to hold
the current. To provide the DC current, a switcher power supply that
operates at 4.5 V and 100 kA was developed and successfully tested
\[5\].

The LER magnet system is intended to be an injector used in support of
the LHC. This means that this magnet system will only pulse for 20
minutes at the time of reloading the LHC. All magnet systems (quad,
trim, fast pulsing and kicker charging supplies) will need to track this
ramp cycle, illustrated in Figure 7.

![](media/image7.wmf){width="3.3465277777777778in"
height="2.0215277777777776in"}

Figure 7. A characteristic current supply ramping scheme for the LER
magnets

The support magnet systems are expected to operate from a 400 V DC bus
and be constructed using standard DC-DC converter designs. The abort
kicker magnets charging power supply system are expected to be CAP
charging power supplies and be of present conventional kicker system
design

A ramping power supply will need to be constructed to provide
+20 V / -18 V @ 72 kA. This voltage assumes that a superconducting dump
switch is designed and installed into the system to avoid the 2 V - 3 V
drop of each conventional switch. The construction of this supply is
expected to extent the present FNAL designs using parallel power
supplies summed through filter chokes to provide this high current. A
reasonable design for this supply would be using four 20 kA supplies
(Figure 8) in parallel with a peak feeder loading 1.1 MVA..

Figure 8. A design for a 20 kA supply

Each of the 20 kA supplies is constructed using two 10 kA bridges
supplied from one Delta-Delta and one Delta-WYE transformer, summed
together through two chokes and then filtered. Using FNAL design
criteria the bridges would be constructed for full current, 10 kA each
while the transformers need only be rated for the half the power between
the RMS and Peak power values. We recommend that the RMS value should be
the injection value rather than zero thus allowing for LER to sit at
injection level for an extended period of time, if needed. Therefore the
RMS value should be 315 kVA and the peak value 1100 kVA, and then the
transformers would need to be rated for 834 kVA (210 kVA each pair) and
braced for the full pulsed current at the output and peak loading at
1.1 MVA.

![](media/image8.wmf){width="3.2930555555555556in"
height="1.5479166666666666in"}This power supply design builds on the
Main Injector dipole power supply system \[10\], which uses 10 kA SCR
bridges in parallel. At the FNAL Magnet Test (MTF) a 30 KA system
exists, that uses 6 power supplies paralleled through chokes and
bus-work. The current feedback for the MTF system is provided by summing
5 kA DCCT's for at the output of the summing chokes but each 12 pulse
supply is used as a regulated source. A 418 V, 10 kA DC supply was
constructed under a development project for the Main Injector power
supply bridges and could be used to further detail the design of this
power supply. The Main Injector power supply bridges are constructed
with three parallel SCRs and use a cable bus system to the transformer
to assist in current balance and sharing. This magnet power supply
system also can operate in a mode where the transformers are pulsed
above the RMS rating. It will need a harmonic filter installed on the
main feeders to control the pulsed current and the power factor. The
design elements for this power supply are available now and need to be
only organized into a system. It is very important to construct the
power leads to be as close as possible to the power supply to limit the
difficulty of building a high current distribution bus

### 3.4 The LER main arc magnet cryogenic support

![](media/image9.wmf){width="3.384027777777778in"
height="1.7916666666666667in"}The cryogenic heat load of the main arc
magnet is generated predominantly from the conductor support inside the
cryostat. This support is made of Ultem™ rings

Figure 9. Cryogenic support ring and shield for the LER main arc magnet
conductor

with 4 pegs every 0.5 m as shown in Figure 9. The inner pegs hold the
conductor, while the outer pegs hold the cryostat vacuum pipe. The
thermal shield at 50 K is also attached to the rings through an aluminum
pipe. Both the shield and the conductor are wrapped with in total 60
layers of the MLI super-insulation. The heat loads due to main arc
magnet conductor are summarized in Table 1. There are eight arcs in the
LHC each with corresponding cryogenic support of an 18 kW cryo-plant.
The total LER main arc magnet conductor heat load is \~ 4 kW, small
enough to be supported from the existing LHC cryogenic system which
consists of 8 refrigerators, each of 18 kW power at 4.5 K, and 600 kW
power at pre-cooler.

Table 1. The LER main arc magnet conductor heat load

### 3.5 Arrangement of the LER magnet ring in the LHC tunnel

The inspection of the LHC accelerator tunnel leaves only one possibility
for the location of the LER magnet, and that is in the space above the
LHC magnet as shown in Figure 10. The return conductor of the LER magnet
is placed in the space above this magnet thus minimizing a potential
effect of LER magnet fringe field on the LHC operation. Fringe field
simulations on the LER magnet are needed to determine if magnetic
shielding is needed to protect the LHC.

![](media/image11.wmf){width="2.5590277777777777in"
height="1.8256944444444445in"}

Figure 10. Arrangement of the LER and LHC magnets in the tunnel

The magnet will be supported from two crossing I-beams fastened to the
top and the side of the concrete wall of the tunnel. The I-beam spacing
is to be determined by an engineering analysis of the strength required
to support the magnet weight of 500 kg/m. There are, however, some
obstacles for the LER magnets, typically located in straight sections
IR1 and IR8. The main obstacles are: (1) the LHC magnet power cables on
the distribution boxes which are fed to the magnet top (\~ 10 m long
space at each side of each IR is used), (2) helium feeds from the top or
from the sides at all IRs (\~ 15 m space), and the power cables for the
accelerating cavities at IR4 (\~ 50 m). Although a re-arrangement or a
bypass of these obstacles may not be easy, it does not appear to be
un-surmountable task.

### 3.6 LER main arc magnets and correctors count

![](media/image12.wmf){width="3.209722222222222in"
height="1.8319444444444444in"}As thoroughly analyzed in \[3\], the main
arc magnets, which are of combined function design, require a set of
correctors on every half-cell. These corrector magnets can be normal
conducting.

![](media/image13.wmf){width="3.2680555555555557in"
height="1.9131944444444444in"}

Figure 11. Arrangement of the main arc dipole corrector magnets

There is about 5.5 m free space between half-cells of main arc magnets.
The correctors use this space where the drive conductor will be moved up
by 230 mm, to be next to the return conductor as indicated in Figure 11.
Each corrector magnet set consists of a sextupole, a quadrupole and
interchangeably a vertical or horizontal dipole for every half-cell. A
summary of the LER magnet count is shown in Table 2.

