**US-LARP Progress on LHC IR Upgrades**

Tanaji Sen, John Johnstone, Nikolai Mokhov, FNAL, Batavia, IL 60510

Wolfram Fischer, Ramesh Gupta, BNL. Upton, NY

Ji Qiang LBNL, Berkeley, CA

Abstract

We review the progress on LHC IR upgrades made by the US-LARP
collaboration since the last CARE meeting in November 2004. We introduce
a new optics design with doublet focusing, and discuss energy deposition
calculations with an open mid-plane dipole. We present the results of a
beam-beam experiment at RHIC. This experiment was the first phase of a
planned test of the wire compensation principle at RHIC.

## Introduction

Increasing the luminosity in the LHC will require upgrades to the
interaction regions (IRs) as well as to the injector chain. US-LARP is
committing resources towards the development of the next generation
magnets and to the optics design for the IR upgrade. Previous reports
and reviews of LARP efforts on the IR upgrades can be found in
references \[1\], \[2\] and elsewhere. Ideas for several alternative IR
designs were proposed and some of their consequences on optics functions
and energy deposition were discussed. Here we will present progress on
the IR designs since the last CARE meeting in November 2004.

Mitigating the impact of the long-range interactions is one of the key
motivations for exploring IR designs different from the baseline design.
Wire- based compensation of the long-range interactions has been
proposed for the baseline optics \[3\]. US-LARP has proposed to carry
out a test of this wire compensation in RHIC which has a layout similar
to that of the LHC. Here we will also report on an experiment performed
at RHIC to test the impact of a long-range interaction on the beams.

## IR Designs

Design and construction of next generation IR magnets with Nb~3~Sn
technology constitutes the major portion of the US-LARP effort on IR
design. The accelerator physics effort here is mainly to provide
guidance to the magnet builders. It is not intended to propose fully
optimized optics designs that satisfy all known engineering and physics
constraints. Due to the complex environment of the LHC IR magnets, beam
optics by itself does not suffice to determine the aperture and gradient
of these magnets. Energy deposition in the IR magnets is a key component
in determining these parameters. The required field quality is another
key input to the magnet designers. An IR design that meets most of the
basic criteria can be useful for initial estimates of the required field
quality. More careful evaluations of the field quality will be required
as we progress towards the final design. Thus apertures, fields, field
quality, demands on correction systems, energy deposition can all be
estimated with preliminary IR designs. That is our purpose here.

In this report we will consider three different designs: the baseline
design and two variants of the dipole first design. One of these
variants is new with doublet focusing which produces elliptical beams at
the IP. It has the promise of higher luminosity but perhaps at the
expense of enhanced beam-beam effects. We will also present energy
deposition results with dipole first optics and the use of a novel open
mid-plane dipole. It should be emphasized that the results shown here
represent work in progress and much remains to be done.

### Optics of the IR Designs

The baseline design features quadrupoles built with NbTi superconductor.
They are placed as close as possible to the IP and designed for β\*=
0.50 m. The promise of Nb~3~Sn for the upgrade is that higher pole tip
fields are achievable. This can be used to either (a) increase the
gradient with the same physical aperture and decrease magnetic lengths
-- allowing the triplet magnets to move closer to the IP or (b) keep the
gradient constant but increase the physical aperture. A previous study
\[4\] had shown that option (b) was the superior path to higher
luminosity.

In the designs to be presented here, we consider the inner triplet
magnets to be at the fixed gradient of 200 T/m at top energy -- the same
as in the baseline optics. The matching section extends from the trim
quadrupole QT13 on the left to the trim quadrupole QT13 on the right.
We've used the LHC optics version 6.2. The optics constraints are the
standard ones. Starting from the left, we match to the required β\* at
the IP keeping α and dispersion and its slope zero at the IP. At the end
of the section, the values of β, dispersion and its slope are matched.
Taking into account both planes, this amounts to 16 matching
constraints. We have not attempted to keep the phase advance the same as
in the baseline optics. In the cases where we've developed solutions
both at injection and collision, we have kept the phase advance across
the section constant. While we recognize that a complete optics design
requires solutions at injection, through different stages of the squeeze
and ending with the collision optics, we have mainly focused on the
design at collision energy to demonstrate the feasibility of the design
and the impact on luminosity.

