3/10/061 Smith- Purcell radiation bunch profile diagnostics / Laser wire Lawrence Deacon, LCUK Meeting, Durham 25/9/06

3/10/062 Bunch profile diagnostics with coherent Smith-Purcell radiation Wade Allison, Victoria Blackmore, George Doucas, Brian Ottewell, Colin Perry and M.F. Kimmitt * (*Dept. of Physics, Univ. of Essex) Collaborators: Lex van der Meer & Britta Redlich (FELIX facility) Michael Johnston & group (Clarendon Lab.) Peter Huggard &group (Space Science & Technology, RAL)

3/10/063 Basic objective The beam in the Linear Collider will be bunched in bunches of about 1ps length. The longitudinal (time) profile of these short, intense e - bunches needs to be determined. This is required for the calculation of the ‘beam-beam’ effects at the Interaction Point. Must done in a way that causes minimal disruption to the beam. One possible approach is to cause the beam to radiate a small amount of energy (but, there are other competing methods….) The bunch can be made to radiate in a number of ways, but the preferred technique is to allow the beam to pass close to a periodic structure, e.g. a grating (Smith-Purcell radiation). If the radiated wavelength is comparable to the bunch length, radiation is coherent, hence greatly increased intensity(  N 2 ). The wavelength distribution of coherent energy depends on the temporal profile of the bunch. Therefore, a measurement of the wavelength distribution allows reconstruction of the bunch temporal profile.

3/10/064 Smith-Purcell radiation Advantages Adjustable wavelength Wavelength dispersion But.. Operation in the far-infrared Expertise from Astronomy & Remote Sensing

3/10/065 Experimental ‘Carousel’ with 3 different gratings + ‘blank’ 11 room-temperature detectors, covering o range. High quality filters for background rejection Use of ‘Winston cones’ for improved light collection efficiency & more monochromatic light. Compact & inexpensive set- up.

3/10/066 Experimental-FELIX Beam energy: 45-50MeV Linac Frequency: 25MHz or 1GHz, i.e. bunch spacing is 40ns or 1ns. Bunch length: nominally 1-3ps Bunch train: 4-5 microseconds Grating periods: 0.5, 1.0 and 1.5mm Our own detector amplifier design.

3/10/067 Recent Results from FELIX Combined results from three gratings. Fitted with two possible profiles, shown below. Data points in shaded area have been excluded from the analysis. Phys. Rev. STAB, (to be published)

3/10/068 Future Plans Runs at SLAC-ESA, end of Jan. & July. 28GeV, N=10 10 in  1ps but different bunch structure (single bunch, 1- 10Hz). Basically the same kit but different detector electronics and better suppression of x-ray background. Additional filters to be able to isolate & measure up to 3 rd order emission from the three gratings. Need to calibrate our detectors at a number of wavelengths in the mm and sub-mm region. Continuing theoretical work (is theory adequate? Can we develop a suitable analysis code?) Moreover: our analysis cannot provide a unique profile; can we use Kramers-Kronig analysis to retrieve the phase information & obtain unique solution? Will need about the same level of support from Electronics, Design Office & Workshop

3/10/069 Laser wire at ATF extraction line Principle of operation Detector Some scan results ATF2 simulation DAQ and hardware Possibility of putting laser wire detectors in ILC polarimeter chicane Lawrence Deacon, LC-ABD meeting, Durham 25/9/06

3/10/0610 Accelerator Test Facility (ATF), KEK 1.3 GeV linac, damping ring and extraction line

3/10/0611 ATF 1.3GeV beam Multi bunch (up to 20 bunches) 2*10^10 electrons per bunch 2.8ns bunch spacing Pulse width 50ps

3/10/0612 ATF Extraction Line Laser Wire Particles lose their transverse momenta and large divergence in the damping ring. Laserwire is a beam profile monitor Laserwire located in extraction. Single shot measurements (ILC beam delivery system) Multiple laserwires together a means of measuring beam emittance (phase space area)

3/10/0613 Principle of Operation Powerful pulsed laser timed to coincide with particle bunches Photons are compton scattered X rays detected Beam profile gaussian. Detector signal plotted as function of laser beam position Laser and electon beam sigma add in quadrature Laser waist size known -> electron beam profile

3/10/0614 Principle of Operation Rate of Compton scatters proportional to spatial overlap Laser sigma and electron beam sigma add in quadrature e e e

3/10/0615 Principle of Operation Beam position changed by tilting mirror Lens on translation stage to change focus position Signal highest where laser waist coincides with electron beam Window in vacuum chamber x y z

3/10/0616 Detector Currently using an aerogel Cerenkov detector. Threshold is 2.68MeV. Removes low energy background from signal. Calorimeter could provide better energy resolution. Calculation and tests to be done. Detector to be calibrated. Response could be non linear.

