Emittance measurements for LI 2 FE electron beams

Emittance measurements for LI 2 FE electron beams LI 2 FE Diagnostic

LI 2 FE Emittance n n n It is a measurement of the quality of the beam. It measures the area of the phase space occupied by the beam A beam of particles can be characterized in detail by its density in the sixdimensional phase space, (x; px; y; py; z; pz), where p is the canonical momentum. LI 2 FE Diagnostic

LI 2 FE Liouville n The utility of a such a description derives from the discovery by Liouville that the density in phase space of a system of non-interacting particles subject to a Hamiltonian (such as that of an electromagnetic field) is constant in time. Accordingly, the extent of the beam in phase space, termed its emittance, is also constant in time, at least under ideal conditions. LI 2 FE Diagnostic

LI 2 FE Definitions of Brightness For many practical application it is more meaningful to know the total beam current that can be in a 4 dimensional trace space V 4. For particle distribution whose boundary in 4 D trace space is defined by an hyperellipsoid [A/(m-rad)2] Normalized Brightness LI 2 FE Diagnostic

LI 2 FE Standard techniques n Quadrupole Scan ¡ ¡ n Multiple screens ¡ ¡ n High energy spread leads chromaticity It needs a lot of space Beam dominated by angular divergence It needs a lot of space Pepper pot ¡ ¡ Problems related to the mask Insensitive to original beam spot size if the mask is placed at more than few mm from the plasma LI 2 FE Diagnostic

LI 2 FE Quadrupole scan P 1 P 2 Beam n k 2 k 3 Quadrupole n k 1 Schermo It is possible to measure in the same position changing the optical functions The main difference respect to the multi screen measurements is in the beam trajectory control and in the number of measurements LI 2 FE Diagnostic

LI 2 FE Pepper pot LI 2 FE Diagnostic

Emittance measurements with pepperpot like structures LI 2 FE To measure the emittance for a space charge dominated beam the used technique is know 1 -D pepper-pot The emittance can be reconstructed from the second momentum of the distribution C. Lejeune and J. Aubert, Adv. Electron Phys. Suppl. A 13, 159 (1980) LI 2 FE Diagnostic

LI 2 FE Design issues n n n The contribution of the slit width to the size of the beamlet profile should be negligible (in our case L greater than few cm) The angular acceptance of the slit cannot be smaller of the expected angular divergence of the beam (in our case l< 1 -5 cm) The material thickness (usually tungsten) must be long enough to stop or heavily scatter beam at large angle LI 2 FE Diagnostic

LI 2 FE Electron range in W LI 2 FE Diagnostic

LI 2 FE In our case… n Any method Must be sensitive to the initial beam size to retrieve the emittance at the beginning LI 2 FE Diagnostic

LI 2 FE A chance? n n n If the correlation at the beginning is zero the measurements of s and s‘ are enough to retrieve the emittance value. Being the angular divergence a constant in a drift it can be measured everywhere The only challenging measurement is the beam spot size at the origin LI 2 FE Diagnostic X’ X

LI 2 FE 3 possibilities so far… n n n Laser Wire Incoherent transition radiation Coherent transition radiation (A. R. Rossi) LI 2 FE Diagnostic

LI 2 FE Laser Wire Rayleigh range of the laser beam : distance between the focus and the point where the laser spot-size has diverged to of its minimum value n n n Not intercepting device Multi shot measurement (bunch to bunch position jitter, laser pointing jitter, uncertainty in the laser light distribution at IP) Setup non easy Resolution limited from the laser wavelength Several effects to take into account I. Agapov, G. A. Blair, M. Woodley, “Beam emittance measurement with laser wire scanners in the International Linear Collider beam delivery system”, Physical review special topicsaccelerators and beams 10, 112801 (2007) LI 2 FE Diagnostic

LI 2 FE Shintake monitor n n Tsumoru Shintake, “ Proposal of a nanometer beam size monitor for e+e- linear collider”, Nuclear Instruments and methods in Physics Research A 311 (1992) 453 Yamanaka et al. “Status of the first commissioning of the Shintake monitor for ATF 2”, Proceeding of PAC 09, Vancouver (Canada) 2009 LI 2 FE Diagnostic

LI 2 FE Critical issues n n n Q-Switched 40 MW 532 nm, pulse length 8 ns (FWHM) Stabilization of the system It needs a lot of space for the setup LI 2 FE Diagnostic

LI 2 FE OTR (Optical Transition Radiation) n n It is emitted when a particle cross the boundary between two medium with different index of refraction The radiation is emitted in a narrow cone with aperture 2/g It is weak (~ 1 photon per 100 electrons in the optical wavelenght) It is linear in the number of particles LI 2 FE Diagnostic

LI 2 FE Equivalence of a metal interface C. B. Schroeder, E. Esarey, J. van Tilborg, and W. P. Leemans, Theory of coherent transition radiation generated at a plasma-vacuum interface, PHYSICAL REVIEW E 69, 016501 (2004) LI 2 FE Diagnostic

LI 2 FE Moving charge At rest moving LI 2 FE Diagnostic

LI 2 FE Radiation field r Natural length The intensity is proportional to the total charge LI 2 FE Diagnostic K 1(x)

LI 2 FE TR angular distribution LI 2 FE Diagnostic

LI 2 FE OTR for beam diagnostic n Both the angular distribution (giving information on the energy and the angular divergence of the beam) and the beam image (giving information on the beam size) can be measured (optical resolution, diffraction limited l/q) LI 2 FE Diagnostic

LI 2 FE Problems so far n n Realization of a compact optical system with at least 10 X Radiation is emitted on axis Electron must be separated from the light Band pass filter to avoid photons from the laser LI 2 FE Diagnostic