Presentation on theme: "CEBAF Photoinjector: What’s new, what’s the same since HAPPEx 2004? What needs to be done? M. Poelker, HAPPEx collaboration meeting, Feb. 18, 2005."— Presentation transcript:
CEBAF Photoinjector: What’s new, what’s the same since HAPPEx 2004? What needs to be done? M. Poelker, HAPPEx collaboration meeting, Feb. 18, 2005
GUN2 Strained layer GaAs GUN3 Superlattice GaAs Air conditioned tunnel New laser room with two 499 MHz Ti-Sapphires (+ one spare at bldg. 58)
Gun2 Status Gun2: Used for most of HAPPEx-H Strained Layer GaAs, pol. ~ 75%. Good QE and long lifetime, ~ stable HC asymmetries. Rebaked (but not vented) Jan QE still good following bakeout. To be tested under HV next week…
Gun3 Status Gun3: Recall we used Gun3 and superlattice photocathode for HAPPEx-He and small portion of HAPPEx-H during Pol. ~ 85%, large active area (no anodization, no mask). Poor lifetime, spot moves daily. Unstable HC asymmetries. “charge limit” behavior, QE not constant with increasing laser power. Questioned new photocathode material, NEG pumps, implementation. Replaced NEG pumps Aug. 2004, Installed new superlattice photocathode under 5 mm dia. tantalum mask. Pol still good ~ 85%, QE is better (~ ½ of lab results) Lifetime is better but still not as good as Gun2 (10’s of Coulombs instead of 100’s Coulombs, Ti-sapp lasers allow weeks of uninterrupted operation). Suspect bad beam envelope (too big) … Start up with gun3. Modify optics to tighten beam envelope and improve lifetime.
Gun2 strained layer GaAsGun3 superlattice GaAs From HAPPEx-H 14 mm Problems with H-cleaning new superlattice material. Installed without anodizing Gun3 lifetime was poor Frequent laser spot moves were necessary QE holes caused HC asymmetry variations Recall HAPPEx 2004 From R. Snyder HALOG entry
Gun3 Ops during Jan mm NEG pumps replaced Aug Active area limited by ta-mask More typical QE scan results. “charge limit” behavior not so pronounced. But charge lifetime ~ 30 C. Should be ~300 C Vacuum bursts along 100 kV beamline. “Something” charging up and moving the beam. This likely contributes to poor lifetime. Beam envelope too large? Why? Or emission from tantalum mask? Ion pump current versus time Two ion pumps near gun QE%
From Riad’s talk at 8 am Meeting, 01 February 2005: Large induced charge asymmetries observed for PZT motion: (compare to typical 30 ppm/V observed during G0 forward angle run) x y strained GaAs (Dec. 2004) 388 ppm/V 238 ppm/V superlattice (before spot move) 3620 ppm/V 948 ppm/V superlattice (after spot move) 973 ppm/V 562 ppm/V This could potentially complicate position feedback; we will watch the HAPPEx experience with superlattice this June; we may want to ask for regular strained if the experience is not good. Matt’s note: The beam size was large at apertures A1 and A2 during these measurements. I suspect we can tighten the beam envelope considerably and reduce HC beam loss. Beam Tests by R. Suleiman
Results of Adiabatic Damping Study: R. Suleiman 1-pass R_PZTx (um/V) R_PZTy (um/V) 1I I I02A L L L R C20A H H00A H00B pass R_PZTx (um/V) R_PZTy (um/V) 1I I I02A L L L R C20A H H00A H00B In X, more damping at 2 pass than 1 pass In Y, less damping at 2 pass.? Overall, but still not as much damping as expected.
Beam Studies by Yu-Chiu Chao Y component of X pzt X component of X pzt X component of Y pztY component of Y pzt The beam is moved at 30 Hz using “common” pzt. Plots show beam displacement at each bpm in the machine, multiplied by square root of beam momentum. Perfect machine would show flat line. Achieved > order of magnitude improvement in position damping at Hall C (5 pass) by implementing a matching solution at 5 MeV (red = before, blue = after).
G0 Backward begins NovDecOctSepJulJunMayAprMarAug HAPPEx resumes Long Shutdown HAPPEx concludes 94 days of beam Hypernuclear Laser Table Opportunity#1 May Laser Table Opportunity#2 July 5, 6 Laser Table Opportunity#3 Aug 8, 9 Facility Development Improve Gun3 Lifetime Laser Room Studies
Between now and July 4, 2005: Use Spare 499 MHz Ti-Sapphire laser (the same laser that you will use in tunnel) Test new HV switch (faster rise/fall times) Test repaired pockels cell Test encoder/translation stage (remote x/y motion of pockels cell). Refine pockels cell alignment procedure? Imaging? IA cell? (alignment, cell selection) What else?
Beam Studies “Beam Loading”. Investigate the correlation between charge asymmetry and induced position asymmetry on neighboring beam. Response time of gun? SRF related? Matching Studies using pzt mirror (ongoing work by Chao) Gun3 qualification. Parasitic, in advance of HAPPEx. Phase trombone. Other Arsenic-capped superlattice samples in house. Good QE ~ 1% at high polarization Might accommodate anodizing. Test in progress at EEL