Applications of Lasers at Accelerators A general overview Matthias Gross for the PITZ team Nakhon Ratchasima, 01. November 2018
Introduction of DESY “ Deutsches Elektronen- Synchrotron”
DESY – Overview Deutsches Elektronen-Synchrotron • National research centre of Germany • Member of the Helmholtz Association • Two sites: Hamburg and Zeuthen • Hamburg since 1959, Zeuthen since 1992 DESY Together 2300 employees + more than 3000 • guest scientists from over 40 countries each year Research Topics • Accelerators • Photon Science • Particle Physics / Astroparticle Physics | Overview | 2018 Page 3
Research in Zeuthen Astroparticle Physics • Role of high-energy particles in the cosmic evolution • Neutrino astronomy/cosmology, gamma astronomy, theoretical astroparticle physics, multimessenger astronomy Particle Physics • Search for the fundamental building blocks of nature and their interaction • Experimental and theoretical particle physics Accelerators • Development of tomorrow's accelerators • Photoinjector Test Facility in Zeuthen (PITZ) | Overview | 2018 Page 4
Latest Science News from DESY in Zeuthen 12 July 2018: Breakthrough in the search for cosmic particle accelerators • Scientists trace a single neutrino back to a galaxy billions of light years away 13 August 2018: World record – Low-draft electron bunches drive high plasma wakes • Scientists achieve highest ratio of acceleration to deceleration in plasma wakes yet 10 October 2018: First CTA telescope inaugurated • LST-1 makes its debut on the northern Cherenkov Telescope Array site | Overview | 2018 Page 5
Lasers and Accelerators Example: FEL – FLASH at DESY, Hamburg site Could be used: Pump-probe Seed laser Laser heater laser Diagnostics (Laser wire…) Photocathode laser Optical Drive laser / synchronization Ionization laser of the whole for setup Plasma acceleration | Applications of Lasers at Accelerators | Matthias Gross for the PITZ team, 01. November 2018 Page 6
Contents Applications of Lasers at Accelerators Introduction / Basics Synchronization • Laser principle / properties • Basics • Important laser types • Example: European XFEL Photoinjector Laser-beam interactions • Example: PITZ • Diagnostics • Laser pulse shaping • Compton Back Scattering Seeding Novel acceleration • Working principle • Introduction • Example: sFLASH • Plasma electron acceleration • Alternative Schemes Pump-probe laser • Working principle Outlook / Summary • Possible experiments | Applications of Lasers at Accelerators | Matthias Gross for the PITZ team, 01. November 2018 Page 7
Introduction / Basics
Laser Working Principle • General setup L ight A mplification by partially Cavity transparent S timulated E mission of R adiation Gain medium Pump | Applications of Lasers at Accelerators | Matthias Gross for the PITZ team, 01. November 2018 Page 9
Properties of Laser Light • They produce highly directional beams. • They have a narrow spectrum (bandwidth). • They are spatially and temporally coherent. • Diagram courtesy Prof. Simon Hooker | Applications of Lasers at Accelerators | Matthias Gross for the PITZ team, 01. November 2018 Page 10
Laser Basics Properties cover a wide range • Wavelength • From THz to X-ray. Most common: visible and near infrared • Temporal structure • cw (e.g. laser pointer) or pulsed (e.g. data transmission) • Accelerator lasers: typically pulsed with lengths of ps to fs • Output power • Very low (e.g. barcode scanner) to very high (e.g. laser welding) Pulse energies for accelerators: typically J (photocathode laser) to J (laser driven plasma acceleration) • • Gain medium • Solid: crystal, glass (most important for accelerators) • Semiconductor: pump laser • Gas, liquid , … • Electrons: free electron laser (FEL) | Applications of Lasers at Accelerators | Matthias Gross for the PITZ team, 01. November 2018 Page 11
Ti:Sapphire Laser Energy level diagram of Ti:sapphire laser Short pulses, high output power • Gain medium: a crystal of sapphire (Al 2 O 3 ) that is doped with titanium ions • Pump: typically a frequency doubled Nd:YAG laser (532 nm) • Tunable over wide wavelength range (650 nm to From: Renk K.F. (2012) Titanium – Sapphire Laser. In: Basics of Laser Physics. Springer 1100 nm); most efficient around 800 nm good for ultrashort pulses (down to a few fs) • Typical setup of Ti:Sapphire laser • High output power reachable with Chirped Pulse Amplification | Applications of Lasers at Accelerators | Matthias Gross for the PITZ team, 01. November 2018 Page 12
