Hall C Compton Polarimeter Working Group Meeting Jefferson Lab August 8, Meeting agenda 2.Compton Polarimeter overview 3.Subsystem status

Meeting Agenda Overview and Status (Gaskell) Fiber Laser (Hansknecht) Beamline Design Update (Benesch) Break Electron Detector (Martin) Interaction Region Instrumentation (all) Discussion –DAQ –Photon detector tests –Slow controls –Schedule and manpower

Meeting Goals Summarize current systems’ status Review milestones and update as needed Identify areas that are lagging Identify areas that as yet have not been addressed

Compton Polarimeter Overview Compton polarimeter required to make continuous beam polarization measurements at same time as data-taking Provides cross-check of Hall C Møller, although (initially) the precision will likely be worse Requires the insertion of a m dipole chicane in the Hall C beam line

Polarimeter Requirements and Constraints Precision –We want a device that can (eventually) reach 1% systematic uncertainty Luminosity –Rapid measurements (~ hours) required to understand the device performance and for cross-checks with Moller Backgrounds –Must be able to either suppress backgrounds or make them small compared to signal of interest Geometry –Must fit in Hall C line –Constrained by requirement that fast raster must be ~15 m from pivot: also, no quads between raster and target!

Compton Subsystems Laser + optics –Pursing development of RF pulsed (499 MHz) 25 W green laser –Luminosity comparable to 90 W low duty-cycle system previously on the table –Background suppression a-priori worse, but makes use of electron detector simpler Photon detector –Prototype lead tungstate detector built by Yerevan –Need to pursue photon beam tests Electron detector –“Diamond” detector to be built by Miss. State and Winnipeg –NSERC funding ~ $85k, DOE funding partial with more available upon completion of working prototype Chicane and beamline –Optics design exists – next job to tackle is physical layout –Dipole design underway

Compton Chicane and Beamline Layout Constraints Compton requires insertion of m dipole chicane Hall C Beamline must be overhauled Constraints/Requirements –Fast raster must be at least 14.5 m from the Hall C pivot –No quadrupoles between raster and target –Polarized target chicane still useable –Compton must work for Q Weak (1.165 GeV) AND at 11 GeV –Compton must accommodate coincidence electron+photon detection In particular, we want to detect the asymmetry “zero- crossing” in the electron detector

Cartoon of Hall C Beamline

Beamline Features Dipoles increased from 1 m to 1.25 m to avoid high fields at 11 GeV Moller quads are part of the beamline –Quadruplet before Compton chicane is used to prepare the beam for the interaction region (3C16, 3C17, 3C18, 3C19) –Quadruplet after the Compton chicane used to prepare beam for target (3C20, 3C21, MQ1, MQ2) No combined optics –Moller quads cannot be at “measurement” values while beam is on target Allowed combinations –Compton + target –Compton + Moller (high current questionable)

Photon Detector (Yerevan/HU) Prototype has been tested in SOS, but pion backgrounds made it difficult to precisely map out response Would like to make some photon beam tests: Can we map out detailed response at Duke FEL (HI  S)?  E  = 50 MeV

Laser Re-evaluation Laser choice for Compton had been a high power, diode- pumped solid state green laser –Example: Coherent-90 Pulse structure: 150 ns at 5-10 kHz Average power = 90 W (at 5 kHz) M 2 ~ 32  beam diverges rapidly from focus Advantages of this system –Easy to use – turn it on and go, no high power caity (a la Hall A) to build and maintain –Low duty cycle (~10 -3 ) strongly suppresses background –Multiple backscattered photons per laser pulse  requires use of “energy-weighted” counting, which has larger figure of merit Disadvantages of this system –Large M 2 may make focusing and transporting beam into beamline vacuum difficult –Multiple backscattered photons per laser pulse (on average 2)  may make use of electron detector difficult

Fiber Laser for Hall C Compton (I) Last year, Matt Poelker proposed a “fiber” laser system for use with the Hall C Compton –Fiber laser has been used in the JLab polarized source for ~2 years with excellent results –Source laser has provided excellent stability and minimal maintenance Proposed laser based on system described in Optics Letters v.30 no. 1 (2005) 67 –high average power: 60W average power (520 nm). –demonstrated high peak power: 2.4KW (d.f. = 30) –almost ideal optical properties: M 2 = 1.33 System not pursued at that time –Significant R&D required –Manpower issues