![](media/image14.wmf){width="3.326388888888889in"
height="1.1340277777777779in"}Table 2. The LER main arc magnet count

## 4 LER-LHC transfer line magnets

### 4.1 Conceptual arrangement of the LER-LHC transfer line

Beam transfer from the LER ring into the LHC ring is the most
challenging task of the LER proposal that needs to be very seriously
dealt with. The vertical separation of the LER and LHC rings can be made
to be 135 cm. This means that the 1.5 TeV beam needs to be bent down (or
up) out of the LER (or LHC) ring, transported, and then bent into the
LHC (or LER) ring over a vertical distance of 135 cm. A free drift space
spans only 70.5 m mostly between D1 and Q5. The basic concept of the
LER-LHC beam transfer is shown in Figure 12, and the beam line design is
presented in Chapter 7. There are two sets of bending magnets. The first
one lifts the beam over the D2 and the second one puts the beam at
1.35 m above the LHC ring. The 1.35 m level is the nominal level of the
LER ring.

Figure. 12. The basic concept of the LER to LHC beam transfer

In the horizontal plane the clock-wise and counter-clock-wise turning
LHC beams have no separation at D1, but they are separated by 194 mm at
D2. The first LER magnet pair of the first vertical bend section must be
placed at a location where the clock-wise turning and counter-clock-wise
turning beams are separated enough to allow for operating these close
neighbouring magnets with a good magnetic field quality. A preliminary
magnetic design suggested that a 110 mm - 130 mm beam separation should
be sufficient. However, with the present LHC design such a beam
separation is only in achieved in about 2/3 of the D1--D2 distance of
86.6 m, making it impossible to accomplish the two first LER beam
transfer bends before the D2 magnet. The required horizontal LHC beam
separation can be only achieved in a shorter distance from D1 by
redesigning the D1 and D2 LHC dipole system. A conceptual proposal of
the new D1 and D2 dipole arrangement together with an outline of the
LER-LHC transfer line magnets is shown in Figure 13.

![](media/image15.wmf){width="3.3229166666666665in"
height="1.7777777777777777in"}

Figure 13. Conceptual arrangement of magnets in the LER-LHC transfer
lines

![](media/image16.wmf){width="3.2930555555555556in"
height="2.113888888888889in"}

Figure 14. Timing sequence for ramping LER and beam transfer to LHC

The LHC D1 dipole magnet is split into two parts, D1a and D1b, with a
beam drift space of about 12.5 m between them. Assuming that a magnetic
field of 8 T is feasible for the new D1 magnets, this drift space will
allow the creation of a separation of the clock-wise and
counter-clock-wise turning beams by up to 130 mm at the exit of D1b,
sufficient for the installation of the V1-5 fast switching magnets. The
integrated field strength of the V1-5 magnets allow for the LHC and LER
beam pipe separation at the end of their path. All the following sets of
transfer line magnets are well above the nominal LHC beam line, and so
they can be in a continuing mode of operation, as their magnetic fields
are not interfering with LHC beams.

In order to understand the choice and arrangement of magnets in the
first bending set we must look at the timing sequence of the SPS-LER-LHC
beam transfer scheme. This is schematically indicated in Figure 14. When
the SPS is ready for beam transfer, all LER magnets, including those in
the transfer lines, are ramped up to the required fields for the
0.45 TeV beam. A ramping time of 90 s is characteristic of the main arc
LER magnets. The stacking of the first LER beam begins and it lasts for
about 3 min. Then the stacking of the 2^nd^ LER beam begins and lasts
for about 3 min as well. As soon as the beam stacking is complete the
LER magnets ramp to 1.5 TeV. The 1.5 TeV beams may circulate for
1 min - 2 min, or so, to allow e.g. for beam tuning if needed, and then
the fast switching LER-LHC transfer line magnets are turned off, forcing
the beams to circulate from now on in the LHC ring only. At this point
all remaining LER magnets are ramped down. The operation of the fast
switching magnets is more complicated, however, due to the fact that for
each beam the magnets on the opposite sides of the IR must work in
tandem. This is illustrated in Figure 15.

![](media/image17.wmf){width="3.3583333333333334in"
height="1.8159722222222223in"}

Figure 15. Timing relation between neighboring fast switching magnets

When the tails of the LER beams pass magnets FSD3 and FSD2, these
magnets have 3 s to be completely turned off before the heads of the LER
bunch trains will reach again this position. But the FSD1 and FSD4
magnets must operate for an additional 86 s until the entire LER beams
are transferred into the LHC ring. So, the FSD magnets on the same side
of the IR have opposite timing sequences during the beam transfer. This
means that the fringe fields of these magnets must not extend into the
beam gaps of their counter-parts in the neighbouring accelerator ring.

### 4.2 Fast switching dipoles for the LER-LHC transfer lines

### 4.2.1 Magnetic design considerations

![](media/image18.wmf){width="3.3in" height="1.4229166666666666in"}The
fast switching magnets must be powered with a single conductor in order
to minimize the inductance. Typically, such magnets can be designed with
an inductance of \~ 1 H for a length of \~ 1 m. In order to minimize the
required current for the 1.5 T field in the 40 mm magnet gap, we
designed a magnetic core that "boosts" the gap field by as much as
factor 2, and so the required power supply current is \~ 50 kA. With a
1 H magnet inductance, the peak voltage drop with a 3 s turn-off time is
then \~ 27 kV, plausibly manageable.

There are two options for the magnetic design: (1) based on parallel
plate conductors, and (2) based on cosine shaped conductors. We selected
the first one as a primary choice due to its simple mechanical structure
and the much lower Eddie current effects. The core is made of Fe3%Si,
100 m thick laminations, to meet the criteria for operating in a
\~ 3 sec time range. The projected B(I) response and laminated magnetic
core design are shown in Figure 16. B-field to current response is
linear up to 1.5 T, the field range of the fast switching magnets.

![](media/image19.wmf){width="3.334722222222222in"
height="2.2395833333333335in"}

Figure 16. A projected response of B(I) for a parallel plate magnet with
a 40 mm gap and a magnetic core of laminated design

### 4.2.2 Conductor and mechanical design considerations

The cross-section area of a normal conductor for the fast switching
magnet is \~ 436 mm^2^. With such a small cross-section, the current
induced Joule heating makes it difficult to operate with a 55 kA current
for a time period of 10 min, the time needed for the LER beam stacking.
The best solution for the conductor Joule heating problem is to use a
superconductor. In this case the heat load is mostly due to the
conductor cryogenic support but the heat generated by Eddie currents
must also be considered. The Eddie currents generate an opposite sign
magnetic field in the magnet gap. This, in turn, slows-down the current
decay time when the magnet is switched off. This effect can be
compensated at the expense of additional power (current) given to the
conductor. A summary of projected Eddie current B-fields with associated
heat loads for a 55 kA magnet current using conductor strand designs of
NbTi, HTS-344S \[3,11\] as well as the OFHC copper strand of RRR = 1000
is shown in Table 3.

