The Advanced Light Source:

What It Is and How It Works

Ina Reichel + Tom ScarvieAFRD

Some viewgraphs and pictures were donated by David Robin, Peter Seidl and Christoph Steier

June 18, 2009

What is the Advanced Light Source?

• A particle accelerator that uses electrons to produce extremely bright light

• Users (several thousand per year) come from all over the world to do experiments using this light

Overview

• Brightness: what it is and why we need it

• How to generate bright beams of x-rays

• How to produce the electron beam required

• What can you study with the light?

What is Brightness?

• Brightness is the number of photons emitted (within some bandwidth) normalized by the emission area and the emission solid angle

The smaller the source and the The smaller the source and the more parallel the beam, the higher more parallel the beam, the higher the brightness.the brightness.

• Total radiated power of the Sun: 1021 times greater than the ALSRsun≈ 100Rearth

RALSbeam ≈ 100micrometers ( = 0.1mm ! )

θsun = 4π ≈ 13 rad2

θALSbeam ≈ 10-8 rad2

areaarea××solid angle factor is about 10solid angle factor is about 103535!!!!!!

The ALS compared to our Sun

• Spectrum of the Sun:“black body radiation” given by surface temperature (5800 K) + absorption lines + some harder UV and x-rays

• ALS covers broad spectrum, including visible and infrared light, with the peak in the ultraviolet and soft x-ray range

•Special magnetic devices extend the spectral range

Why do we need high brightness?

• Samples studied at the ALS are small (protein crystals, cells, computer memory structures, atomic beams…)• Detectors have limited acceptance • Smaller beams enable higher precision experiments, coherence, etc…

Bright beam means smaller source still has plenty of light

Why do we use soft X-rays?

• Wavelength of light has to be less-than or equal to the size of what you want to look at (distance of atoms, molecules,…)

ALS spectrum covers from ~0.1 nanometers (1 Angstrom)

to several millimeters

• Probing the electronic structure of materials requires the energy of soft x-rays

• X-rays can penetrate matter, can probe surface as well as bulk

Synchrotron radiation is electromagnetic radiation emitted when charged particles are accelerated radially

Radio waves (KALX, KFOG) are created by accelerating electrons up and down a radio antenna

Both cases are manifestation of the same physical phenomenon:Both cases are manifestation of the same physical phenomenon:

Charged particles radiate when acceleratedCharged particles radiate when accelerated..

What is Synchrotron Radiation?

•Quantum field theory: particles moving in free space are surrounded by a cloud of virtual photons that can appear and disappear but travel along nearby, until…

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• Some acceleration acts as a “kick” that can separate the particle from the photons, which then become real and independently observable.

In a synchrotron, charged particles are moved on a curved trajectory using magnetic fields. The transverse acceleration liberates the photons, which are then called synchrotron radiation.

Why do particles radiate under acceleration?

Lighter particles are “easier” to accelerate and radiate photons more efficiently than heavier particles, which is why we like to use electrons.

• Historically, the whole theory was developed well before quantum mechanics was even conceived:

• The description of synchrotron radiation presented in the previous viewgraph used quantum field theory (whatever that is!)

PowerLarmor

ac

qP 2

30

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6- in 1897 Joseph Larmor derived the

expression for the instantaneous total power radiated by an accelerated

charged particle.

- and in 1898 Alfred Lienard extended Larmor’s result to

the case of a relativistic particle undergoing

centripetal acceleration in a circular trajectory

(this was before Einstein’s relativity theory!)

1857-19421857-1942

1869-19581869-1958

The Classical Picture

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dt

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dt

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qP

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radiuscurvatureq

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Radiated power for transverse accelerationRadiated power for transverse acceleration increases dramatically with energyincreases dramatically with energy. . This sets a practical limit for the maximum energy obtainable with a storage ring, This sets a practical limit for the maximum energy obtainable with a storage ring, but makes the construction of synchrotron light sources extremely appealing!but makes the construction of synchrotron light sources extremely appealing!

Transverse vs. Longitudinal Acceleration

Why do we care about power?

Radiation Simulator

Program written by Tsumoru Shitakecan be downloaded at

http://www-xfel.spring8.or.jp/

History of Synchrotron Radiation

• Historically in accelerators used for high energy physics, it was an annoyance requiring shielding and lots of RF power

• As has happened before in science, what was once a problem became an extremely important tool!

• First generation synchrotron light sources were high energy physics accelerators after they were no longer needed for high energy physics research

• Later people started building dedicated accelerators which are more optimized to being a light source (e.g. smaller beam size)

In a particle storage rings, charged particles circulate around the ring in bunches for a large number of turns.

