14/11/2007
sc
Thoughts on LHeC: A 70 GeV (e-) X 7 TeV (p) Collider
At a luminosity of 10^33 cm-2s-1
SCENARIO:
 Linac-Ring
 Collider energies:
Electron: 70 GeV
Proton: 7 TeV
 Collider luminosity: 1033 cm-2s-1
 Assumptions about collision: in the linac-ring scenario, all
geometric and dynamical beam-beam effects at collision are
ignored. Luminosity is simply calculated as the product of the
intensities of the colliding bunches normalized to the collision
overlap cross-sectional area and enhanced by the collision
repetition rate – nothing else, no geometrical or dynamical
form factors have been applied.
 Assumptions about the proton beam :
 By the time the electron-proton collider is built, the
injector chain of the LHC complex is assumed to have
been upgraded so that an intensity of 1.7X1011 protons
per bunch can be supported in the LHC ring.
 The proton beam emittance is assumed to be 0.5
nanometre in each plane, vertical and horizontal.
 The horizontal and vertical beta* are assumed to be
0.25 meters (25 cms.) each, according to the
upgraded IP parameters at ATLAS and CMS planned
by 2012 via newly designed quadrupole triplets.
 Bunch spacing of beams in the proton ring is 25
nanoseconds.
 Assumptions about the electron beam:
 Electron beam optics and bunch spacing are exactly
matched to the proton beam with round beams
at the IP with spot sizes of approximately 11 microns
X 11microns colliding every 25 nanoseconds.
 Electron beam intensity is kept at a value consistent
with the required` luminosity (approx. 2X108 per
bunch)
 Electron beams are provided by a superconducting
CW linac, either of the ILC type at 1.3 GHz modified
from ‘pulsed’ to ‘CW’ operation, as envisaged at the
Cornell facility or of the Jefferson lab type at 1.5 GHz
ready for CW operation to start with. Both versions are
assumed to be able to support CW currents of a few
milliamps (already demonstrated at Jlab).
COMMENTS:
 The CW electron beam power in this scenario is modestly
high: about 50 MW.
 Assuming that in a superconducting microwave structure
most of the power fed into the structure goes to establish the
electromagnetic fields and into the beam in contrast to room
temperature structures where only approximately 50% is
available for this purpose (the other half lost in heating the
structure walls) and taking into account a CW klystron wallplug to RF power overall efficiency of 50% including all
waveguide and other losses, the linac power consumption is
expected to be in the range of 100 MW.
 Provision for this extra site power will have to be made if
nothing but a straight forward CW linac option is used brute
force.
POTENTIAL for INNOVATIVE USE of ENERGY RECOVERY
towards ENHANCED LUMINOSITY, REDUCED POWER and
SYNERGY with OTHER CERN PROGRAMS:
 To reduce the energy demand, one can imagine using
energy recovery with two opposing linacs, the spent bunches
from one after collision traversing the opposing linac in 180
degrees out of phase, allowing the linacs to recover the
energy spent in the acceleration of the opposing bunches.
Taking into account the energy lost at the low energy beam
dump depending on the energy at which the beams are
dumped, the overall energy efficiency, minus the dump
energy, can be as high as 99.5 %, based upon demonstrated
routine recovery at a few milliamps at a few hundreds of MeV
energy achieved at JLab.
 Potentially the luminosity can be higher, first by a simple
factor of two due to the two linacs and suitable arrangement
of IPs, second due to the large capacity of added linac
current possible with energy recovery, potentially by an
additional factor of 10.
 The linacs for LHeC could fit synergistically with other
possible CERN programs e.g. future electron-positron
colliders, etc.
SWOT ANALYSIS:
Strengths
 Simple collision optics and dynamics
 Potentially higher eventual luminosity by a factor of 10
 No interference with LHC ring
 No synchrotron radiation and cryogenic worries
 Possibility of expansion into higher energies
 Possibility of multiple IPs
Weaknesses:
 No one has ever built a linac-ring collider to date, so there is
the risk of the unknown
 Additional linac power needed on site
 Additional space for linacs on site
 If power not affordable, need to rely on innovative energy
recovery technology which is a risk, pending further R&D
 Energy recovery will double the cost of the linacs
 Energy recovery of the type needed for the collider – namely
two opposing linacs --- is sufficiently different from existing
tested prototypes or envisaged future ones, that a detailed
conceptual and technical development is warranted.
Opportunities:
 Tremendous opportunity for an innovative collider
 Opportunity to enhance the ultimate reach in luminosity
 Opportunity to expand into an electron-positron collider
 Opportunity to broaden European possibilities via an
expanded CERN portfolio with circular and linear
accelerators on site for hadrons and leptons, including
possibilities into neutrinos physics.
Threats:
 Ring-ring collider option is straight forward and conventional,
implementable in a decade, though it will be ultimately limited
by its energy and luminosity reach.
 Possible long development time.