The LHC: an Accelerated Overview
Jonathan WalshMay 2, 2006
LHC in a nutshell
• LHC beam from start to finish
• Expected beam statistics• What is luminosity, and
what can it do for me?• Beam properties and
difficulties unique to the LHC
Overview: staging in LHC beam production
• Duoplasmatron: 300mA beam current at 92 keV• RFQ: to 750 keV• Linac 2: to 50 MeV• PSB: to 1.4 GeV• PS: to 28 GeV• SPS: to 450 GeV• LHC: to 7 TeV at 180mA beam current
Increase factors:RFQ: 8.2Linac: 66.7PSB: 28PS: 20SPS: 16LHC: 15.5
Duoplasmatron: H+ source• Hydrogren gas is fed into a cathode chamber with electrons• The hydrogen dissociates and forms a plasma confined by magnetic fields• The plasma is constricted by a canal and extracted through the anode• The plasma is allowed to expand before forming the proton beam• The LHC Duoplasmatron operates at 100 kV
The Duoplasmatron
gas feed
cathode anode
canal
expansion cup
RF Quadrupole: shaping the beam
• 4 vanes (electrodes) provide a quadrupole RF field• The RF field provides a transverse focusing of the beam• Spacing of the vanes accelerates and bunches the beam
Linac-2: the MeV weapon of choice
Linac Tank: RF accelerator• The linac tank is a multi-chamber resonant cavity tuned to a specific frequency• RF is sent into the tank by waveguides, and normal modes can be excited in the cavity• These normal modes create potential differences in the cavities that accelerate the particle
Resistive losses in RF cavitiescan overwhelm accelerators
• The walls of a linac tank or other RF cavity begin converting input RF power into heat due to finite wall resistance
• Solution: make the cavity superconducting
Linac 2 is already at LHC spec
• LHC spec (achieved):– 180 mA beam current (192 mA)– 30 μs pulse length (120+ s)– 1.2 μm transverse rms emittance (1.2 μm)
Down to the Proton Synchrotron Booster (PSB)
• The beam line to the PSB from the Linac is 80m long
• 20 quadrupole magnets focus the beam along the line
• 2 bending and 8 steering magnets direct the beam
• The PSB will boost the protons up to 1.4 GeV (factor of 28)
The Fellowship of the Rings
PSB: Proton Synchrotron BoosterPS: Proton Synchrotron
SPS: Super Proton SynchrotronLHC: Large Hadron Collider
The PS Booster• Output energy has been increased to 1.4 GeV from 1 GeV for the LHC• 16 sectioned synchrotron consisting of bending magnets, focusing magnets, and RF cavities• PSB upgrades are largely to the high power RF system for the energy boost
Proton Synchrotron: Last low energy step synchrotron
• The PS has been upgraded for 40 and 80 MHz RF operation and new beam controls have been added• The PS is responsible for providing the 25 ns bunch separation for the LHC
PS accelerating sections
SPS: Converted for LHC
• The SPS boosts protons up to 450 GeV for LHC injection
• SPS was the injector for the LEP system, and the injection system was upgraded as well as the RF systems (at 200, 400, and
800 MHz)• SPS is fully LHC dedicated during fills
(1-2 per day)
LHC Injection Chain
• 81 bunch packets produced in the PS with 25 ns spacing• Triplets of 81 bunches are formed in the PS and injected into the SPS, taking up ~27% of the SPS beamline• The total LHC beam consists of 12 “supercycles” of the 243 bunches from SPS
LHC: The Lord of the Rings
LHC acceleration and beam steering system
• Entire beamline run cold• RF cavities run at 400 MHz• 1232 Dipole magnets for beam steering• 386 Quadrupole focusing magnets• Many (thousands) of small correcting
magnets also in place
The LHC Dipole Magnet
An RF Cavity…shiny
Luminosity: the other key to the puzzle
N = σIL
N: number of expected events of a certain typeσ: cross section of those types of eventsIL: integrated luminosity
Calculating luminosity from beam parameters
Intersecting storage ring, identical beams
kb: number of bunches, Nb: protons per bunch
fr: revolution frequency, εn: emittance
β: beta function at intersection
€
L = kbNb2 f r
4πεnβ
LHC luminosity goals
In the first year, the expected LHC luminosity is 1033 (cm2 s)-1: 5 times
that of Fermilab
Target luminosity is ten times this value, believed to be achievable in the second year, with 25 times in
the future
Beam Parameters
Beam Difficulties
• Magnet quenching is a real danger, with only a small fraction (10-6) needed
to quench a SM• A quenched dipole will require a
beam dump in a single turn - 7 TeV (690 MJ) dissipated in 89 μs!
• An error in dumping the beam will expose accelerator components to
serious radiation risk
The future of particle accelerators
Ring accelerators are on their way out - the strongest magnets (8.33 T) are
employed to steer the LHC beam
The ILC has the brightest future (more than the VLHC), with wakefield plasma
acceleration achieving limited gradients of 1 GeV/m
References1. M Benedikt (ed.), “The PS Complex as Proton Pre-Injector for the
LHC - Design and Implementation Report”,CERN 2000-03, 20002. G Arduini et. al., “Beams in the CERN PS Complex After the RF
Upgrades for LHC,” Proc. EPAC, 20043. P Collier, “The SPS as Injector for the LHC,” CERN-SL-97-07-DI,
19974. K Schindl, “The Injector Chain for the LHC,” Chamonix IX,
CERN, 19975. N Tahir et. al., “Impact of 7 TeV/c large hadron collider proton
beam on a copper target,” J. Appl. Phys. 97, 20056. C. Rembser, “LHC - Machine and Detectors,” CERN, 2005
Photos courtesy of CERN