Transcript ilc_brief_collider

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
Elements of the linear collider (the electron half of it):
pre-accelerator
few GeV
source
KeV
damping
ring
few GeV
few GeV
bunch
compressor
250-500 GeV
main linac
extraction
& dump
final focus
IP
collimation
Electron
source
pre-accelerator
few GeV
source
KeV
damping
ring
few GeV
few GeV
bunch
compressor
250-500 GeV
main linac
extractio
& dump
final focus
IP
collimation
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.
Damping
Ring
pre-accelerator
few GeV
source
KeV
damping
ring
few GeV
few GeV
bunch
compressor
250-500 GeV
main linac
extractio
& dump
final focus
IP
collimation
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.
Bunch
compressor
pre-accelerator
few GeV
source
KeV
damping
ring
few GeV
few GeV
bunch
compressor
250-500 GeV
main linac
extractio
& dump
final focus
IP
collimation
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 mm.
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.
pre-accelerator
Main Linac
few GeV
source
KeV
damping
ring
few GeV
few GeV
bunch
compressor
250-500 GeV
main linac
extractio
& dump
final focus
IP
collimation
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.
* Superconducting to reduce
power lost due to resistance in
cavity walls.
accelerating electron
Electric
field
direction of motion
Main Linac
rf power
pre-accelerator
few GeV
source
KeV
damping
ring
few GeV
few GeV
bunch
compressor
250-500 GeV
main linac
extractio
& dump
final focus
IP
collimation
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 Linac
rf power
Modulator (328
per linac)
pre-accelerator
few GeV
source
KeV
damping
ring
few GeV
few GeV
bunch
compressor
250-500 GeV
main linac
extractio
& dump
final focus
IP
collimation
Klystron
(328 per
linac)
Coupler (3934 per linac)
Superconducting
cavity (7868 per linac)
Final focus and
extraction
pre-accelerator
few GeV
source
KeV
damping
ring
few GeV
few GeV
bunch
compressor
250-500 GeV
main linac
extractio
& dump
final focus
IP
collimation
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
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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.
head
room
{
parameter 1
max
luminosity
operating plane
Design to run on the
operating plane, allowing
flexibility to solve
unexpected problems.
Relative cost of subsystems
operations (4%)
cryo operations
cryogenics (4%)
4%
4%
instrumentation (2%)
instrumentation
2%
controls
controls (4%)
4%
vacuum
civilcfconstruction (31%)
vacuum
(4%)
4%
31%
magnets
magnets (6%)
6%
installation & test
(7%)
7%
installation&test
systems engineering
(8%)
systems_eng
8%
accelerating cavities (18%)
rfrf power
12%
(12%)
structures
18%
Pay most attention to the cost drivers – the civil
construction and the main linac components.
Optimizing the cost
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).
Accelerating electric field
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