The Linear Collider –accelerator design

A particle beam accelerator is a microscope – the resolution is inversely proportional to the energy of the beams. Seeing fine detail requires very high energy.

Making new particles of matter requires high energy (E=mc2)

Using two high energy beams that collide with each other allows much higher available energy than one high energy beam impinging on a stationary target (due to conservation of momentum).

Particle acceleration relies on strong electric fields, phased to boost the energy of beam particles as they traverse an accelerating structure.

Acceleration by travelling

radiofrequency waves

The Linear Collider –accelerator design

Older electron-positron colliders are racetracks, with particles circulating repeatedly through an accelerating structure. But as the energy increases, the radiated energy grows dramatically (this is the basis for synchrotron light sources!).

Thus for the ILC, make linear accelerators that bring bunches of particles into collision just once, then transport to a beam dump. The beams must be very small (nanometer scale) to give the needed high rate of collisions.

Collision energy needed for ILC physics is 500 – 1000 GeV (two 250 – 500 GeV beams).1 GeV = billion electron volts = energy gained by an

electron traversing a billion volt battery potential.

Elements of the ILC

main linacbunchcompressor

dampingring

source

pre-accelerator

collimation

final focus

IP

extraction& dump

KeV

few GeV

few GeVfew GeV

250-500 GeV

Elements of the linear collider (the electron half of it):

main linacbunchcompressor

dampingring

source

pre-accelerator

collimation

final focus

IP

extraction& dump

KeV

few GeV

few GeVfew GeV

250-500 GeV

Electron source

Can get electrons boiled out from a heated filament; but to have electrons polarized (spins all pointing the same way) shine a polarized laser on a gallium arsenide crystal. The emitted electron spin direction can be altered pulse to pulse.

Collect these electrons in accelerating cavities, arrange them into bunches, and further accelerate to about 5 GeV.

The energy spread within the bunch and the angular divergence are still large – far too large to produce the nanometer sized beams at collision.

main linacbunchcompressor

dampingring

source

pre-accelerator

collimation

final focus

IP

extraction& dump

KeV

few GeV

few GeVfew GeV

250-500 GeV

Damping Ring

The Damping Ring is needed to reduce the angular beam spread. This is done by sending the beam around a racetrack so that electrons emit synchrotron radiation. Both transverse and longitudinal (along the beam direction) momentum is lost. Restore longitudinal momentum using accelerating electric fields. The net effect is a drastic reduction of the transverse momentum (thus the angular divergences) while keeping the beam energy fixed.

For the ILC, the train of bunches is very long (~ 300 km); bunches are folded into a tight pattern to fit into the DR. Fast kicker magnets extract single bunches at the appropriate interval for subsequent acceleration.

main linacbunchcompressor

dampingring

source

pre-accelerator

collimation

final focus

IP

extraction& dump

KeV

few GeV

few GeVfew GeV

250-500 GeV

Bunch compress

or

The beam bunches from the Damping Ring are tightly controlled in angular spread, but are too long to give high brightness at collision. The Bunch Compressor shortens the longitudinal dimension from 6 mm to 300 m.

Electron bunches from the damping ring are manipulated to have the lowest energy particles at the front. Each bunch is passed through a magnetic dog-leg so that the low energy electrons travel further and arrive back at the same time as the higher energy electrons at the back of the bunch.

The bunch shortening is done at the expense of increased energy spread.

main linacbunchcompressor

dampingring

source

pre-accelerator

collimation

final focus

IP

extraction& dump

KeV

few GeV

few GeVfew GeV

250-500 GeV

Main Linac

The Main Linac accelerates electrons from 5 GeV to 250 GeV (for colliding energy of 500 GeV), over about 20 km.

Electrons passing through a superconducting* niobium cavity feel a travelling radio frequency (1.3 GHz) electromagnetic wave whose electric field boosts their energy by about 30 MeV per meter of cavity.