Table 3. Some properties of the conductors considered for FSM magnets

![](media/image20.wmf){width="3.3180555555555555in"
height="2.388888888888889in"}The Eddie current effect is minimized by
suppressing magnetic flux which crosses the conductor. The CERN kicker
magnet \[12\] is a good example of a design where the Eddie current
induced field distortion during the 3 s ramp to 0.6 T field is at most
5%. The magnetic design shown in Figure 15 is consistent with the CERN
design, and so we expect only minor field distortions due to Eddie
currents as indicated in Table 3. As the magnetic field distortion
occurs only during the gap between bunches (when one of the magnets is
turned off) it will not affect the particle beam, provided that the
power supply was successfully turned off and so the magnet current has
fully decayed. The strength of the Eddie current B-field may have strong
impact on the mechanical structure of the conductor assembly and
therefore it should be minimized. As the duration of the Eddie currents
is limited to an \~3 s time period, it has only a small effect on the
overall conductor heat load.

Figure 17. A conceptual design of current leads for the FSM magnet

FSM magnets using superconductors, will have their total heat load
dominated by the current leads. Current leads based on HTS
superconductors offer the lowest heat load, and therefore rather low
liquid helium consumption as demonstrated recently in \[13\]. We project
for the FSM magnet current leads, operating at a current of 60 kA, that
the heat load will be \~ 6 W per lead, and the total heat load for all
fast switching magnets in IR1 and IR5 will be \~ 0.7 kW, which is easily
supported from the existing LHC cryogenic system. A conceptual design of
the FSM magnet current leads is shown in Figure 17. In this design the
power connection to the magnet is made using two-step current leads. The
HTS leads must have enough superconductor material to carry the current
required for the magnet up to 70 K. At this temperature conventional
leads will make the connection to the power supply. The liquid helium
flow is used only to cool the conductors inside the magnet, and
conduction cooling is used for the HTS leads. At the location where the
HTS leads are spliced to the conventional Cu leads, the 60 K helium gas
flow is used to provide cooling. The length of the copper leads
(\~ 0.8 m) is calculated so that at the connection point to the power
supply they are at about room temperature. In Figure 18 two possible
arrangements of ![](media/image21.wmf){width="3.2993055555555557in"
height="1.4458333333333333in"}NbTi and HTS-344S superconductors into
conductor bars are shown.

![](media/image22.wmf){width="3.254861111111111in" height="2.23125in"}

Figure 18. Arrangement of NbTi and HTS (344S) strands into a conductor
bar

![](media/image23.wmf){width="3.338888888888889in"
height="1.6652777777777779in"}Figure 19. A conceptual arrangement of the
FSM magnet pair

Each strand is electrically insulated using a polyimide tape of 0.025 mm
thickness. The conductor strands are held inside a 316LN case, which is
insulated for a voltage larger than 100 kV with 10 layers of 0.3 mm
Nomex tape. A possible mechanical arrangement of the FSM magnet pair is
shown in Figure 19. These magnets would use a ceramical beam pipe and
the conductor bars will be thermally isolated from the magnetic core
with help of G11-CR supports.

### 4.2.3 The FSM magnet list

Using the above magnetic and mechanical considerations for the FSM
magnets and the transfer line design described in Chapter 6, we derive
the list of transfer line magnets as shown in Table 4. The guiding
principles were: (1) - not to exceed the 1.5 T B-field, and (2) - to
keep the single magnet inductance under \~ 1.5 H. This low inductance
allows for keeping the peak voltage below 45 kV during the magnet
turn-off time, and thus being well below the electrical insulation
design limit at 100 kV.

Table 4. The FSM dipole magnet list

### 4.2.4 Fast switching magnet power supply considerations

![](media/image24.wmf){width="2.384027777777778in"
height="1.7097222222222221in"} Turning off the magnet's supply current
in a time span of 3 s is the most challenging part of the LER transfer
line proposal. A short current decay time constant must be enforced on
the system. Typically, an IGCT (Integrated Gate Commutating Thyristor)
device installed in-line with the magnet leads, is used for such
applications. A new idea is proposed for enforcing fast magnet current
decay that takes advantage of the cold environment around the magnet,. A
conceptual arrangement of such a system is shown in Figure 20, and its
electronics circuitry is shown in Figure 21

Figure 20. Conceptual arrangement of FSM magnet, current leads and pulse
kicker PS

![](media/image25.wmf){width="3.1729166666666666in"
height="1.6430555555555555in"}

Figure 21. The FSM magnet pulse kicker power supply arrangement

This power supply system concept combines three elements of presently
used superconducting magnet systems: a forced commuted dump switch, a
kicker power supply and a switcher supply. One of the current leads to
the magnet is instrumented with a pulsed commutation inductor (CI). This
inductor will be smaller if it is situated in the cold area but it could
also be in the warm area inside the high voltage tank of the kicker
power supply. If installed in the cold area, an additional pulsed
current lead would need to be brought out of the magnet to connect to
the kicker pulser. In any case the inductance added to the system by its
physical constraints will need to be minimized using either strip-line
or micro-strip methods in the HTS magnet conductor leads.

In order to stop the magnet current, the CI must provide a higher
impedance than that of the magnet and the common point is pulsed from a
kicker style power supply to 30 kV and 50 kA for 3 s. This increases the
current in the inductor from 50 kA to 58 kA and the current in the
magnet from 50 kA to zero in 3 s. At the time the current reaches zero,
the IGBT switch is turned off. This allows for zero current switching of
the IGBT and helps to reduce the blocking voltage needed for the switch.
To protect the switch from turning the device off at high current, a
self-retrigger circuit will be installed. The superconductor may heat-up
due to Eddie currents during the pulse and stop being superconducting,
forcing the current to commutate into the SS substrate of the HTS
conductor and in this way creating a shunt resistor. This shunt
resistance increases the series resistance of the magnet, which may
decrease the turn-off time. The speed of this commutation is unknown but
should have little effect on the overall speed of the system.

