Introduction to electron and photon beam physics. Zhirong Huang SLAC and Stanford University

Transcription

1 Introduction to electron and photon beam physics Zhirong Huang SLAC and Stanford University August 03, 2015

2 Lecture Plan Electron beams (1.5 hrs) Photon or radiation beams (1 hr) References: 1. J. D. Jackson, Classical Electrodynamics (Wiley, New York, third edition, 1999). 2. Helmut Wiedemann, Particle Accelerator Physics (Springer-Verlag, 2003). 3. Andrew Sessler and Edmund Wilson, Engine of Discovery (World Scientific, 2007). 4. David Attwood, Soft X-rays and Extreme Ultraviolet Radiation (Cambridge, 1999) 5. Peter Schmüser, Martin Dohlus, Jörg Rossbach, Ultraviolet and Soft X-Ray Free- Electron Lasers (Springer-Verlag, 2008). 6. Kwang-Je Kim, Zhirong Huang, Ryan Lindberg, Synchrotron Radiation and Free- Electron Lasers for Bright X-ray Sources, USPAS lecture notes Gennady Stupakov, Classical Mechanics and Electromagnetism in Accelerator Physics, USPAS Lecture notes Images from various sources and web sites. 2

3 Electron beams Primer on special relativity and E&M Accelerating electrons Transporting electrons Beam emittance and optics Beam distribution function 3

4 Lorentz Transformation 4

5 Length Contraction and Time Dilation Length contraction: an object of length Dz* aligned in the moving system with the z* axis will have the length Dz in the lab frame z = z γ Time dilation: Two events occurring in the moving system at the same point and separated by the time interval Dt* will be measured by the lab observers as separated by Dt t = γ t 5

6 Energy Energy, Mass, Momentum E = T + mc 2 Kinetic energy Rest mass energy Electrons rest mass energy 511 kev (938 MeV for protons), 1eV = Joule Momentum p = γβmc Energy and momentum E 2 = p 2 c 2 + m 2 c 4, E = γmc 2. 6

7 Momentum change Relativistic acceleration Beam dynamics drastically different for parallel and perpendicular acceleration! Negligible radiation for parallel acceleration at high energy 7

8 Maxwell s Equations D = ε 0 E B = μ 0 H c = (ε 0 μ 0 ) 1/2 Wave equation Lorentz transformation of fields 8

9 Field of a moving electron In electron s frame, Coulomb field is In lab frame, space charge fields are

10 Lorentz force Lorentz Force Momentum and energy change Energy exchange through E field only = 0 No work done by magnetic field!

11 Lorentz force Guiding beams: dipole Centrifugal force F cf = γmc2 β 2 ρ (opposite for e - ) Bending radius is obtained by balance the forces 1 = eb ρ γβmc 2 1 ρ [m-1 ]= B[T] βe[gev] 11

12 Cyclotron If beam moves circularly, re-traverses the same accelerating section again and again, we can accelerate the beam repetitively 12

13 Lawrence was my teacher when I built the first cyclotron. He got a Nobel prize for it. I got a Ph.D. (- S. Livingston, years later) 13

14 From Cyclotron to Synchrotron Cyclotron does not work for relativistic beams. 14

15 Synchrotron GE synchrotron observed first synchrotron radiation (1946) and opened a new era of accelerator-based light sources. 15

16 Electron linac 16

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19 SLAC linac 35 Total 12-m CryoModules for LCLS-II

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21 Livingston Plot for High-Energy Accelerators

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23 Beam description Beam phase space (x,x,y,y,dt, Dg) Consider paraxial beams such that 23

24 Linear optics for beam transport Transport matrix Free space drift Quadrupole (de-)focusing 24

25 Beam properties Second moments of beam distribution rms size rms divergence correlation 25

26 Beam emittance Emittance or geometric emittance Emittance is conserved in a linear transport system Normalized emittance is conserved in a linear system including acceleration Normalized emittance is hence an important figure of merit for electron sources Preservation of emittances is critical for accelerator designs. 26

