%


% briefly summarise parameters and design \\
% emphasise synchronous running with LHC and luminosity potential


The LHeC aims at colliding the high-energy protons and heavy ions 
circulating in the LHC with 60-GeV polarized electrons 
and possibly also positrons. 
The LHeC is realized by adding to the LHC a separate 9-km 
racetrack-shaped recirculating superconducting (SC) 
energy-recovery linac (ERL).
The key components of the LHeC are the two 1-km 10-GeV SC linacs of the ERL,
comparable in scale to the 17.5-GeV SC linac 
of the European XFEL presently under construction. 
The LHeC ERL provides a design  
lepton beam current of 6.4 mA at the $ep$ collision point,  
which is taken to be at IP2 of the LHC.   
Aside from the IP2 interaction region, the LHeC underground infrastructure 
is fully decoupled from the existing LHC tunnel. 
Two of the access shafts could be located on the CERN Prevessin site. 
 
The LHeC is designed to operate with simultaneous LHC 
$pp$ (or $AA$) collisions.
LHeC operation is fully transparent to the other LHC experiments thanks to 
the low lepton bunch charge and resulting minuscule beam-beam tune shift 
experienced by the protons,  
together with the choice of the LHeC circumference 
to be equal to a third of the LHC's in order  
to allow for ion-clearing gaps in the ERL without perturbing LHC steady-state operation.  

% The development of a CW SC recirculating energy-recovery linac (ERL) for LHeC
% would prepare the ground, the technology and the infrastructure for many possible
% future projects, e.g., for an International Linear Collider, for a Muon Collider, 
% for aneutrino factory, or for a proton-driven plasma wake field accelerator.
% A ring-linac LHeC would, therefore, promote a variety of conceivable long-term
% high-energy physics projects, while pursuing an attractive forefront 
% high-energy physics programme in its own right

\begin{table*}[htb]
\begin{center}
\begin{tabular}{|l|cc|}
\hline
parameter [unit] &  \multicolumn{2}{|c|}{LHeC} \\ 
\hline
species & $e$ & $p$, $^{208}  $Pb$^{82+}$ \\    
beam energy (/nucleon) [GeV] & 60 & 7000, 2760 \\
bunch spacing [ns] & 25, 100 & 25, 100 \\
bunch intensity (nucleon) [$10^{10}$]  & 0.1 (0.2), 0.4 & 17 (22), 2.5  \\
beam current [mA] & 6.4 (12.8) & 860 (1110), 6 \\
rms bunch length [mm] &  0.6 & 75.5  \\
polarization [\%] & 90 ($e^{+}$ none) & none, none \\  
normalized rms emittance [$\mu$m]  & 50  & 3.75 (2.0), 1.5  \\
geometric rms emittance [nm] & 0.43  & 0.50 (0.31)  \\
IP beta function $\beta_{x,y}^{\ast}$ [m] & 0.12 (0.032) & 0.1 (0.05) \\   
IP spot size [$\mu$m]  & 7.2 (3.7) & 7.2 (3.7)\\   
synchrotron tune $Q_{s}$ &  --- & $1.9\times 10^{-3}$ \\ 
hadron beam-beam parameter &   \multicolumn{2}{|c|}{0.0001 (0.0002)} \\ 
lepton disruption parameter $D$  & \multicolumn{2}{|c|}{6 (30)} \\ 
crossing angle &   \multicolumn{2}{|c|}{0 (detector-integrated dipole)} \\ 
hourglass reduction factor $H_{hg}$  & \multicolumn{2}{|c|}{0.91 (0.67)} \\ 
pinch enhancement factor $H_{D}$  & \multicolumn{2}{|c|}{1.35 (0.3 for $e^{+}$)} \\ 
CM energy [TeV] &  \multicolumn{2}{|c|}{1.3, 0.81} \\ 
luminosity / nucleon [$10^{33}$ cm$^{-2}$s$^{-1}$] &  \multicolumn{2}{|c|}{1 (10), 0.2} \\ 
\hline
\end{tabular}
% \vspace*{-8 mm} 
\end{center}
\caption{LHeC parameters. The numbers give
 the default values with optimum values for maximum
 $ep$ luminosity in paranthesis and values
 for the $ePb$ configuration separated by a komma.}
\label{tab1}
\end{table*}



LHeC has been designed under the constraint that 
the total electrical power for the LHeC lepton branch 
should not exceed $100$\,MW 
(about half the present maximum CERN site power).  
The LHeC electrical power budget is dominated by the RF and by the 
cryo power for the two 1-km long SC linacs. 
%  as is illustrated in Table \ref{tab3}. 
The cryo power required and, therefore, also the  
size of the cryoplants (as well as the maximum lepton current) 
are directly linked to the unloaded quality factor of the cavities, $Q_0$. 
With a $Q_{0}$ of $2.5\times 10^{10}$, the total  
main-linac cryopower amounts to 23 MW. The RF power needed for 
RF microphonics control is about 24 MW, and the 
extra-RF power needed for compensating SR losses 
at 6.4-mA current also to 23 MW. 
The remaining components, like injectors 
or arc magnets, require a few MW each.  

Together with rather conservative assumptions for most parameters, 
the $100$\,MW power limit yields  
the LHeC $ep$ target luminosity of $10^{33}$~cm$^{-2}$s$^{-1}$. 
However, extensions to significantly higher luminosity, e.g., 
$10^{34}$~cm$^{-2}$s$^{-1}$, are possible by a 
combination of improvements, namely   
(1) by considering a normalized proton beam emittances of 2 $\mu$m
(as achieved in 2011/12 LHC operation) instead of
3.75~$\mu$m; (2) by a further reduction of the 
proton IP beta function from 0.1\,m down to 0.05\,m,
which should be possible by using a variant of 
the so called ATS optics;  
(3) by increasing the proton bunch intensity  
from $1.7\times 10^{11}$ to the HL-LHC
25\,ns target value of $2.2\times 10^{11}$ 
[for the 50-ns HL-LHC scenario it would be even 
$3.3\times 10^{11}$ with a possible further factor 2.5 
increase of luminosity, to more than $2\times 10^{34}$~cm$^{-2}$s$^{-1}$]; 
and (4) by doubling the lepton beam current, 
which should be possible without exceeding 
the 100-MW power limit if the unloaded 
$Q_{0}$ value of the SC RF cavities can be raised to 
$4\times 10^{10}$ (as it is assumed for the similar eRHIC design). 
Table \ref{tab1} shows LHeC parameters, including, in parentheses,  
values for a higher-luminosity variant. 

The LHeC represents an interesting possibility for 
further efficient exploitation of the LHC infrastructure investment.
%At $60$\,GeV lepton beam energy and 
%using the $7$\,TeV proton (and few TeV/nucleon or ion) beam, 
%centre-of-mass collision (CM) energies in the TeV range are attained. 
The development of a CW SC recirculating 
energy-recovery linac for LHeC would 
prepare for many possible future projects, e.g., 
for an International Linear Collider, 
for a neutrino factory, for a proton-driven plasma wake field accelerator,
or for a muon collider. 
With some additional arcs, using 4 instead of 3 passes 
through the linacs, a machine like the LHeC ERL (without energy recovery) 
could also operate as a Higgs factory $\gamma\gamma$ collider (SAPPHiRE). 
%  \cite{lhechiggs}.  