A more detailed block diagram of the FSM magnet power supply system is
shown in Figure 22. The left side of the diagram represents the switcher
power supply and the output filter. This switcher supply is a subset of
the holding supply, which was developed and tested for the VLHC. Due to
the low inductance and the low ramp rate of the system, the output of
the supply is bypassed with diodes that are used to decrease the
reflected inductance to the kicker pulse which lowers the required
voltage. The bypass diode will need to be located close to the magnet to
minimize the stray inductance, but allowing in this way for the switcher
to be located farther away from the magnet. The energy stored in the
switcher power supply filter inductance is discharged at a slow rate
through these diodes and the bus resistance. The IGTB is turned off at
the zero current point, which then blocks the positive voltage on the
bypass diodes, +18 V, and ‑18 V on the ring down diode across the pulsed
inductor which is the result of the high current in the diodes. The 36 V
would try to refill the magnet resulting in a non-zero current for the
decay time of the stored energy in the inductors. Even though the IGBT
is a fast turn-off device the need for voltage protection will require a
ZNR and snubber cap that will prevent the switch from acting as if truly
open to the magnet. The details of this protection will have to be well
understood to insure that the voltage on the device is kept low during a
false turn off and that the current leakage back into the magnet is at a
minimum.

The kicker pulse circuit is shown on the right side of the diagram. This
system uses a proven design that incorporates a solid-state turn on
switch to pulse the current and the voltage. There have to be two of
these switches installed in the system in a parallel arrangement. The
high-speed forward conduction path of the kicker is from the kicker cup
through the kicker switch on the magnet to the IGBT, and then to the
bypass diode.

![](media/image26.jpeg){width="3.363888888888889in"
height="2.092361111111111in"}

Figure 22. More detailed schematic of the FSM magnet power supply system

This current path will need to have a strip-line construction to keep
the stray inductance as low as possible. When the kicker switch turns
off at the end of the pulse the commutation inductor current has to
switch fast into the commutation diode to minimize the re-applied
voltage to the magnet and the IGBT switch. The high-speed current
switching into the bypass diode, kicker diode and commutation diode will
require that the diode stacks are constructed using kicker element
designs.

With the physical size of the system being rather large there is little
space left for the DCCT for high current feedback. Continuation of the
development of cold a DCCT at a 60 kA level that could be installed on
the power leads would allow to considerably reduce the space and to
minimize the bus work.

The theory of operation of this system is that the kicker pulser will
receive a trigger timed by the 3 s abort gap reaching the first FSM
magnet in the LER-LHC transfer line, and start taking the magnet current
to zero. At the same time the drive to the switcher power supply is
turned OFF and the output voltage will change from 4.5 V to - 0.55 V,
using the bridge diodes as a bypass. The kicker current pulse will force
to commutate the current in the magnet to zero and divert the switcher
filter current into the bypass diodes. When the current in the IGBT
switch is at zero, it is turned off and the magnet circuit is open. At
the same time the kicker stops conducting and the commutation choke
current is switched into the commutation diode.

![](media/image27.wmf){width="3.3229166666666665in"
height="1.5229166666666667in"}The two elements with high current flowing
will decay over 9 ms - 10 ms time, leaving the magnet with zero current.
As one can see in the simulated functions of the FSM magnet power supply
in Figure 23, the current is rising both in the commutation and bypass
diodes and therefore a parallel path for sharing and lowering the stray
inductance will be required. The current in the commutation diode has an
oscillation that is the result of the current in the kicker pulse
inductance, cap bank diode and the commutation inductor ringing.

Figure 23. Simulation of currents and voltages of the FSM magnet power
supply during turn-off

### 4.3 Other LER-LHC transfer line magnets

The last pair of fast switching dipoles, V5 ( see Figure 12), separates
the LER beam path from the LHC beam path by 46 mm. This separation is
increased to 75 mm by allowing the beam to drift for about 4 m. The
75 mm separation is sufficiently wide to install the LHC beam pipe
inside the V6 magnet core without affecting its magnetic properties. The
arrangement of the first pair of the V6 magnet string with LHC beam
pipes inside their cores is shown in Figure 24. When the LER beam is
deflected vertically to about 115 mm, the LHC beam pipes are totally
outside of the V6 magnets cores (Figure 12). As the V6 magnets carry
only the LER beam, a beam pipe with a diameter of 30 mm is sufficient.
This in turn, allows to increase the magnetic field to 1.95 T while
keeping the magnet current at 55 kA. The entire string of V6 magnets is
energized by a single power supply. The string of six V6 magnets lifts
the LER beam to about 170 mm above the LHC beam path. This gives enough
separation from the LHC beam path to install the V7, high-field type
magnets. In order to fit the entire LER-LHC transfer line in the beam
path ending before the Q5 magnet of the LHC, all V7 magnets have to
operate at no less than 8 T field. The V7-type magnets are vertical
bending dipoles that have shared magnetic cores.

Figure 24. A pair of V6-type magnets with LHC beam pipes embedded in
cores

As shown in Figure 25, the LHC beam pipes are well outside the core but
for the first string of V7 magnets the LHC beam pipes may have to be
inside the cryostat.

![](media/image29.wmf){width="3.1506944444444445in" height="2.7in"}

Figure 25. Conceptual arrangement of V7-type magnets at closest distance
to LHC beam pipes

The new D1a and D1b are part of both LHC and LER. As discussed in detail
in \[2\] these new D1 dipole magnets, with large apertures up to 140 mm,
can be built to operate at a 8 T field using existing technology based
on NbTi superconductors. In Figure 26 we illustrate the feasibility of
such magnets using an analytical approximation of B~c~ (the central
dipole field when the peak field is at the short sample limit) presented
in \[14\]. The 8 T dipole, with an inner radius of 70 mm, is feasible
using a 2-layer cable of 15 mm ‑ 16 mm width and under the assumption of
a reality factor of 85%. The feasibility of D1a and D1b from a
mechanical and quench protection point of view is based on the
experience with MFRESCA \[15\], a large aperture (88 mm bore) dipole
operating at 10 T. As the projected magnetic force ratio of D1 is almost
equal to the magnetic force in MFRESCA a similar mechanical design will
be adequate. MFRESCA has only 60% of the projected stored energy of D1
at maximum field, but the energy density is very close because of the
larger coil volume of D1. An adequate quench protection could be
provided by using a fast quench detection system, with a large heater
coverage (in order to spread the energy over the whole coil volume), and
by a selecting large strand diameter (in order to reduce the inductance
and therefore peak voltages).