Optical Parametric Chirped Pulse Amplification (OPCPA) Reaching very high pulse energies • Amplifying an ultrashort laser pulse up to petawatt level • Typical: Ti:Sapphire laser as seed • Main idea: keep laser pulse intensity at a manageable level in the amplifier crystal • After compressor: light transport in vacuum only (filamentation) Source: Wikipedia | Applications of Lasers at Accelerators | Matthias Gross for the PITZ team, 01. November 2018 Page 13
Solid State Photocathode Laser at PITZ Built and maintained by Max Born Institute • Basic principle • Solid state: Yb:KGW oscillator, Yb:YAG amplifier, 2x frequency doubling • Basic parameters • Wavelengths: 1030/515/257 nm Pulse length: 2…25 ps • • Pulse energy: <5 µJ in the UV • Repetition rate: 10 Hz (1 MHz in burst) • Manufacturer Laser pulse timing structure: • Max Born Institute, Berlin (custom product) 100ms (10 Hz) • Application • Photocathode laser 1 s up to 600 pulses | Applications of Lasers at Accelerators | Matthias Gross for the PITZ team, 01. November 2018 Page 14
Fiber Laser • Optical fiber: guiding light by difference in refractive indices of core and cladding • Advantages: • Compact • High electrical and optical efficiency • Reliable (24/7 operation of accelerators) • Excellent beam quality Use as oscillator of solid state laser systems • http://www.fiberlaser.fujikura.jp/eng/products/about-fiber-laser.html | Applications of Lasers at Accelerators | Matthias Gross for the PITZ team, 01. November 2018 Page 15
Diode Laser Used as pump laser for solid state laser systems active zone • Advantages active zone • High efficiency ( direct current conversion from electrical current to light ) p n • Cheap F n • Small eV • Direct modulation is possible hf F p • Disadvantages metal • Low beam quality cleaved facet • Not easy to produce ultrashort pulses Schematic of a p-n junction forward p-n diode laser biased with voltage V | Applications of Lasers at Accelerators | Matthias Gross for the PITZ team, 01. November 2018 Page 16
Free Electron Laser (FEL) Principle SASE = self amplified spontaneous emission • Highly energetic electrons are forced onto a slalom Power course in a magnetic structure (undulator) and are emitting synchrotron radiation • Photons and electrons interact, leading to a density modulation in the electron bunch Distance • Radiation is stimulated in the disks, leading to lasing | Applications of Lasers at Accelerators | Matthias Gross for the PITZ team, 01. November 2018 Page 17
Photoinjector
Photoinjector Photo cathode RFgun: Main solenoid, (Cs 2 Te) L-band (1.3 GHz) Setup at PITZ (Photoinjector Bz_peak~0.2T nc (copper) Test Facility at DESY, Zeuthen Site) standing wave 1½-cell cavity Cathode laser • Photocathode laser must be 257nm stable and locked to RF. ~20ps (FWHM) • Light needed at energies Vacuum >photocathode work function, mirror UHV generally UV . • -> Laser needs third or fourth harmonic frequency conversion. • Laser requirements: Coaxial RF coupler • High pulse energy Bucking solenoid • Reliable running • Excellent pointing stability Laser pulse timing structure: Electron bunch • Properties of the bunch can be 1nC, ~6.7MeV/c controlled by the laser pulse shape in time and space. | Applications of Lasers at Accelerators | Matthias Gross for the PITZ team, 01. November 2018 Page 19
Laser Pulse Shaping Important to optimize electron bunch quality (at PITZ and elsewhere) Projected emittance: PITZ holds WR on Experiments (projected emittance): lowest measured projected emittances Simulated slice emittance (1nC) 1.2 60 emittance slice emittance (mm mrad) (Gaussian) 1 50 0.8 40 current (A) emittance (flattop) 0.6 30 emittance 0.4 20 (3D-ellipsoidal) 0.2 10 current (flattop) 0 0 z-<z> (mm) -5 -4 -3 -2 -1 0 1 2 3 4 5 6 • Laser shaping key for optimizing photoinjector brightness (Q/ x y z ) . • Ellipsoidal laser shaping benefits high bunch charge beams or CW guns (lower gun gradients). | Applications of Lasers at Accelerators | Matthias Gross for the PITZ team, 01. November 2018 Page 20
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