We discuss the baseline design first. All positions and lengths of
magnets are assumed to be unchanged. A design that reduced the β\* to
0.25m had been presented earlier \[5\]. A drawback in that design was
that the gradient in one magnet exceeded 200 T/m. In the present design
all gradients are at or under 200 T/m. Figure 1 shows the beta functions
in the matching section.

![](media/image1.png){width="2.85in" height="2.5in"}

Figure 1: Beta functions in the baseline design with β\* = 0.25m.

The maximum beta values occur in Q2b and Q3 and are twice the values
with β\* = 0.50m. Magnet coil apertures and pole tip fields can be
extracted from this solution. We calculate the coil aperture as follows:

$\begin{array}{r}
\text{Aperture} = 1\text{.}1(\text{Beamsep} + 2 \times \text{Beamenv})ο + \\
2(\text{Orbit} + \text{beampipe} + \text{Hechan} + \text{beamscr})
\end{array}$ (1)

The factor 1.1 accounts for a 20% β beating, the beam separation (in
units of σ) is 10, the beam envelope is 9, orbit distortions total 8.6
mm including contributions from on-momentum errors (3mm), dispersion (4
mm), and mechanical alignments (1.6 mm), the beam pipe thickness is 3
mm, the liquid He channel is 4.5 mm and the beam screen thickness is 1
mm. From the aperture and the gradient we calculate the pole tip field
without assuming any additional margins. These are shown in Table 1 for
the baseline optics.

While the gradients are not exactly left-right symmetric, the
differences are less than 15%. Table 1 shows the maximum values. The
realistic pole tip fields will likely be higher when margins are added.
It is clear therefore that even with the baseline optics, Nb~3~Sn
technology will be required for the inner triplet magnets whose pole tip
fields exceed 9 T.

Table 1: Gradients, apertures and pole tip fields for the baseline
optics shown in Figure 1.

We now discuss the first of the two dipole-first layouts. This is the
same layout discussed in previous reports \[1, 2, 6, 7\]. The separation
dipoles D1 and D2, each with strength of 13.4T, are placed right after
the TAS absorber to separate the beams early and minimize the number of
long-range interactions. Figure 2 shows the conceptual layout of the
separation dipoles followed by the triplet focusing channel for both
beams on both sides of the IP. The optics is anti-symmetric about the
IP.

![](media/image2.png){width="2.74375in" height="2.0180555555555557in"}

Figure 2: IR layout with dipoles-first and triplet focusing. The
focusing is anti-symmetric about the IP for each beam. The TAS and TAN
absorbers are not shown.

Energy deposition in the magnets downstream of the dipoles is a major
issue with this optics \[1, 2, 7\]. The MARS15 energy deposition
calculations with the open mid-plane dipole design for D1 show (see
below) that an integrated field of 20 T-m is necessary in order for most
of the energetic particles to be deflected into an intermediate
absorber. Therefore, D1 originally 10-m long is split into two pieces:
D1A 1.5m long (integrated strength of 20T-m) and D1B 8.5m long with the
TAS2 absorber placed between them. Another absorber TAN, for neutral
particles, is estimated to be 5m long and placed after D1B. Any
realistic optics design has to incorporate these absorber lengths from
the outset. Table 2 shows the relevant lengths and distances for the
triplet version of the dipole-first optics. The first focusing element
Q1 starts at 55.5m from the IP compared to 23 m from the IP in the
baseline optics.

Table 2: Relevant lengths up to the inner triplet in the
triplet-focusing version of the dipole-first optics.

In order to make minimal changes to the insertion, the positions of the
downstream magnets Q4 to QT13 have been kept at the same positions as in
the baseline optics. This is strictly not necessary -- the magnets Q4 to
Q7 before the start of the dispersion suppressor could be placed
differently. In future iterations we will make use of this flexibility.
Figure 3 shows the beta functions across the matching section.