![](media/image30.wmf){width="3.342361111111111in"
height="2.1319444444444446in"}Figure 26. Projected central field (T)
versus coil width (mm), when the peak field is at the short sample
limit, at 4.2 K (red), at 1.9 K (blue), and at 1.9 K assuming 85%
reality factor (green) for a large bore D1 dipole

## 5. The LER RF system

The LER accelerator must have its own RF system, so the acceleration of
the LER beams can be performed independently of the status of the LHC.
However, as shown in Chapter 7.2, the LER RF system requires twice the
voltage of the LHC RF (32 MV/m versus 16 MV/m) if the LER beam losses
caused by bunch coalescing should be kept down to an acceptable 5%
level. Recently, the superconducting cavities operating at
35 MV/m - 40 MV/m fields, were successfully tested \[16\], and a strong
development of high-field superconducting and normal conducting cavities
is being vigorously pursued at present. For the symmetry with the LHC RF
system the LER RF system should also be placed at IR4, but other
locations, such as e.g. IR6, may also be considered. A possible
arrangement of the LER RF system is shown in Figure 27. Although the two
sets of LHC and LER cavities can certainly fit together in the IR4
space, adding there the LER RF associated power supplies may be very
difficult. Consequently, one should plan for expanding the tunnel area
including building alcoves, etc.. LER at IR4 will have its own set of
dipole and quadrupole magnets facilitating proper horizontal clock-wise
and counter-clock-wise beam separation

![](media/image32.jpeg){width="3.3465277777777778in"
height="2.2916666666666665in"}

Figure 27. Possible arrangement of the LER RF system at IR4

## 6 The LER lattice design

A preliminary optics for the LER is presented in \[17\]. The arc and
dispersion suppresser cells were designed to replicate both the LHC
optics and the accelerator footprint. The deduced parameters for the LER
main arc magnets are shown in Table 5.

Table 5. Magnet parameters at 1.5 TeV in the arc and dispersion
suppresser cells

The B-fields and dimensions are smaller than the corresponding ones in
the VLHC design \[3\], thus suggesting the possibility of an even higher
field quality as well as facilitating easier fabrication and
installation in the tunnel.As pointed-out earlier (Chapter 4.1), at IR1
and IR5 the LER and the LHC must share the LHC Q(1-3) triplets and first
separation dipole D1. In the present LHC design the divergence of the
beams exiting D1 is so gradual (1.11 mm/m) that the LER-LHC transfer
line magnets can not be installed until just \~ 37 m from the face of
D2. This distance is much to short to allow the LER beams to clear D2/Q4
magnet combo vertically. Consequently, the D1 dipole has to be replaced
with a high-field, \~ 8 T, D1a magnet and at a 12.5 m distance, with an
opposite polarity strong dipole (\~ 8 T) D1b. Both LER and LHC beams
require short D2 magnets to bring the horizontal separation of 150 mm
for LER and 194 mm for the LHC. A list of LER-LHC transfer line magnets
is shown in Table 4 (Chapter 4.2.3) and the list of the new D1 and D2
magnets is given in Table 6.

Table 6. Parameters of D1 and D2 dipoles for 1.5 TeV beams

![](media/image33.wmf){width="3.3625in" height="0.9493055555555555in"}

The LER lattice functions at IR1 and IR5 extending through the first
full arc cell on each side of the straight section are illustrated in
Figure 28. The LER optics is matched to the LHC injection ^\*^ = 18 m,
and  (max) = 400 m. The corresponding quadrupole parameters of the IR1
and IR5 straight sections for 1.5 TeV beams are listed in Table 7.

![](media/image34.wmf){width="3.342361111111111in"
height="1.9388888888888889in"}

Figure 28. LER lattice functions at IR1 and IR5 straight sections.

Table 7. Straight section LER quadrupole parameters at IR1 &IR 5 for 1.5
TeV

![](media/image35.wmf){width="2.2993055555555557in"
height="1.8722222222222222in"}

There are two more quadrupoles in LER on each side of the IR than in the
LHC. This is due to additional LER constraints of cancelling vertical
dispersion and its derivative. The quadrupole # 9 is located directly
above the LHC Q7.

## 7. LER accelerator physics issues

### 7.1 The LER accelerator parameters and beam stability

The LER accelerator parameters are listed in Table 8. For comparison the
parameters of the SPS are also presented as the LER is intended to
replace the SPS as the main injector for the LHC. In addition the
injection and extraction energies the main difference between the two
machines is the size of the beam pipe as well as the average bunch
current.

Table 8. LER vs SPS parameter set

![](media/image36.wmf){width="3.3465277777777778in"
height="2.0416666666666665in"}

A comparison of the predicted LER instabilities versus those of the SPS
is shown in Table 9. One can see that the TMCI and the space charge
effects are considerably better with the LER. Also the LER resistive
wall coupled bunch is improved as compared to the SPS one. The AC tune
shift, however, must be improved e.g. by using a special AC quad, or via
special loading. In summary, with special care the LER-LHC system may be
able to handle twice the bunch current. The LER must use an Al beam pipe
of a = 14 mm and coated with a 50 m layer of copper or silver.

![](media/image37.wmf){width="2.6020833333333333in"
height="1.1284722222222223in"}Table 9. Beam stability of LER versus SPS

Several methods are conceptually available to increase the beam
intensity in the LER. These include bunch coalescing, momentum stacking
and slip stacking. All of these processes are used in the FNAL injectors
but only slip stacking seems practical in the LER. For example, bunch
coalescing requires that the bunches which are to be coalesced should
ideally be in adjacent buckets while the bunches in the LER will arrive
with the nominal ten buckets spacing foreseen for the LHC bunches.
Momentum stacking requires a very large horizontal aperture so that
bunches can be injected far off momentum.

Slip stacking is a process whereby two adjacent batches are combined to
form a single batch of approximately twice the bunch intensity. In a
collider the luminosity is given by:

![](media/image38.wmf){width="1.8069444444444445in"
height="0.4736111111111111in"}where, *f~rev~* is the revolution
frequency, \* is the beta function at the IP, is the emittance, *M* is
the number of bunches, *N~b~* is the bunch intensity and *F* is a form
factor which depends on the crossing angle and the bunch length ~s~. The
luminosity is increased four-fold if the bunch intensity is doubled
while keeping the same number of bunches. The expected luminosity
increase will be slightly less since there will be losses during the
slip stacking process. For example in the FNAL Main Injector, where slip
stacking is part of everyday operations, losses are typically about 7%.

