A laser-wire to measure the beams size at the ILC

1. A laser-wire to measure the beams size at the ILC • LBBD collaborators • Beam delivery at the ILC • Laser-wire as a beam diagnostic tool • The PETRA laser-wire • The laser-wire at the ATF extraction line • Future plans Nicolas Delerue, University of Oxford for the Laser Based Beam Diagnostic collaboration 1/10

2. People and institutes involved in our project • University of Oxford:Nicolas Delerue, Brian Foster, David Howell, Armin Reichold • Royal Holloway University London:Ilya Agapov, Grahame Blair, Gary Boorman, John Carter, Chafik Driouichi, Michael Price, Thorsten Kamps (now at BESSY) • University College London:Stewart Boogert, Steve Malton • DESY:Klaus Balewski, Hans-Christoph Lewin, Siegfried Schreiber, Kay Wittenburg(with the kind help of all the PETRA group) • KEK:Alexander Aryshev, Hitoshi Hayano, Pavel Karataev, Kiyoshi Kubo, Nobuhiro Terunuma, Junji Urakawa (with the kind help of all the ATF group) • SLAC:Josef Frisch, Marc Ross Project web page: http://www.hep.ph.rhul.ac.uk/~lbbd/ 2/10

3. Beam delivery at the ILC • The ILC will accelerate electrons up to an energy of 250 GeV per beam (upgradable to 500 GeV) • After acceleration, the beams are not ready to deliver the full luminosity required for the physics studies: • The beam size must be reduced from 1 micrometer at the end of the acceleration unit to just a few nanometers at the interaction point • The beams need to go through several optics correction to allow such strong focusing • This is done in the beam delivery section of the ILC. This section beam delivery section is a few kilometers long. • A factor 2 increase in resolution on the beam size in the beam delivery section allows a factor 4 better resolution on the βfunction and thus helps to reduce the BDS length dramatically. • No mechanical device can achieve this resolution nor stand the ILC’s high currents: a novel beam size monitor is needed. 3/10

4. Laser-wire as a beam diagnostic tool • A laser-wire allows non destructive measurement of the electron beam size. • The method was pioneered at SLC but was never used as a beam diagnostic tool. • It seems to be the best suited tool to measure the ILC beams’ size. • R&D is in progress to make laser-wire scans a routine before the ILC startup. • The UK laser-wire group is working at 2 different locations: • PETRA (DESY, Hamburg, Germany) well advanced • ATF (KEK, Tsukuba, Japan) just started 4/10

5. How does a laser-wire work? • A laser-wire uses a high power laser to shoot short laser pulses on a particle beam. • A movable mirror is used to "scan" the beam:each laser pulse is deflected by the mirror with a different angle. • When the pulse hits a bunch of electrons, Compton photons are produced. • A calorimeter is installed downstream after a bend in the electrons trajectory. • This distribution of the energy gives the beam profile. • From the beam profile, it is possible to extract the size of the electron beam. Laser wavelength: 532 nm (Nd:YAG) Laser power: 1MW@PETRA 1GW@ATF 5/10

6. The PETRA laser-wire • PETRA (currently used as injector for HERA at DESY): • 7 GeV, • beam size of the order of ~100 micrometers • Laser: refurbished from the LEP polarimeter • First laser-wire scan obtained in December 2003 • DAQ recently upgraded • New beam pipe installed in PETRA with a window to reduce the photons absorbtion and enhance the signal • New system tested in February (One scan now takes less than a minute)… 6/10

7. Recent result:February 2005 scan Gaussian width= Beam size  Laser spot Gaussian width: 81.83 micrometers Laser spot: 34.92 micrometers Beam size: 74.01 micrometers 7/10 Plot by Steve Malton (UCL)

8. Laser-wire at the ATF extraction line • The ATF (Accelerator Test Facility) at KEK has been built to demonstrate the feasibility of the small emittance beams required for the ILC • Ideal location for R&D projects • Our laser-wire is still in the planning stage • Trip this month to study how to install it • Goal: have wire size of the order of 1 micrometer (comparable to the beam size) • A spot size of 1 micrometer is only twice the laser wavelength => very challenging! Requires a lens with a big aperture (f#/1)and resistant to the high power delivered by the laser. 8/10

9. This year’s plans for the ATF extraction line laser-wire • March 2005:“study trip”: Understand the possible setup [optics/infrastructure], prepare our DAQ, study the laser,… • May/June 2005:Install the laser transport and delivery (Optics, Scanning,…), laser measurements, study the beam optics, validate the DAQ • September 2005:Install our vacuum vessel at the ATF and final focusing lens • November/December 2005:Laser-wire run Our latest plans for our work at the ATF are posted at: http://www-pnp.physics.ox.ac.uk/~delerue/laserwire/atf.php 9/10

10. Future plans • PETRA: • The laser at PETRA will soon be upgraded => more reliable scans. • Work is in progress to add a second dimension of scanning. • PETRA III (light source) will include a wire-scanner as a standard diagnostic tool. • ATF • We hope to achieve our first scan at the ATF by the end of the year. • A 1 micrometer spot with a green laser is very challenging! • The ATF will be extended to test the ILC final focusing, we plan to install some laser-wires as diagnostic tools. • The collaboration is expanding: we are recruiting two more research assistants next month. • Multidisciplinary project involving lasers, optics, accelerator physics... 10/10