Particle Storage Rings

Particle bunchesOptics elements

quad

rupo

les

dipo

les

RF cavity

The ALS

The ALS consists of two pre-accelerators, a storage ring,and many beamlines for experiments

Photon energy

• Doppler shift shifts radiation into x-ray range (like pitch of sound of fire engine higher when approaching, lower when departing)

Bending Bending MagnetMagnet

critical energy: εεcc(keV) = 0.665 B(T)E(keV) = 0.665 B(T)E22(GeV)(GeV)

Continuous bend magnet spectrum Continuous bend magnet spectrum characterized by characterized by cc = critical energy = critical energy

bending magnet

undulator - coherent interference

How Synchrotron Radiation is Generated in How Synchrotron Radiation is Generated in Storage RingsStorage Rings

Undulator gives quasi-Undulator gives quasi-monochromatic (laser-like) monochromatic (laser-like)

spectrumspectrum

The photon energy depends The photon energy depends on the period and the field on the period and the field

strengthstrength

RF Acceleration

• Particles are accelerated using RF fields

• Particles are also longitudinally focused by the RF wave

• Similar to surfer on a wave

Typical Magnet Types

There are several magnet types that are used in storage rings:

Dipoles used for guiding Bx = 0 By = Bo

Quadrupoles used for focusing Bx = Ky By = -Kx

Sextupoles used for chromatic correction

Bx = 2Sxy By = S(x2 – y2)

The Electron Gun

• All electrons are generated at the electron gun

• Similar to television tube, but: higher voltage and much higher temperatures to generate more electrons

• Cathode out of (thoriated) tungsten: 800-1000 degree C

• Can be compared to hottest spots on space shuttle when re-entering atmosphere!

The LINAC (first pre-accelerator)

• After exiting the gun with 50-100 keV (about 3-5 times more than in television), the LINAC accelerates the electrons to 50 MeV 99.995% of the speed of light within just a few meters (and about a tenth of a millionth of a second)

• Microwaves (RF) are used to accelerate the electrons (they are ‘surfing’ the electric waves): Peak RF power is about 50 MW, about 50,000 microwave ovens !

The Booster (second pre-accelerator)

• After leaving the LINAC, the electrons are further accelerated in the booster synchrotron (another, smaller ring)

• They are accelerated to 1.9 GeV (billion Volts!) (99.99999% of the speed of light) within 0.1 s!

• Acceleration again uses microwaves, which oscillate 500,000 thousand times a second, or at 500 MHz.

• During the 0.1 s, the electrons complete 500,000 turns, about one tenth of the distance to the moon!

• Strong magnets keep the electrons circulating.

The Storage Ring

• Finally, the electrons are transferred to and stored in the storage ring at 1.9GeV

• The electrons in the storage ring are so close to the speed of light, that they would lose a race to the moon against light by only 14 m!

• In the storage ring, the 1012 (or one million * one million) electrons circulate for about 8 hours, slowly getting lost while producing synchrotron radiation

• After eight hours they have traveled a distance of 9*1012 m (22,000 times the distance to the moon or 60 times to the sun)

Vacuum System

• Accelerators need good vacuum to avoid electrons scattering off gas molecules

• In air, electrons would be lost after about 1/3 turn

• Atmospheric pressure: 760 Torr

• Pressure at 100km altitude: 10-3 Torr

• Pressure on surface of the moon: 10-11 Torr

• Pressure inside the ALS vacuum chamber: ~10-10 Torr

Undulators, Wiggler, Superbends

Many types of sources are used to generate the light: bending magnets; wigglers; undulators; Superbends (superconducting bend magnets)

Protein Crystallography

• Proteins are essential parts of organisms and participate in every process within cells

• Understanding their molecular structure can help understand their function

• Freeze protein into crystal

• Diffraction pattern from shining X-rays on crystal allow study of 3D atomic model

Magnetism and magnetic materials

• Magnetic storage devices limited by how small you can make the space required to store one bit

• Studies how to get more bits in the same area can lead to smaller storage devices

2 Megabytes in 1975! 8 Gigabytes in 2008!

Spectroscopy

• Binding energies of inner-shell electrons for an atom vary depending on the electron density around that atom

• Furthermore, the molecule's geometry can remove the degeneracy of inner-shell electrons

• Investigation of inner-shell photoionization in molecules

This technique is very sensitive to molecular environment

Tour of ALS

ALS tours are scheduled for

Monday, June 22 at 1pm, 2pm, and 3pm.

Monday is a great day for a tour since the machine will be completely open for maintenance