The Laser: A Solution Looking for A Problem

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The Laser A Solution Looking for A Problem ?
Graeme HirstSTFC Central Laser Facility

• Cockcroft Institute Laser Lectures
• April 2008

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This Lecture Series

• TODAY
• 1. The Laser A Solution Looking for a Problem
• 2. Radiation Sources Horses for Courses
• APRIL 21st
• 3. Laser Science Come on, How Hard Can It Be ?
• 4. Laser Technology or Why is My Ray-Gun so
  Expensive ?
• APRIL 28th
• 5. Lasers as Accelerator Subsystems or Whats
  the Problem Bob ?
• I Dont Know Alice, It Must Be The Laser.
• 6. Laser Accelerators The Technology of The
  Future (They Always Have Been and They Always
  Will Be ?)

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Why Should I Care ?
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Health Warning !
There are two kinds of laser scientist Those
who regard the laser as a black box out of which
comes light, And those who regard the
universe as a black box into which goes light
! It is important to establish as
quickly as possible which kind youre talking
(or listening) to ...
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Lecture 1 Plan

• Introduction
• Whats the thing about lasers ?
• What types of laser are there ?
• Where do lasers fit with respect to accelerators
  ?
• What else are they used for ?
• What about safety ?

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The Thing(s) About Lasers
STIMULATED EMISSION
In lasers the light acts on the laser medium
tostimulate the coherent emission of more light
COHERENCE
Because laser amplification is coherent,
selectivetechniques can be used to generate
well-definedbeams from optical noise with high
efficiency
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The Tree of Life
Simplified from http//www.tellapallet.com/tree_of
_life.htm
Bacteria
Archaea
PondScum
Plants
SlimeMoulds
Fungi
Animals
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The Tree of Life
Lasers
Simplified from http//www.tellapallet.com/tree_of
_life.htm
Bacteria
Archaea
PondScum
Plants
SlimeMoulds
Fungi
Animals
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The Diode Laser Business
Other
Laser pumping
Revenuegrowthhas beenachievedby
raisingvolume asunit pricehas fallen
Optical storage (CD, DVD)
Telecommunications
Diode laser sales (M)

• Data from Laser Focus World magazine

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The Tree of Life
Lasers
Simplified from http//www.tellapallet.com/tree_of
_life.htm
Bacteria
Archaea
PondScum
Plants
SlimeMoulds
Fungi
Animals
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The Non-Diode Laser Business
Other
Instrumentation
Researchlasersmake up6 of thenon-diodemarket
Basic research
Medical therapeutics
Materials processing
Diode laser sales (M)

• Data from Laser Focus World magazine

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The Tree of Life
Lasers
Simplified from http//www.tellapallet.com/tree_of
_life.htm
Bacteria
Archaea
PondScum
Plants
SlimeMoulds
Fungi
Animals
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Laser Light Sources
A general-purpose laboratorylight source might
give
100fs pulses at 800nm 10nJ/pulse at 80MHz
and2mJ/pulse at 1kHzi.e. Pave ? 1 watt Near
transform-limited anddiffraction-limited
output Tunability from gt2000nm tolt300nm with
conversionefficiency from lt1 to gt10
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Lasers in Accelerators
Modern accelerators incorporate lasers for
Photogeneration ofelectron bunches
Heating of the particle beam
Spatial beam diagnosisby laser wire
X-ray generation byCompton scattering
Temporal beam slicing
Ultra-low noise clockgeneration and distribution
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Lasers as Accelerators
High intensity laser pulses induce large electron
density changes in plasmas The E-field arising
from the electron-ion charge separation can be
GV/cm Electrons travelling with the plasma wave
can acquireGeV energies with DE/E of 1 and low
emittance
Issues include

• shot rate i.e. average current
• shot-to-shot reproducibility
• further reduction of DE/E
• length scaling

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Very Short Pulse Lasers
The shortest direct pulses come from
laserswhose media have gain over a wide
spectralbandwidth (Fourier) and whose
opticalcavities contain elements which (phase)
lockmany cavity modes
Commercially available lasers can deliver7 fs
pulses at 800 nm where the period is 2.7 fs
Time (fs)
E field
Intensity
Intensity envelope
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Very Temporally Stable Lasers
Atomic transitions with linewidths ltlt1Hz are now
known and can beused as timing references with
stability (much) better than a part in 1015 An
issue is the development of an optical
clockwork to divide the opticalfrequency down
to a countable value One approach uses the
frequency comb produced by modelocked lasers
wref sets the absolute frequencyand beating w1
with 2w1 allowsthe comb spacing to be stabilised
(See 2005 Nobel Prize for physics)
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A Very Large Laser Schematic
The NationalIgnition Facilityat
LawrenceLivermoreNational Lab
Laser amplifiers
Laser pulsegenerator
1.8MJ will bedelivered in192 beamsto a
sub-mmDT cryo target
Spatial filters
Beam transport
Targetchamber
Nanosecond ablation ofthe target surface will
compressthe interior, heating it to a
temperaturesufficient to ignite a nuclear fusion
reaction
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A Very Large Laser
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A Very Large Laser
France is also buildingone of these
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Very High Average Power Lasers
Average laser power is limited by

• Efficiency of conversion from pump to output
• Ability to cool the laser medium

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Very High Average Power Lasers
Laser diodes can be gt70 wall-plug efficient and
optically-pumpedsolid state stages can be nearly
as good
The SSHCL at LawrenceLivermore National Labhas
delivered 67kWfor 10 seconds and canrun at 10
duty cycle
Connections toMW battery
Pump diodes
NdGGG orNdYAG/ceramicslabs
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SDI - Star Wars
In 1983 President Reagan challenged the
scientific community to give usthe means of
rendering ... nuclear weapons impotent and
obsolete
One option, discussed by Edward Teller, was an
array of space-basedDirected Energy Weapons
using X-ray lasers pumped by nuclear explosions
An independent review estimated the required
on-target fluence to be 3 kJ/cm2and listed many
problems with generating and projecting this
This idea was judged weaker than Brilliant
Pebbles
Rev Mod Phys 59 (3) S1, 1987
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Star Wars - Pt 2
S P Bugaev et al,A 2-kJ,wide-apertureXeCl
laser, Quantum Electronics34 (9) 801, 2004
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Laser Safety
Laser hazards include

• High power electrical supplies in close proximity
  to cooling water
• Tripping over cables in darkened laboratories
• Laser eye damage
• Crushing by heavy equipment
• Laser skin damage (burns, UV)
• Toxic substances
• Fire
• Collateral radiation (ionising, optical)

Maximum permissible exposuresare only 5 times
below the 50damage threshold The corneal MPE
for a sub-10ps,800nm beam is 2.410-4 J/m2 The
NWSF laser on ERLPgenerates 280 J/m2
unfocused(0.8J, f60mm, 100fs)
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Laser Safety
Laser hazards include

• High power electrical supplies in close proximity
  to cooling water
• Tripping over cables in darkened laboratories
• Laser eye damage
• Crushing by heavy equipment
• Laser skin damage (burns, UV)
• Toxic substances
• Fire
• Collateral radiation (ionising, optical)

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Conclusions

• The range of laser types and capabilities is
  extremely wide
• A multi-billion dollar industry supports diverse
  applications in telecoms, IT, medicine and
  manufacturing
• In 45 years lasers have gone from a scientific
  curiosity toan essential laboratory tool
• Significant developments continue on timescales
  of justa few years
• So whats the next problem ... ?