PBPL Study of X-ray Harmonics of the Inverse Compton Scattering Experiment at UCLA Particle Beam Physics Lab UCLA Adnan Doyurana, Joel Englanda , Chan Joshib , Pietro Musumecia , James Rosenzweiga, Sergei Tochitskyb , Gil Travisha, Oliver Williamsa a UCLA/Particle [email protected] Beam Physics Laboratory, b UCLA/Laser Plasma Group, Los Angeles, CA 90034, USA Design Electron, Laser and Scattered Photon Beam Parameters Abstract Electron and Laser Beam Parameters We propose an Inverse Compton Scattering experiment, which will investigate nonlinear properties of the scattering utilizing the terawatt CO2 laser system in Neptune Laboratory at UCLA. When the normalized amplitude of the vector potential a0 is larger than unity the scattering occurs in the nonlinear region; therefore, higher harmonics are also produced. We discuss the experimental setup and procedure to measure the angular and spectral distribution of the higher harmonics scattered from 14 MeV electron beam. We also present a calculation tool for the Double Differential Spectrum (DDS) distribution and total number of photons produced for both head-on and 90o scattering. We decided to do the experiment at 90o to avoid complications due to strong diffraction of the incoming laser. We discuss the electron and laser beam parameters for the experiment. Scattered Photon Beam Parameters Parameter Electron Beam Energy Beam Emittance Electron Beam spot size (RMS) Value 14 MeV 5 mm-mrad 25 m Beam Charge 300 pC Bunch length (rms) 4 ps Inverse Compton Scattering UCLA/NEPTUNE Laboratory Accelerator consists of a photo-injector RF Gun (BNL/SLAC/UCLA), a solenoid, PTW Linac, a chicane compressor, Three triplets for final focusing, 10 steering magnets, two integrating current transformers (ICT) for charge measurements and a number of pop in monitors for beam measurements. The accelerator can deliver up to 15 MeV Electron beams with ~5 mm-mrad emittance and 300 pC charge. We plan to do the experiment with 14 MeV electrons. The photo-injector gun is driven by Achieved beam parameters for NEPTUNE. 10 ps long UV laser at 266 nm. A 1024 nm mode locked Nd:YAG Parameter Value Beam Energy 15 MeV laser is amplified by Nd:Glass Energy spread <0.5% regenerative amplifier and then Energy jitter <1% quadrupled which yields up to 200 Beam size (rms) 100 µm J pulse energy. Beam Charge 300 pC Bunch length (rms) CO2 Laser Power 4 ps 500 GW CO2 Rayleigh range 2.2 cm Pointing jitter <100 µm Alignment precision <1 mrad UCLA/NEPTUNE Laboratory houses a Terawatt CO2 laser system which delivers 10.6 m CO2 pulses with 200 ps pulse length and up to 100 J energy pulses (500 GW) I0 is the intensity and 0 is the wavelength of the incident laser a0 0.85 105 0 [m] I 0 [W / m2 ] 0 2 2 (1 z 0 ) 2 a0 1 2 2 2 Frequency of the scattered photons where 0 is the frequency of the incident laser, is the relativistic factor of electron beam, z0 is component of the electron initial velocity in the direction of laser pulse. (z0=1 for head on scattering and z0=0 for 90o scattering. 10 CO2 laser wavelength 10.6m Number of Photons per electron 0.11 CO2 laser Rayleigh Range 0.75 mm Total Number of Photons CO2 Laser Power 500 GW Opening angle (deg) 15 CO2 waist size (RMS) 25 mm bandwidth (%) 10 CO2 laser pulse length 200 ps rb n High intensity CO2 laser can be focused to achieve the normalized laser strength parameter a0 1 . Therefore it is possible to investigate the nonlinear properties of the Inverse Compton Scattering. In order to avoid the additional nonlinearities due to diffraction we choose a transverse scattering geometry (I=90o). a0 1 0 n 2 2 (1 z 0 ) Frequency of harmonics 2 a0 1 2 2 2 2 2 2 d In e k 20 d d c 2 sin k 0 2 2 [ A J A ( J / b ) ] 2 n t 1 n k Radiation Spectrum for circularly polarized laser beam* 75 m waist radius using an F3 and 50 m waist radius using F2 focusing geometries are achieved experimentally in Neptune. The M factor is estimated to be 1.93. For F2 geometry the Rayleigh range is 0.75 mm. 500 GW laser yields an intensity of 1.25x1016 W/cm2 and a0 1. Since a0 is proportional to 0 wavelength it is advantageous to use CO2 laser compared to YAG lasers. For F3 geometry the Rayleigh range is 1.7 mm. 500 GW laser yields an intensity of 5.5x1015 W/cm2 and a0 0.67 . 