PQB Photocathode Analyzing Power Study May 19, 2009.

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Presentation transcript:

PQB Photocathode Analyzing Power Study May 19, 2009

GaAs Analyzing Power  The GaAs crystal has preferred axis for absorption of linearly polarized light, Linear Dichroism (LD): difference in absorption of light polarized parallel and perpendicular to an orientation axis (Phys. Lett. A 212, 231 (1996))  Circular Dichroism (CD): difference in absorption of left- and right-handed circularly polarized light. CD is zero for GaAs.  Measure the super-lattice photo-cathode analyzing power (QE anisotropy), turn off Pockels Cell (PC) and do a Rotatable Half Wave Plate angle (θ) scan:

IHWP IN IHWP OUT E Field

Not Perfect Circular Polarization?  Residual linear polarization causes charge asymmetry when coupled to photocathode analyzing power  Measure the charge asymmetry (turn on Pockels Cell and do a RHWP scan)

RHWP Scans TermCaused by c0c0 Vacuum Window Phase Shift c1c1 RHWP Phase Shift c2c2 Pockels Cell Residual Phase Shift & Imperfect Initial Linear Polarization  Did RHWP Scans with: I.Insertable Half-Wave Plate: IHWP IN or IHWP OUT II.Insertable Linear Polarizer: ILP IN or ILP OUT I. ILP OUT: Spot 900/900 II. ILP IN: Spot 850/800, to run from same spot as OUT; ILP moved the beam by about 2 mm III.Pockels Cell HV set for maximum circular polarization (PITA = 0 V) or set for elliptical polarization (PITA = -120 V)

Summary of RHWP Scans o IHHWP IN → IHWP OUT: A q → -A q  IHWP does not cancel helicity correlated effects caused by residual linear polarization o ILP has small effect  The initial polarization is highly linear  Because of the way the three laser beams are combined, ILP can only be used during beam studies o How to zero charge asymmetry?  Use c 2 to compensate for offset term or  Zero offset term by rotating photocathode

Photocathode Rotation  Measure Offset term (c 0 ) as a function of photocathode angle (θ c )  Choose angle where Offset is zero

Why use IHWP?  IHWP cancels helicity-correlated changes due to: 1.Position differences caused by Pockels Cell steering o PC alternately pulsed to +HV and -HV to change from right to left circularly polarized light and vice-versa o PC behaves alternately as converging and diverging lens o If beam is off-center, it can be steered o Induces helicity-correlated position differences 2.Electronic cross talk