3/10/0617 Scan Signal plotted against laser vertical position. Vertical electron beam size approx 1 micron Plot shows 15 micron sigma So laser sigma currently 15 microns- too large to measure e beam profile Effectively measurements made on laser beam using electron beam New lens currently being tested at Oxford Laser M^2 measurements are underway

3/10/0618 Multi Scan Scans were taken at multiple focus positions Data used to determine Raleigh length of laser beam w0 is the beam waist=2sigma b is Rayleigh length, where w is increased by a factor of sqrt(2) Rayleigh length useful for calculating M^2 (laser beam quality factor). Limits degree to which beam can be focused for a given divergence angle laser light lens electron beam waist x y

3/10/0619 Multi Scan Each point is one vertical scan Lens x position plotted against sigma y Rayleigh length 520 +/ 10 microns

3/10/0620 ATF2 Simulation Currently simulating ATF2 extraction line Signal and background Sources of background: halo, synchrotron radiation. Results will be useful for both laser wire and Shintake monitor Simulations of ATF will be compared with experimental results

3/10/0621 ATF2 Simulation Modeled quadrupole geometry in detail include gaps between pole tips where laser wire signal can pass through Compton scattered photon beam properties (profile, energy spectrum, energy fluctuations) determined Next steps: include all correct apertures and do complete background simulation including halo effects Initial results show 10 GeV signal at first position and 8GeV at second position BH 5 QD 6 QF 5 SF 5 Possible Detector Locations BH 5 QD 6 QF 5 SF 5 Possible Detector Locations BH 5 QD 6 QF 5 SF 5 Possible Detector Locations BH5 QD6 QF5 SF5 Possible Detector Locations

3/10/0622 Quadrupole Model Laser wire signal collimated when it passes through gaps between pole tips of quadrupole QD6

3/10/0623 ATF2 Simulation - acceptance 1 st position (between dipole and quadrupole). E plotted against detector size Note: assumes uniformly sensitive detector plane

3/10/0624 ATF2 Simulation – statistical precision deltaE/E plotted against detector width Minimum at 1.5cm However, energy at 40% maximum. Larger detector may be preferable due to increased signal to background ratio

3/10/0625 DAQ and hardware Last running period, semi- manual control of mirrors, data taking etc DAQ being improved to automate scans for next running period (begins Autumn) Network file system has been set up and user interface will soon be on a single machine Data taking (laser power meter, Cerenkhov, beam position monitors), mirror/lens movements, wire scanner control synchronized beam position monitors detectors analogue to digital converter camac crate controller computer (server) wire scanner laser power meter windows computer lens/mirror motors APD shutter linux machine (client) message wall current monitor User interface

3/10/0626 DAQ and hardware Recently wire scanner and power meter have been included in user interface Temperature logging system has been set up- problems with laser performance last running period possibly due to temperature fluctuations. Considering improving temperature control. beam position monitors detectors analogue to digital converter camac crate controller computer (server) wire scanner laser power meter windows computer lens/mirror motors APD shutter linux machine (client) message wall current monitor User interface

3/10/0627 Laser wire detector in polarimeter chicane? Has been suggested that laser wire detector at ILC be placed in polarimeter chicane instead of it's own separate chicane laserwires 10m Polarimeter Compton IP Polarimeter detectors Laserwire detector ?

3/10/0628 Laser wire detector in polarimeter chicane? In current design, space between outer surface of beam pipe and centre line of Compton photons coming from laser wires enough for ~10mm radius detector Not enough space for crystal calorimeter- need 2-4cm due to Moliere radius Enough space for Cerenkov emitter/detector May have to use Cerenkov detector anyway due to low energy background

3/10/0629 Laserwire detector in polarimeter chicane? Will simulate whole polarimeter chicane region in some detail This will determine whether possible backgrounds created by the laser wire signal will be acceptable for the polarimeter detectors and vice versa

3/10/0630 Future Work Working with collaborators to upgrade the laser beam system Will operate with the new final lens which is currently being tested in Oxford Will start working on scheme to extract the emittance from the multi laser wire system Doing simulation of ATF2 including background Will begin simulation of ILC polarimeter/ laserwire chicane