Electric field

direction of motion

accelerating electron* Superconducting to reduce power lost due to resistance in cavity walls.

main linacbunchcompressor

dampingring

source

pre-accelerator

collimation

final focus

IP

extraction& dump

KeV

few GeV

few GeVfew GeV

250-500 GeV

Main Linac rf power

Line ac power is fed to a Modulator (capacitor switching device) that provides a high voltage dc pulse. This pulse is sent to a Klystron (high current electron tube that converts the power to high voltage radiofrequency (rf) power (10 MW at 1.3 GHz).

Klystron rf power is fed through an input Coupler to a Superconducting Cavity, forming the travelling wave that accelerates the beam.

Low level rf system controls the timing.

Coupler

main linacbunchcompressor

dampingring

source

pre-accelerator

collimation

final focus

IP

extraction& dump

KeV

few GeV

few GeVfew GeV

250-500 GeV

Main Linac rf power

Modulator (328 per linac)

Klystron (328 per linac)

Coupler (3934 per linac)Superconducting cavity (7868 per linac)

main linacbunchcompressor

dampingring

source

pre-accelerator

collimation

final focus

IP

extraction& dump

KeV

few GeV

few GeVfew GeV

250-500 GeV

Final focus and

extraction

Over the last 2 km, powerful focussing magnets bring the beam to a small spot size (10 nanometers (nm) high by 1000 nm wide) within the experimental detector at the collision point. Bunches arrive every 300 ns over a 1 ms interval. Collimators remove the background particles in the halo.

Instrumentation before and after the collision point allows measurement of beam energy, polarization and intensity.

Beams are channeled from the interaction region to specially designed beam dumps to handle the intense radiation load.

The positron side

The positron accelerator is a mirror image of the electron side …

except that we can’t make positrons by extracting them from ordinary matter.

Create positrons from the accelerated electrons by passing them through ‘wiggler magnets’ that create photons which impinge upon a target, creating positron – electron pairs. Collect the positrons, channel them to the positron damping ring, and proceed as for electrons.

Keeping the beams harnessed

The key to high brightness (high luminosity) is keeping the beam sizes very small from the damping rings to interaction point.

This demands very tight alignment tolerances, control of disruptive effects of the beam on itself, and remote positioning devices to correct for vibration & ground motion.

Designing the collider

Use a parametric approach to machine design to allow a range of parameters stressing different aspects of the machine.

parameter 2

parameter 1

{head room

max luminosity

operating planeDesign to run on the operating plane, allowing flexibility to solve unexpected problems.

Relative cost of subsystems

cf31%

structures18%rf

12%

systems_eng8%

installation&test7%

magnets6%

vacuum4%

controls4%

cryo4%

operations4%

instrumentation2%

civil construction (31%)

accelerating cavities (18%)

rf power (12%)

systems engineering (8%)

installation & test (7%)

magnets (6%)

vacuum (4%)

controls (4%)

cryogenics (4%)operations (4%) instrumentation (2%)

Pay most attention to the cost drivers – the civil construction and the main linac components.

Accelerating electric field

Cost

Optimizing the cost

Varying the many machine parameters allows a parametric cost optimization.

Shown is the cost vs. accelerating field – higher field cuts civil and cavity costs but increases cryogenic costs. The minimum is quite shallow (4% cost increase going from 40 to 30 MV/m).

Critical issues

The ILC has a large cost: a substantial R&D program is necessary to assure cost optimization and reliability. Among the key issues:

Reliable high gradient superconducting cavities; streamline fabrication procedure for industrial production; compact packaging into cryomodules.

Efficient, reliable, low cost rf power elements (modulator, klystron, coupler)

Optimize the civil design – 1 vs. 2 tunnels (linac and service) or just 1 ? Cut-and-fill or tunneling? Laser straight or follow earth curvature?

Control of beam dynamics – avoid beam blowup through self interactions, residual gas. Develop beam alignment techniques.

Instrumentation for beam position measurement, machine protection system, beam extraction.

Conclusions

The International Linear Collider design has been developed over the past 12 years

The basic technological choices have been made

Working prototypes exist for all major systems

The project is technically achievable

Test facilities exist at labs around the world

R&D funding is critically needed over the next three years to optimize costs, improve reliability, do

value engineering, and develop industrialization