1 n n N 0 2 Bandwidth where N0 is the number of periods that electrons interact. 10% bandwidth is estimated for the fundamental (1 a0 / 2) n s 2 e k0 sin k 0 k g b1 b2 2 [ B B 2 B ] 0 1 2 d d 4 2 c k k0 2 2 d I n 2 2 e k0 sin k 0 b1 b2 2 [ B 2 B ] 1 2 d d 4 2 c k 2 2 2 d 2I n d d d 2 I n d d d 2 I n d d Radiation Spectrum for linearly polarized laser beam* 2 2 n Half opening angle of the harmonics 15 mrad half angle is estimated Permanent Magnet Quadrupole (PMQ) System for Final Focus For circularly polarized laser at 90o geometry n=2 Four permanent magnet cubes (NdFeB) are positioned to produce Quadrupole field at the axis. Radia Program is used to design the magnet to produce 110 T/m gradient. Octagonal iron yoke provide proper field flow and hyperbolic iron tips produce perfect quadrupole field inside the magnet. The prototype was build and it is good agreement with the simulation. 1% field error in the magnetization of the cubes in worst orientation causes 10 m axis offset which is quite reasonable. The cubes are measured and sorted to reduce possible errors. n=3 Permanent Magnet Dipole (PMD) for Energy Spectrometer Beam Transport for Inverse Compton Scattering A Permanent magnet Dipole is designed to serve as energy spectrometer and dump for the electron beam. It bends the beam by 90o. The geometry is chosen so that beam always exits the dipole at 90o for various energies only with some offset. Iron Yoke is designed for proper field flow Magnets are made out of NdFeB high grade magnets which can yield 1.2-1.4 T magnetization. The Magnetic field inside the gap is ~0.85 T. For 14 MeV energy design electrons the bend radius is about 55mm For linearly polarized laser at 90o geometry where electron’s velocity is parallel to plane of polarization PMD F-Cup IP Beam size plot along the Neptune beamline for Inverse Compton experiment simulated by MAD program. Angle of the trajectory Trajectory of the electron beam in PMD For linearly polarized laser at 2 Experimental Setup Normalized intensity distribution of the scattered photons of the first three harmonics The detector is centered at zo along z (=0) axis. The distances in x and y are measured in units of (x/zo)~ 90o geometry 2.00E+08 Wavelength of harmonics n d 2 I n *Ride, Esarey, Baine, Phys. Rev. E, Vol 52, p 5425 (1995) Nonlinear Harmonics Spectrum n=1 0.33 ps No of periods that electrons see Note: All the variables are defined in the paper a0 is the normalized amplitude of the vector potential of the incident laser field (just like K, undulator parameter) Interaction time Nonlinear Harmonics Inverse Compton Calculations e A0 a0 me c 2 Value 10.7 nm 117.7eV 10 ps 25 m Laser beam size at the IP (rms) Introduction Parameter Scattered Photon wavelength Scattered Photon Energy Scattered Photon Pulse duration where electron’s velocity is perpendicular to plane of polarization Inside the Magnet Inside the Magnet X`[rad] X[mm] Outside the Magnet Outside the magnet Field distribution inside the PMD gap simulated by Radia Trajectory of the 14 MeV electron beam in the PMD gap. Beam enters the magnet by 45 o angle and exits by 45 o angle. Y axis is the length of the magnet and x axis is width. Angle of the 14 MeV electron trajectory in the PMD gap. Angle linearly changes from /4 to -/4 inside the magnet.