The slip stacking process to coalesce two adjacent bunches in the LER is
shown schematically in Figure 29 by the RF frequency curves. After
injection, the first batch is accelerated to an energy E above the
reference energy using a group of cavities tuned to a frequency f~rf~
 - f~rf~ slightly below the reference RF frequency f~rf~. This frequency
difference f~rf~ is chosen so that the buckets for the two batches are
well separated in energy. For our studies we chose f~rf~ = 5 f~s~, where
f~s~ is the synchrotron frequency. This corresponds to an energy
difference E = 0.81 GeV at the injection energy 450 GeV.

![](media/image39.wmf){width="3.1458333333333335in" height="1.6625in"}

Figure 29. Schematic of a slip stacking process to capture 2 adjacent
bunches

The second batch is injected on-momentum and both batches are then
decelerated by an energy E/2. This preserves their relative energy
difference of E. The batches are allowed to circulate in the ring for
the time it takes for the lower energy batch to nearly catch up with the
slower moving higher energy batch. The energy difference between the
batches is decreased by changing the RF frequencies. At the moment that
they are exactly aligned, they are captured in a single RF system at the
reference frequency f~rf~.

The energy difference just before recapture and the voltage of the RF
capture system need to be chosen very carefully to minimize losses and
emittance growth. Detailed simulations of the injection, slipping and
capture process have been performed with the ESME code. These
simulations show that emittance growth and beam loss during the slipping
is negligible. The losses occur mainly during the final stages of
reducing the energy difference and recapture. Figure 30 shows the net
beam loss as a function of the capture voltage obtained after careful
optimization.

![](media/image40.wmf){width="3.161111111111111in"
height="1.7159722222222222in"}

Figure 30. Beam losses versus capture RF voltage during batch recapture
process

At 16 MV, the losses are about 12% and fall with voltage to around 4% at
32 MV. Increasing the voltage further does not significantly reduce the
losses. The losses are then determined by the pre-capture stage. The
drawback of higher capture voltages (besides cost and space) is that the
longitudinal emittance of the captured bunch increases with the capture
voltage as √V. Thus an RF capture voltage of 24 MV where the losses are
about 6% might be sufficient.

Extraction from the LER can only be done once for each beam so all
batches have to be coalesced at once. The batch length from the SPS will
have to be adjusted so that the abort gap in the LER is at least as long
as or slightly longer than the batch length. We assume that 12 SPS
batches fill the LER. After the first set of 12 batches is injected,
they are accelerated by E. Then the next set of 12 batches is injected,
one at a time into the LER abort gap. After both sets of 12 batches are
injected, they are allowed to slip and then captured when they align as
described above. Following the capture, these higher intensity bunches
are accelerated to top energy and extracted to the LHC.

## 8. Detector and accelerator component safety with LER

### 8.1 The LER beam dump system

The LER accelerator will use its own beam dumps systems. They will be
located in IR6 where also the LHC beam dumps are situated. As the LER
ring is 1.35 m above the LHC ring, the beam dump magnetic components
will not interfere with the LHC ones, as indicated in Figure 31.

![](media/image41.wmf){width="3.3229166666666665in"
height="1.6493055555555556in"}

Figure 31. Conceptual arrangement of the LER beam dump system for the
counter-clock beam

The LER beams will be dumped into the upper level tunnel locations on
both sides of the IR6. The magnetic bending power to send the LER beams
into a dump is considerably smaller than that for the LHC beams due the
5 times lower energy and the fact that the LER ring is already at very
high elevation as compared to the LHC.

### 8.2 The LER pilot beam bunches

The CERN PS is able to inject a single low intensity bunch into the SPS,
which after ramping to an energy of 0.45 TeV is then injected to the LHC
(pilot bunch). This allows testing of the LHC readiness to accept the
high intensity beam at 0.45 TeV. The 0.45 TeV beam stacking in both LHC
rings takes about 20 minutes, after which time the LHC magnets are
ramped to 7 TeV. With the LER injector the beam stacking takes place in
the LER rings instead of the LHC ones. Consequently, the single PS/SPS
pilot bunch that is now injected to the LHC can be injected to the LER,
and then used for testing the common LER and LHC accelerator systems in
a "pre-stacking" time period.

![](media/image42.wmf){width="3.279861111111111in"
height="1.3291666666666666in"}Figure 32. The LER beam bunch train and
the excitation function of the LER-LHC transfer line magnets to transfer
a pilot beam batch to LHC

![](media/image43.wmf){width="3.263888888888889in"
height="1.99375in"}The stacked beam bunch train in the LER will also
contain a short (e.g. 1 µsec long) beam batch separated by 3 µsec gaps
on both sides. As shown in Figure 32, by using a sine-wave function for
the excitation of the fast switching LER-LHC transfer line magnets it
will be possible to inject this beam batch as the LHC pilot beam to test
the LHC at the 1.5 TeV level, before the transfer of a full intensity of
the stacked LER beam to the LHC is enforced.

Figure 33. Possible sequence of pilot beam transfers for testing the LER
and LHC

### 8.3 Detector and accelerator emergency protection from a failed LER beam

In the situation of a major catastrophic failure of the LER accelerator
components e.g. just before transferring the 1.5 TeV beams into the LHC,
and with a danger that large portion of the beam could not be extracted
in time into the IR6 beam dump, one may resort to a localized LER beam
dump system. Such local beam dumping systems can only be placed in the
accelerator straight sections. One of possible locations is the LER beam
drift space at IR1 and IR 5 between the LER Q5 and Q6 (equivalent to the
LHC ones) as shown in Figure 34.

![](media/image44.wmf){width="3.311111111111111in" height="1.64375in"}

Figure 34. Conceptual arrangement of a local beam dump for the
counter-clock LER beam

This beam dump will consist of a fast kicker magnet string (FKM) and a
beam absorber. The FKM magnet string has to lift the LER beam by at
least 25 mm to be 10 mm above the LER beam pipe before entering the
dump. With a FKM magnet B-field of 1.5 T, a 13 m long string of magnets
will lift the 1.5 TeV beam by 28 mm, and a beam dump of 5 m length is
sufficient to stop the beam and to contain a large part of the induced
radiation. The total length of the beam dump system is then \~ 18 m,
much less than the available space of 27 m between the Q5 and Q6. The
FKM magnetic field will have the rise-time of 3 s, and its flat top will
have to last for at least half of the LER beam circulation time, or
45 s.

These local beam dump systems at IR1 and IR5 provide an important
improvement of the protection not only for the detectors but also for
the LHC and LER components in those areas. Any malfunctioning of the LER
transfer line magnets can trigger the powering of the FKM magnets. As
these magnets are very close to the LER-LHC transfer line, the only
delay in beam dumping is their rise time of 3 s. This means that at most
1/30 (3 s / 89 s) of the failed LER beam will potentially scatter into
the transfer line magnets.