![](media/image3.png){width="2.85in" height="2.5in"}

Figure 3: Beta functions across the matching section at collision in the
triplet-focusing version of the dipole-first optics.

The peak beta functions are about 27 km in the triplet quadrupoles
compared to about 9 km in the baseline optics for the same β\*=0.25 m.
The optics is not left-right symmetric beyond Q7. This can be improved
with changes such as repositioning the Q4 to Q7 quadrupoles. Table 3
shows the important parameters that can be extracted from this solution:
the apertures and the pole tip fields. Only one beam needs to be
accommodated in each aperture and there is no need to include the beam
separation factor in Equation (1) or the factor of 2 before the beam
envelope term.

Table 3: Gradients, coil apertures and pole tip fields for the
triplet-focusing version of the dipole-first optics.

### 

### 

### 

### 

Compared to the apertures and pole tip fields seen in Table 1 for the
baseline optics, the values for this optics are higher. Nb~3~Sn magnets
will be required even for the first quadrupole Q1 in the triplet in this
optics.

Finally we discuss the doublet focusing optics for the insertion. Such
focusing has conventionally been used in e^+^e^-^ colliders with 2
rings. We can explore the feasibility of doublets in the dipole-first
option where the focusing occurs in separate channels. We require
symmetric focusing around the IP in order to have nearly equal
beta-functions in both planes upstream and downstream of the IP. The
transverse beam sizes are unequal at the IP but they can still be
matched between the beams provided each beam sees the same focusing
sequence in the doublets. The crossing plane determines the polarity of
the quadrupole Q1 nearest to the IP. The polarity is chosen so that we
maximize the overlap between the beams. If the crossing plane is
horizontal, then the horizontal beam size should be larger to increase
the overlap.

![](media/image4.png){width="2.7416666666666667in"
height="2.017361111111111in"}

Figure 4: IR layout with dipoles-first and doublet focusing. In contrast
to the triplet focusing, the focusing is symmetric about the IP from Q1
to Q3 for each beam. On a given side of the IP, the quadrupole
polarities are opposite for the two beams. This figure should be
compared with Figure 2.

This implies that the nearest quadrupole Q1 should be vertically
focusing -- this argument assumes that β^max^ in the two planes are
nearly the same. Maximizing the overlap leads to an important advantage
in luminosity as will be shortly seen. Figure 4 shows the layout with
the doublet focusing.

Unequal beam sizes imply that the head-on beam-beam tune shifts will
also be different in the two planes. However, with alternating crossing
planes, this is easily resolved. At IP1, where the crossing plane is
vertical, the vertical head-on beam-beam tune shift is larger while at
IP5 with a horizontal crossing plane, the horizontal head-on tune shift
is larger resulting in equal head-on tune shifts. This requires that the
quadrupole nearest to the IP for a given beam have the opposite
polarities at IP1 and IP5.

Another benefit of the dipole focusing is that with fewer magnets it is
cheaper than using triplets and also results in lower nonlinear fields
on the beams.

![](media/image5.png){width="2.65in" height="2.5in"}

### ![](media/image6.png){width="2.65in" height="2.5in"}

Figure 5: Matched optics at injection (top) and at collision (bottom)
across the insertion for the doublet-focusing version of the
dipole-first optics.

The solution that has been developed uses a doublet Q1 and Q2 with the
same lengths and strengths. An additional trim quadrupole Q1Trim is
required for matching purposes. The quadrupoles Q3, Q4 form another
doublet. Figure 5 shows the matched optics at injection and collision.
All quadrupoles from Q1 to Q6 are at different locations compared to the
baseline optics. The insertion has to match to an anti-symmetric arc
with respect to the IP. The insertion is symmetric about the IP up to Q3
but anti-symmetric from Q4 onwards. At collision the β\* values in the
two planes at IP5 are β~x~\* = 0.462 m and β~y~\* = 0.135 m whose
geometric mean is β\* = 0.25 m. At IP1 the β\* values are interchanged.
For magnet designers, the important quantities are the apertures and the
pole tip fields -- these are shown in Table 4.