Fiber Laser for Hall C Compton (II) Matt Poelker proposed a fiber laser option again this year, with some differences: –modest average power: 20 W (532 nm). –Pulsewidth ~ 30 ps at 499 MHz –almost ideal optical properties: M 2 = 1.33 This system is more straightforward and uses the following components –Seed laser (1064 nm) –Fiber amplifier (50 W output at 1064 nm) –Frequency doubling cavity Polarized source group is willing to build this laser (with help from Shukui Zhang from FEL for the frequency doubling)

Luminosity from Fiber Laser Average power from fiber laser modest (20 W)  does this equal factor of 5 reduction in luminosity compared to 100 W laser? No – we can actually get about a factor of 4 improvement –For laser pulsed at electron beam repetition rate (499 MHz) and comparable pulse width (on the order of ps), the luminosity is increased by a factor: For typical JLab parameters, this yields about a factor of 20 improvement in luminosity for  = 20 mrad

Luminosity from Fiber Laser Fiber laser pulse-width about 15 times larger than electron beam – no problem! 2.0 cm 2 1 cm 2  e =  laser = 100  m,  = 20 mrad Luminosity gain only weakly dependent on laser pulse width  for laser pulses ~ 10’s of ps

Laser Figure of Merit laser P E max rate t (1%) option (nm) (W)(MeV)(KHz) (%)(min) Hall A UV ArF UV KrF Ar-Ion (IC) DPSS Fiber laser Fiber laser (counting mode) Energy-weighted asymmetry

Figure of Merit FOM comparisons on previous slide not entirely fair to fiber laser –DPSS laser crossing angle limited to 0.85 degrees for s=200 um spot size –Fiber laser can achieve crossing angle of 0.5 degrees for 100 um spot size Comparing “best” scenarios for each laser: –Fiber: Compton rate = 46 kHz –DPSS: Compton rate = 11 kHz Both systems can get an extra factor of 2 just by re- circulating the beam once

Fiber Laser Development Source group will develop 20 W laser in their lab –PO for 50 W fiber laser amplifier signed Matt Poelker and John Hansknecht will set up laser to deliver 50 W IR Shukui Zhang will undertake frequency doubling Hall C Compton group will develop laser transport –Transport into vacuum beamline, focusing –Alignment system (R. Jones) –Linear  circular polarization –Polarization monitoring, determination of “transfer function” This system should be relatively low risk – however, this particular configuration never attempted at JLab before –In the worst case, fall back to original plan (low duty cycle, pulsed laser)

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Interaction Region

Open Items: Photon Detector Photon detector tests Zeroth order tests in photon beam would be useful Check global response Stability Signal speed Can we characterize detailed response (linearity) adequately at HI  S? –How well is the photon energy known?

Open Items: Data Acquisition We need to read out at least 2 detectors –Photon detector –Electron detector –What are the detailed requirements for each (channels etc.)? –Rates are high – assumedly we need detailed info event-by-event Other signals –Laser diagnostics –Beam conditions (epics readout fast enough?) Software and analysis

Open Items: Slow Controls Will need slow controls for: –Laser system (on-off, mirror adjustment?, focusing?) –Electron detector (motion mechanism) –Anything else?

Manpower Beamline (JLab): Competing with 12 GeV, other experiments for resources here Chicane (MIT-Bates) Laser system (JLab/UVa/?): – “JLab” (Matt, John, Shukui) will deliver laser –“JLab”/UVa (Dave, Kent, or designated alternates) will handle transport, diagnostics Electron Detector (Canada/Miss. State): remarkable progress in short amount of time Photon detector (Yerevan/HU): prototype in hand DAQ/software (?) Slow controls (?)  subsystems develop their own?

Schedule If we can meet these milestones, we should be ok – is this realistic? TaskResponsible InstitutionsPlanned Completion Date Dipole magnet construction MIT-BatesJanuary 2008 Finalize chicaneMIT-BatesJuly 2008 Photon detector testsYerevan/HUApril 2008 Photon detector final construction Yerevan/HUJanuary 2009 Fiber laser low power prototype JLab/UVaOctober 2007 January 2008? Final laser choiceJLab/UVaJanuary 2008 Laser transport setupJLab/UvaJuly 2008 Electron detector fabrication Winn./Man./TRIUMF/ Miss.St. October 2008 Compton InstallationJLabSpring 2009