The Q1,2,3 magnets are the ones most exposed to radiation caused by a
failed LER beam in the area of the fast switching magnets. In the
vertical plane the FSM's are transporting the LER beam between 0 mm and
45 mm separation from the LHC nominal beam path. As these magnets are
nearly 90 m away from the interaction region their total failure will
send beams undisturbed through the large bore HD1b and HD1a LHC dipoles,
and then into the absorbers placed in front of the Q1,2,3 triplet, as
indicated in Figure 35. The situation is more subtle, however, when only
the FSM magnet closest to the HD1b dipole fails. In this case there is a
possibility that portion of the beam will be scraping the collimator,
and then it may also be scraping the beam pipe inside the Q1,2,3
triplet. A detailed study of the failed beam trace is needed to evaluate
the level of danger to the Q1,2,3 magnets as well as to the detector
itself arising from such a situation. One way to reduce the effect of
the beam scraping inside the LHC beam pipe is to dilute the beam energy
density while the beam is still scraping the beam pipe within the FSM
magnet string. The dilution can be achieved with a help of the carbon
scatter block placed inside these magnets as shown in Figure 36.

![](media/image45.wmf){width="3.342361111111111in"
height="1.8319444444444444in"}

Figure 35. Possible arrangement of collimators for the FSM magnets
string

The carbon scatter block is about 1 cm thick, and it covers the entire
length of the beam pipe in the FSM magnet string. For the counter-clock
beam the scatter block is below the beam pipe (as shown in Figure 35),
and for the clock-wise beam it will be above the beam pipe. The
introduction of the scatter block widens the magnet vertical width. This
does not affect the current strength required for a given B-field, but
it does increase the magnet inductance, and so the peak voltage value
during the turn-on or turn-off operation is also slightly increased.

![](media/image46.wmf){width="3.2402777777777776in"
height="2.370833333333333in"}

Figure 36. A conceptual arrangement of a carbon scatter block inside the
FSM magnet.

## 9. Major LER component cost, fabrication and installation schedule 

### 9.1 Major LER component list and cost estimate

The cost in 2006 \$ was estimated by scaling down by a factor of 10 from
the VLHC proposal \[1\], and then applying the current price of the raw
materials. The cost of the power supply systems, cooling water, etc. is
included in the cost of the components.

![](media/image47.wmf){width="3.263888888888889in"
height="2.201388888888889in"}Table 10 LER cost estimate

The cost of the magnet support fixtures in the tunnel, and the cost of
some necessary modifications (e.g. possible need for enlargement of the
tunnel height/width at the transfer line locations as well as alcoves at
IR4 for the LER RF system electronics) are not included in the above
cost estimate.

### 9.2 Schedule for LER component fabrication and installation in the LHC tunnel 

A task flow-chart is shown in Figure 37, and the very tentative schedule
(see Table 11) is very speculative.

![](media/image48.wmf){width="3.252083333333333in"
height="1.363888888888889in"}Table 11 Tentative schedule

![](media/image49.wmf){width="2.5236111111111112in"
height="2.7333333333333334in"}Items 1--3 and the items 4-5 can proceed
simultaneously, but item 4 must follow items 1-2, so the magnet
production cannot start sooner than 2 years from the time "zero". As
soon as some arc magnets are produced and tested, the installation in
the LHC tunnel may begin. The overall lapsed time for the LER completion
work will be determined, however, by the number of months per year
allowed for LER installation, and the number of crews working
simultaneously on the installation in the tunnel. We assumed that 20
crews of 6 people should be able to install 40 magnets per week, or 1200
magnets in 30 weeks (\~ 8 months). Hence with one 4-months break of the
LHC operation per year, the LER installation in the tunnel may be
completed in a period of two years. In summary, the LHC operation with
the LER as injector could be ready in 6 years from time "zero".

![](media/image50.wmf){width="3.283333333333333in"
height="2.6152777777777776in"}

Figure 37. A task flow-chart for LER accelerator design and construction
based in part on VLHC Stage 1 construction schedule proposal \[3\].

## 10. Applying the LER magnet technology to a Fast Cycling SPS 

One can consider an application of the LER magnet technology described
above in other type of accelerators. Below we describe briefly a
proposal for a fast cycling SPS (SF-SPS, Super-Ferric SPS) at CERN. The
main characteristic of the LER (or the VLHC Stage 1) accelerator is that
it consists of a string of magnets energized by a common conductor (e.g.
transmission-line type) with one common power supply connected to the
magnet string via one set of high-power current leads, as shown earlier
in Figure 6. This arrangement simplifies the accelerator structure as
well as its operation, and in the process helps to significantly reduce
the overall cost.

In the VLHC and LER designs, the magnetic core determines the strength
of the B-field in the magnet gap. A field of 2 T is a practical limit
where saturation effects can be suppressed to an acceptable level. With
a 2 T field the SF-SPS can be designed to accelerate beams to 500 GeV.
The layout of the PS-SPS-LHC accelerator complex is shown in Figure 38.

Figure 38. The layout of the CERN accelerator complex

One can see that the proton beams in the PS and the SPS circulate in
only one direction. This implies using magnets with beam gaps where
their B-fields are oriented in the same direction. This requirement does
not allow application of the VLHC-type magnetic design as the field in
the gaps is of opposite direction, but the fast switching magnet design
proposed for the LHC-LER transfer lines can be adapted to the SF-SPS
(and the PS for that matter). As the energy of the beams in PS and SPS
is low, the beam pipe size is rather large (40 mm high and 70 mm wide)
and so the magnet gap has to be no less than 40 mm. This implies using a
conductor current in the range of 100 kA for a 2 T field. If we limit
ourselves to this maximum current value than there have to be two
independent magnet rings for the SF-SPS two-beam accelerator. A possible
arrangement of such a set of magnets is shown in Figure 39.

![](media/image51.wmf){width="3.1708333333333334in"
height="3.277083333333333in"}

Figure 39. Arrangement of a two-dipole magnet set for the SF-SPS.

![](media/image52.wmf){width="3.252083333333333in"
height="1.2409722222222221in"}One can easily see that it is also
possible to have 2 sets of the C-type magnets sitting side by side, and
then using only one conductor loop to generate the B-field in both
magnet gaps. This arrangement is shown in Figure 40. Although there is
only one conductor loop in the second case the number of conductor
strands is doubled of that in the first case as the current to power the
magnet for the same B-field has to be doubled. Consequently, there is no
saving in the conductor amount. In addition, the magnetic design of the
C-type magnets is considerably more difficult, especially in the B-field
range close to the saturation.