Table 4: Gradients, coil apertures and pole tip fields for the
doublet-focusing version of the dipole first optics.

+--------------+-------------------+-------------------+---------------+
| Quad         | Gradient          | Aperture          | Pole tip      |
|              |                   |                   | field \[T\]   |
|              | \[T/m\]           | \[mm\]            |               |
+--------------+-------------------+-------------------+---------------+
| Q1           | 198               | 103.6             | 10.3          |
|              |                   |                   |               |
| Q2           | 198               | 103.6             | 10.3          |
|              |                   |                   |               |
| Q1T          | 125               | 96.9              | 6.1           |
|              |                   |                   |               |
| Q3           | 46                | 62.9              | 1.4           |
|              |                   |                   |               |
| Q4           | 50                | 61.5              | 1.5           |
|              |                   |                   |               |
| Q6           | 155               | 49.3              | 3.8           |
|              |                   |                   |               |
| Q7           | 31                | 44.2              | 0.7           |
|              |                   |                   |               |
| Q8           | 147               | 42.9              | 3.2           |
|              |                   |                   |               |
| Q9           | 205               | 41.3              | 4.2           |
|              |                   |                   |               |
| Q10          | 198               | 40.6              | 4.0           |
|              |                   |                   |               |
| QT11         | 98                | 40.2              | 2.0           |
|              |                   |                   |               |
| QT12         | 44                | 40                | 0.9           |
|              |                   |                   |               |
| QT13         | 108               | 40                | 2.2           |
+--------------+-------------------+-------------------+---------------+

In both versions of the dipole-first optics, coil apertures in excess of
100 mm will be required for the quadrupoles that are next to the
separation dipoles.

Even taking into account the space required for the coil and yoke
assembly, there should be enough space between the two apertures given
that the center to center distance between beams is 194 mm. However, at
larger apertures, magnet design issues such as Lorentz forces and
stresses and cross talk between the two apertures impose an upper limit.

Consider now the impact on luminosity with elliptical beams at the IP.
The beam separation is kept at 10σ with a crossing angle that scales
inversely with the square root of β\*. Since β\* is larger in the
crossing plane, the required crossing angle with elliptical beams to
achieve the same separation is smaller than with round beams by the
factor

> $\phi_{E} = \sqrt{\frac{\beta \ast_{R}}{\beta \ast_{E}}\phi_{R} = 0\text{.}\text{74}\phi_{R}}$
> (2)

where the subscripts E and R refer to elliptical and round respectively.
This directly increases the luminosity, a simple estimate of the change
can be found from using the expression

> $\frac{L_{E}}{L_{R}} = \left\lbrack \frac{1 + (\phi_{R}\frac{\sigma_{s}}{\sigma_{x,R}^{}})^{2}}{1 + (\phi_{R}\frac{\sigma_{s}}{\sigma_{x,R}^{}})^{2}(\frac{\beta_{x,R}^{}}{\beta_{x,E}^{}})^{2}} \right\rbrack^{\frac{1}{2}}$
> (3)

This yields a factor of 1.38 but does not take into account the
hourglass effect which is important in the vertical plane at IP5 where
the β\* is comparable to the bunch length of 7.5cm. A more complete
luminosity calculation taking into account the overlap in both planes
yields

> $\frac{L_{E}}{L_{R}} = 1\text{.}\text{33}$ (4)

This luminosity increase is a major advantage of using elliptical beams
with crossing angles.

The head-on beam-beam tune shift with alternating crossing planes is the
same as with round beams. The long-range beam-beam tune shifts also need
to be examined. Here round beams have an advantage. With alternating
crossing planes, the negative (zero-amplitude) horizontal tune shifts at
large amplitudes beyond 4σ at the IP with horizontal crossing are almost
exactly cancelled by the positive horizontal tune shifts at the IR with
a vertical crossing plane. There is a similar cancellation of the
vertical tune shifts. However, with elliptical beams, the vertical tune
shifts are large and positive at the IR with horizontal crossing while
at the other IR with vertical crossing, the vertical tune shifts are
negative but not nearly as large. Therefore the cancellation is not
nearly as good. As a consequence the long-range beam-beam tune shifts at
all amplitudes are larger. In the dipole first optics, there are 6
long-range interactions on either side of the IP before the beams are in
separate channels. The tune footprints with all 24 long-range
interactions from IR1 and IR5 have been calculated analytically with the
expressions in Reference \[8\] and are shown in Figure 6.

