Prospective New Accelerator Projects
The Physics of Beams will experience vibrant development in the future:
after the completion of Europe's Large Hadron Collider, the sights are
set towards a Next Linear Collider for electron- and positron beams, possibly
followed by a Muon Collider. And in the distant future lies the dream for
a Very Large Hadron Collider, dwarfing all previous accelerators in size.
The Next Linear Collider
The NLCTA, a Test Accelerator for the Next Linear Collider, was designed
to integrate the new linear accelerator technologies being developed for
a normal conducting, TeV-scale linear collider. The photo shows four 1.8m
long accelerator sections installed in the NLCTA linac. The four sections
shown all utilize a cell-to-cell gaussian detuning technique to suppress
transverse wakefields. Two of the sections shown also employ damping to
suppress the transverse wake. In commissioning operations to date, the
Test Accelerator has achieved gradients up to 50 MV/m, and beam energies
up to 200 MeV. We are about to start conditioning the third of three klystron
stations, which powers the last two sections in the photo. When fully conditioned,
the maximum beam energy will be 450 MeV.
The Advanced Accelerator Program at SLAC
The advanced accelerator research program at SLAC is looking at future
accelerators operating at energies beyond those achievable with technology
in current use. To escape a trend toward longer accelerators and the added
expense associated with their construction, technology incorporating high-gradient
acceleration and low-emittance beams is required. Technologies must be
identified for accelerating electron beams with 10-100 pC charge at a GeV/m
over a substantial length. The Accelerator Research Department B at SLAC
is working on the design, construction and comissioning of a 1 GeV accelerator
with a gradient in excess of 1 GeV/m and with a technology employable for
a 5 TeV collider. ARDB collaborates with physicists from universities,
other laboratories and the private sector.
The main advantages of muons, as opposed to electrons, for a lepton collider
The synchrotron radiation that forces high energy electron colliders to
be linear, is negligible in muon colliders. Thus a muon collider can be
circular. In practice this means it can be smaller.
Because the muon collider can be circular, the muon bunches can collide
many times. The number of such interactions is limited by the muon lifetime
and is related to the average bending field in the muon collider ring,
where the number of collisions is approximately equal to 150 times the
average B-field in Tesla.
Synchrotron radiation emitted by particles of one bunch as they pass through
the opposite bunch (Beamstrahlung) is suppressed by the higher mass of
the muon. This allows the use of larger bunches of muons and reduces the
energy spread of the interactions.
The idea of muon colliders was introduced by Skrinsky et al and Neuffer
and has been studied in more detail since 1993. Muon Colliders are promising,
but they are far less developed than hadron or electron-electron machines.
No muon collider has ever been built.
|But there are problems with the use of muons:
Muons are obtained from the decay of pions, made by higher energy protons
impinging on a target, but in order to obtain enough muons, the proton
source must have a high intensity and very efficient capture of the pions
The selection of fully polarized muons is inconsistent with the requirements
for efficient collection. Polarizations only up to 50% are practical, and
some loss of luminosity is inevitable (e-e machines can polarize the electrons
up to approximately 85%).
Muons made with very large emittance must be cooled, and this must be done
before they decay. Conventional synchrotron, electron, or stochastic cooling
is too slow. Ionization cooling is the only clear possibility, but does
not cool to very low emittances.
Because of their short lifetime, conventional synchrotron acceleration
would be too slow. Recirculating accelerators or pulsed synchrotrons must
Because they decay while stored in the collider, muons radiate the ring
and detector with decay electrons. Shielding is essential and backgrounds
will be high.
Neutrino radiation is an issue for muon colliders with center of mass energies
above about 3 TeV. At such high energies the flux of neutrinos from muon
decay can represent a significant hazard.