![](media/image53.wmf){width="3.2125in"
height="1.9715277777777778in"}Figure 40. Possible arrangement of C-type
magnets for 2 beam SF-SPS accelerator

The only gain is in the mechanical simplification as both magnets are
placed inside the common cryostat. In both type of magnets we installed
carbon scatter blocks to protect the conductors from failed beams. In
designing fast switching magnets we followed the principles of the
transmission line super-ferric magnet design, except for changing the
structure of the superconductor, and limiting the magnet string length
to a few meters in order to minimize the inductance, and in this way to
help accommodating the power supply fast switching-off operation.

With accelerator cycling time of 1 s - 3 s, the magnet string can be
very long, and as it turns-out even equal to the accelerator
circumference in case of the SF-SPS. The inductance for the magnets in
Figure 36 is estimated to be \~ 3 H per meter. The inductance of the one
entire SF-SPS accelerator ring is then \~ 20 mH, and the voltage
drop/rise for 1 Hz operation at 90 kA current is \~ 1.6 kV, a very
manageable value. The key then to the successful SF-SPS operation is the
level of the tolerable Eddie current induced B-fields, both in the
conductor and the magnet lamination. As these magnets must continually
cycle there is no time for recovery (except of a short flat top,
10 ms - 100 ms to stabilize and then extract beams), and so each newly
injected beam must meet the proper magnetic field. In Table 12 we
summarize preliminary assessment of the Eddie current B-fields for the
NbTi and HTS(344S) conductors, as well as for the Fe3%Si laminations of
1mm and 100 m thickness, respectively. These predictions are for 0.33 Hz
and 1 Hz magnetic field cycling operations.

Table12. Projected Eddie current B-fields for the SF-SPS magnets

The conclusion from the above table is that using a HTS(344S)
superconductor is superior to a NbTi one. One should also note that
during ramping up the induced Eddie B-field has the opposite sign to
that generated in the followed-up ramping down. This may help to keep
the "average" magnet B-field at the proper value. Careful simulations of
the beam transport and acceleration in the SF-SPS must be done to find
out what are the acceptable limits for the induced Eddie current
B-fields. If necessary the magnetic core can be made of 50 m thick,
Fe3%Si lamination, thus lowering the Eddie current B-field in the
laminations to a value below 1 mT for the 3 s cycle type of the SF-SPS
operation.

The use of a superconducting line to energize the SF-SPS magnets
strongly suppresses the required cryogenic support. The heat load for
the SF-SPS conductor should be in the same range as for the LER main arc
magnets. For two accelerator rings of \~ 7000 m each, the projected heat
load is then \~ 2.1 kW at a 4.5 K - 6.0 K level, and \~ 42.5 kW at a
50 K - 75 K level, which is about 1/10 of the one LHC cryo-plant power.
Consequently, the cryogenic demands of the SF SPS could be supported
from the existing LHC cryogenic system, especially that the SF-SPS
operations will not likely overlap in time with those of the LHC.

The estimated cost (by scaling from the LER) for the SF-SPS accelerator
based on a NbTi conductor is \~ \$ M100, and based on a HTS (344S)
conductor at 2006 price doubles to \~ \$M 200, but possibly only \$M 120
if the HTS(344S) conductor price drops by a factor of 5 in 2008, as
indicated by the American Superconductor Corporations \[11\].

## 11. Conclusions

We made a preliminary assessment of the feasibility for installing an
injector accelerator ring (LER) in the LHC tunnel using the VLHC Stage 1
transmission line magnets. We believe that there are no serious
obstacles in principle for such an undertaking. The transfer of the beam
from the LER ring to the LHC one is challenging, but it seems feasible.
A re-arrangement and re-design of the D1 LHC dipole is however needed,
to facilitate the installation of the LER-LHC transfer line. A
preliminary cost analysis of the LER suggests that it is below the LHC
yearly operational expenditure and thus very much affordable. In
addition, the LER components can be manufactured and then installed in
the LHC tunnel during the regular LHC shut-downs in a reasonable time
span of 5-6 years without interference with the ongoing LHC physics
program in the same time period.

With a second accelerator in the LHC tunnel the complexity of the
overall LHC system is significantly increased raising concerns about the
detector and accelerator components safety. A limited use of pilot
bunches provided by the SPS is possible and may potentially improve the
safety of the LHC-LER complex. In addition, installation of dedicated
kicker magnets and beam dumps in the straight sections (especially in
the IR1 and IR5 ones to protect detectors) will allow for a safe removal
of the LER beams from the circulation in case of a catastrophic failure
of the transfer line magnet system (or other significant LER component)
when a standard beam dumping at IR4 may not be sufficiently effective.
The radiation implications of such a system still have to be evaluated.

A feasibility study of bunch slip stacking in the LER suggests the
possibility of a luminosity increase of up to a factor of 4. The
combination of this luminosity increase with a potential improvement of
the LHC beam transport and acceleration due to the increased injection
energy to 1.5 TeV may lead to the overall LHC luminosity increase in an
order of magnitude. Such an improvement of the LHC performance would
greatly facilitate the working of the detectors allowing for a
significant increase in streaming of the hard-collision data relative to
the multiple interaction backgrounds. In this way the boundaries of the
LHC physics reach will be expanded much beyond the present expectations.
In a longer term the implementation of the LER will provide a very
strong boost to the LHC energy upgrade proposal (DLHC).

It appears that the magnet technology developed for the VLHC combined
with the one for the proposed fast switching magnets of the LER also
offers a new option for an SPS application allowing it to become a fast
cycling accelerator while not being used for the injection of beams to
the LHC. This would enhance prospects for the fixed target physics at
CERN, especially neutrino physics, which requires proton beams of a very
high integrated intensity. It would only be natural to extend the
proposed fast cycling magnet technology to the PS, upgrading its beam
energy to 50 GeV in the same time. The proposed fast switching/cycling
magnet technology, however, needs a vigorous R&D effort to prove
expectation of the performance, and to prove its high
cost-effectiveness.

## Acknowledgements

We greatly profited from several illuminating discussions with
Gianluigi Arduini, Jean‑Pierre Koutchouk, Yvon Muttoni, John Osborne,
Germana Riddone of CERN, with William Foster, Vladimir Kashikhin of
Fermilab, and with Masayoshi Wake of KEK.

We are also greatly indebted to Steve Holmes, Robert Kephart,
Peter Limon and Steve Peggs for their inspiration and support.

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