![](media/image7.wmf)

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Figure 6: Tune footprint to 6 σ with head-on and long-range interactions
from IP1 and IP5 (calculated analytically) for the two versions of the
dipole first optics. There are 12 parasitics per IR, 24 in all from IR1
and IR5.

As an example, the zero amplitude tune shifts with round beams are 2ξ
(the beam-beam parameter) while with the elliptical beams used in this
design, the corresponding tune shift is nearly 2.7ξ. It is possible that
exploring other layouts, such as inclined plane crossings, may mitigate
this effect but at the expense of luminosity. It remains true
nonetheless that the long-range beam-beam effects are more of a concern
with the elliptical beams and needs to be resolved. One possibility is
to use wire-based compensation \[3\].

Finally we consider the chromaticity of the insertion in the three
designs considered here. We use exact expressions for the chromaticity
of thick quadrupoles. We show the chromaticity of the insertion and the
inner magnets for each design in Table 5.

Table 5: Chromaticity of an insertion and of the inner magnets for the
three optics designs at collision optics.

### 

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### 

### 

### 

This table shows the chromaticity of a single IR. Clearly the inner
triplet and inner doublet dominate the chromaticity. If we include both
IR1 and IR5 then (a) the chromaticity of dipoles first with triplets is
99 units larger per plane than the design with quadrupoles first, (b)
the chromaticity of dipoles first with doublets is 31 units larger per
plane than dipoles first with triplets. The reason for (a) is simply the
much larger beta functions in the inner magnets with the dipole-first
optics.

### ![](media/image8.png){width="2.65in" height="2.5in"}

### 

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### 

### 

Figure 7: Horizontal chromaticity contributions from individual
quadrupoles in the IR for the 3 designs. With the anti-symmetric optics
(red and green), the chromaticities upstream and downstream of the IP
have opposite signs. For the symmetric optics (blue) the chromaticities
upstream and downstream have the same signs.

A closer look at the chromaticity contributions from individual
quadrupoles shows the reason for (b). With anti-symmetric optics
upstream and downstream quadrupoles have opposite chromaticities and
tend to cancel. With symmetric optics: upstream and downstream
quadrupoles have the same sign of chromaticities. This can be seen in
Figure 7 where the horizontal chromaticity contributions from the
quadrupoles are plotted for the three designs. It remains to be checked
whether the linear and nonlinear chromaticities of the different IR
designs can be adequately corrected with the available LHC chromaticity
sextupoles.

### Energy Deposition

Energy deposited by particles affects accelerator operation in at least
four distinct ways \[9\]. Quench stability is determined by the peak
power density. The dynamic heat loads on the cryogenics is determined by
the amount of power dissipated in the magnets. Hands-on maintenance is
determined by the residual dose rates. Finally, the lifetime of
components is determined by the peak radiation dose and the lifetime
limits which vary for different materials.

For some time now it has been recognized that energy deposition and the
spray of particles from the IP will be the major issues for a ten-fold
luminosity upgrade \[7, 9\]. At a luminosity of 10^35^ cm^-2^ sec^-1^,
the debris power will be 9kW. All of this debris power will be directed
towards the IR magnets which have to be well protected. In the baseline
design with quadrupoles first, 1.6 kW will be absorbed within the
triplet. At these dosages, the lifetimes of conventional insulators,
used for the magnet ends, is estimated to be only several months.

A significant part of the US-LARP magnet effort is therefore focused on
developing more radiation hard materials. The energy deposition problem
is more severe in the dipole-first layout -- so some effort has been
invested in developing a dipole design that can withstand the expected
radiation \[7, 10\].