27 Beam optics function Optics functions (Twiss parameters) Given beta function along beamline 27

28 Single particle Free space propagation Beam envelope Analogous with Gaussian laser beam 28

29 FODO lattice Multiple elements (e. g., FODO lattice) 29

30 FODO lattice II Maximum beta Minimum beta When f >> l 30

31 Electron distribution in phase space We define the distribution function F so that is the number of electrons per unit phase space volume Since the number of electrons is an invariant function of z, distribution function satisfies Liouville theorem equations of motion 31

32 Gaussian beam distribution Represent the ensemble of electrons with a continuous distribution function (e.g., Gaussian in x and x ) For free space propagation Distribution in physical space can be obtained by integrating F over the angle 32

33 Photon or radiation beams Introduction to radiation Radiation diffraction and emittance Coherence and Brightness Radiation intensity and bunching Accelerator based light sources 33

34 Photon wavelength and energy 34

35 Single pass FELs (SASE or seeded) Synchrotron radiation Various accelerator FEL oscillators Undulator radiation and non-acc. sources (High-average power) 35

36 Radiation from Accelerated Electrons The field outside of the circle of radius ct does not know that the charge has been moved. If the charge was moved twice, then the field lines at time t > t1 would look like this there will be two spheres, with the radiation layers between them

37 Three forms of synchrotron radiation

38 Shintake Radiation Demo Program

39 Radiation propagation and diffraction Wave propagation in free space Angular representation General solution Paraxial approximation (f 2 << 1)

40 Gaussian beam and radiation emittance Single electron radiation can be approximated by Gaussian beam Gaussian fundamental mode at waist z=0 At arbitrary z Analogous with electron beam 40

41 Coherence E(x,t) at location z z R. Ischebeck 41

42 Transverse (Spatial) Coherence Transverse coherence can be measured via the interference pattern in Young's double slit experiment. Near the center of screen, transverse coherence determines fringe visibility 42

43 Phase space criteria for transverse coherence Initial phase space area 4pR >> l Final phase space area Coherent flux is reduced by M T Show this criteria from physical optics argument 43

44 Temporal Coherence 44

45 Chaotic light Radiation from many random emitters (Sun, SR, SASE FEL) Correlation function and coherence time 45

46 Temporal mode and fluctuation Number of regular temporal regions is # of coherent modes Intensity fluctuation DW W Same numbers of mode in frequency domain 1 M L Fourier limit, minimum longitudinal phase space Longitudinal phase space is M L larger than Fourier limit Total # of modes 46

47 Light Bulb vs. Laser Radiation emitted from light bulb is chaotic. Pinhole can be used to obtain spatial coherence. Monochromator can be used to obtain temporal coherence. Pinhole and Monochromator can be combined for coherence. Laser light is spatially and temporally coherent. A. Schawlow (Nobel prize on laser spectroscopy), Scientific Americans, 1968

48 Brightness D Dx Photons in unit spectral range in unit time B 2 (source size divergence) Units: photons/s/mm 2 /mrad 2 /0.1%BW Peak Average 48

49 Incoherent radiation from many electrons Such a beam can be described by the convolution of the coherent Gaussian beam with the electron distribution in phase space Effective source size and divergence When electron beam emittance s x s x >>l/(4p) x # of transverse modes x

50 Evolution of X-ray Light Sources GE synchrotron (1946) opened a new era of accelerator-based light sources. These light sources have evolved rapidly over four generations. The first three-generations are based on synchrotron radiation. The forth-generation light source is a game-changer based on FELs. The dramatic improvement of brightness and coherence over 60 years easily outran Moore s law.

51 Synchrotron Radiation Facilities NSLS-II (2015) SSRF (2009) MAX-IV (2016) State-of-art storage rings have pulse duration ~10 ps, emittance ~1 nm. Diffraction-limited storage rings with emittance ~10 pm are under active R&D.

52 Radiation intensity What if emitters are not random in time For an electron bunch with rms bunch length s e When intensity from many electrons add incoherently (~N e ) 52

53 Bunching and coherent radiation If the bunch length is shorter than the radiation wavelength Form factor or bunching factor Radiation intensity from many electrons add coherently (~N e2 ) Another way to produce bunching from a relatively long bunch is through so-called microbunching 53