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The LHC: an Accelerated Overview
Jonathan Walsh
May 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 s pulse length (120+ s)– 1.2 m transverse rms emittance (1.2 m)
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 Booster
PS: Proton Synchrotron
SPS: Super Proton Synchrotron
LHC: 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 = IL
N: number of expected events of a certain type
: cross section of those types of events
IL: integrated luminosity
Calculating luminosity from beam parameters
Intersecting storage ring, identical beams
kb: number of bunches, Nb: protons per bunch
fr: revolution frequency, n: emittance
: beta function at intersection
€
L =kbNb
2 f r4πεnβ
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 s!
• 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
References
1. M Benedikt (ed.), “The PS Complex as Proton Pre-Injector for the LHC - Design and Implementation Report”,CERN 2000-03, 2000
2. G Arduini et. al., “Beams in the CERN PS Complex After the RF Upgrades for LHC,” Proc. EPAC, 2004
3. P Collier, “The SPS as Injector for the LHC,” CERN-SL-97-07-DI, 1997
4. K Schindl, “The Injector Chain for the LHC,” Chamonix IX, CERN, 1997
5. N Tahir et. al., “Impact of 7 TeV/c large hadron collider proton beam on a copper target,” J. Appl. Phys. 97, 2005
6. C. Rembser, “LHC - Machine and Detectors,” CERN, 2005
Photos courtesy of CERN