![](media/image9.png){width="2.65in" height="2.4in"}

### 

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### 

### 

Figure 8: Sketch of the open mid-plane dipole design with no coils in
the mid-plane. The warm targets are tungsten rods at liquid N~2~
temperature.

An open mid-plane design was developed with no coils in the mid-plane,
seen in Figure 8. A major part of the particle spray is transported to
tungsten rods at liquid nitrogen temperatures, placed outside the coils.
Several technical challenges were addressed including obtaining good
field quality even with a large gap between the coils and supporting the
coils against the large Lorentz forces on them. Energy deposition
analysis with this dipole split into D1A and D1B (as mentioned earlier)
showed that TAS protects D1A quite well. Higher energy charged particles
that are not absorbed in TAS and those generated in D1A are absorbed
efficiently in TAS2. The minimum integrated field required before TAS2
is 20 T-m. The calculations show that the peak power density in the
superconducting coils is \~0.4mW/g, below the quench limit. The dynamic
heat load to D1 is drastically reduced with this design. The estimated
lifetime based on displacements per atom is \~10 years. These initial
calculations suggest that if this design proves to be realistic, then it
might survive the radiation environment long enough to be useful. Due to
budgetary constraints however, the US-LARP magnet program has decided to
focus entirely on building the next generation quadrupoles and to
postpone further work on the dipoles for the LHC IR.

## Beam-beam phenomena

RHIC has the same geometrical layout as the LHC with two rings, called
yellow and blue. The optics of RHIC corresponds to the dipoles first
layout with triplet focusing discussed above. Within the common IR where
the beams share the same beam pipe, there can be no more than two
parasitic interactions at the current bunch spacing in RHIC. Due to
adverse phase advances, only a single parasitic can potentially be
corrected in RHIC. In order for RHIC to be a practical test bed of the
wire compensation principle, we first have to demonstrate that this
single parasitic interaction has an observable effect to be compensated.

In April 2005 an experiment was performed at the injection energy of
24.3 GeV with a single proton bunch in each beam. This choice of energy
allowed several experiments with both bunches at full intensity. The
experiment consisted of changing the vertical separation between the
beams at one parasitic interaction while the beam losses were observed.
The separation at the diametrically opposite parasitic in the ring was
kept constant at \~10σ. The experiment was done four times with four
different tunes. For the first three tunes only the blue beam suffered
losses as the separation was reduced below a certain value. The yellow
beam suffered very few losses at these tunes. In the fourth case, the
tunes of the two beams were chosen to be symmetric about the diagonal.
In this case, as seen in Figure 9, there were losses in both beams at
separations smaller than 7σ. This experiment showed that there is indeed
an effect to compensate, at least at injection energy, but the
phenomenon is very tune-dependent.

![](media/image10.jpeg){width="2.75in" height="2.25in"}

## 

## 

## 

Figure 9: Losses in the blue beam, yellow beam (in red) and the beam
separation (in green) for the fourth tune. There was a sharp increase in
losses in both beams as the separation dropped below 7σ.

In the next phase of this program the experiment will be repeated at
collision energy. At this energy the phase advance between the parasitic
location and the possible location of the wire is 6^o^, perhaps still
small enough for the wire compensation to work.

Over the next year, it is planned to build and install two wires in the
two rings of RHIC, downstream of Q3 for each beam in IR6. Figure 10
shows the schematic layout on one side of IR6. The single parasitic
occurs before the separation dipole DX.

## ![](media/image11.png){width="2.647222222222222in" height="1.8680555555555556in"}

Figure 10: Sketch of IR6 in RHIC where the wire compensator will be
placed, roughly 41m from the IP.

The first tests with these wires will likely take place during 2007. We
also aim to test robustness of compensation with respect to current
ripple, alignment errors etc.

A separate effort has been the development of a strong-strong beam-beam
simulation PIC style code Beambeam3D \[11\]. In the past it has been
used to study the emittance growth when one beam is swept around the
other, as would be done for the luminosity monitor designed and built at
LBNL. Recently more physics has been added to this simulation code
including crossing angles, and long-range interactions.

