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