Numerical noise remains a difficult issue to resolve -- the calculated
growth rate depends on the number of macro-particles M. Studies to
extract growth rates asymptotic in M continue. Very recently the
emittance growth with round and elliptical beams at the IPs has been
studied.

## Magnet R & D

We will discuss the US-LARP magnet program very briefly. At the LARP
collaboration meeting in April 2005 it was decided to concentrate the
effort on the quadrupole program and postpone the dipole program. The
major goals that were set were (a) demonstrate by 2009 that Nb~3~Sn
magnets are a viable choice for the upgrade and (b) demonstrate that
these magnets with the required aperture, field, and length can be built
with reproducible performance. A plan to realize these goals has been
defined. Over the near term the plan is to build short quadrupoles (1 m
long) with 90 mm aperture and 200 T/m gradients. These will be followed
by longer quadrupoles (4 m long) with other parameters the same. Higher
gradient (250 T/m), short (1 m long) quadrupoles will also be built.
Other aspects of the magnet program include supporting R&D to build
magnets with different geometries, test the capability to reach higher
fields, develop radiation hard insulators etc. See reference \[12\] for
details.

## Summary

We have discussed three optics designs for the upgrade: the baseline
with quadrupoles first, dipoles-first with triplet focusing, and
dipoles-first with doublet focusing. All three optics require magnets
with apertures larger than 100 mm and pole tip fields greater than 10 T.
This implies that only Nb~3~Sn magnets will suffice to realize the
optics under the assumptions made here. The doublet focusing optics we
discussed is new and has features both positive and negative. It creates
elliptical beams at the IP. The resulting luminosity is about 33%
greater than that obtained with round beams for the same effective β\*.
This is partly due to the fact that the required crossing angle to
achieve the same beam separation is smaller. The nonlinearities seen by
the beams are smaller because of the smaller transverse orbit excursion
and fewer magnets at high fields. On the negative side, the long-range
beam-beam tune shifts are larger compared to round beams and the IR
chromaticity with optics symmetric about the IP is larger than with
anti-symmetric optics.

We mention here that other options were discussed at this Arcidosso
workshop. These included moving magnets closer to the IP and installing
a dipole magnet close to the detectors to start the beam separation as
early as possible \[13\]. If realistic, these possibilities could be
incorporated into the optics discussed here. Immediate benefits would be
to reduce the requirements on aperture and pole tip fields.

Energy deposition studies with the open mid-plane dipole design showed
that the severe radiation issues in the dipole-first optics can be
mitigated. This reinforces again the importance of early inclusion of
energy deposition calculations in the designs of IR optics and magnets.

Beam-beam phenomena are also an important part of the IR upgrade. Test
of the wire compensation principle is a new LARP program. The design of
the wire compensation has begun and first tests of the wire compensation
are planned for 2007.

# Acknowledgements

We thank O. Bruning, J.P. Koutchouk, S. Peggs, F. Ruggiero, G.L. Sabbi,
F. Zimmermann and A. Zlobin for valuable insights and useful
discussions.

# References

\[1\] J. Strait et al, Proceedings of PAC 2003, page 42

\[2\] J. Strait, N. Mokhov and T. Sen, CARE-HHH 2004 Proceedings

\[3\] J.P. Koutchouk, LHC Project Note 223 (2000)

\[4\] T. Sen et al, Proceedings of PAC 2001, page 3421

\[5\] T. Sen et al, Proceedings of EPAC 2002, page 271

\[6\] O. Bruning et al, LHC Project Report 626 (2002)

\[7\] N. Mokhov et al, Proceedings of PAC 2003, page 1748

\[8\] T. Sen et al, Phys, Rev, Spec. Top. AB, 7, 041001 (2004)

\[9\] N. Mokhov et al, Fermilab-FN-732, LHC Project Report 633 (2003)

\[10\] R. Gupta et al. Proceedings of PAC 2005, page 3055

\[11\] J. Qiang et al, Phys, Rev, Spec. Top. AB, **5**, 104402 (2002)

\[12\] LARP R&D document
http://uslarp.lbl.gov/workshops/051102/files/RandD_v1c.pdf

\[13\] J.P. Koutchouk, this workshop