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

GUN2Strained layer GaAs

GUN3Superlattice 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 2004.• Strained Layer GaAs, pol. ~ 75%.• Good QE and long lifetime, ~ stable HC asymmetries.• Rebaked (but not vented) Jan. 2005. • QE still good following bakeout.• To be tested under HV next week…

Gun3 StatusGun3:• Recall we used Gun3 and superlattice photocathode for HAPPEx-He and small portion of HAPPEx-H during 2004.• 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. 2005

5 mm

• NEG pumps replaced Aug. 2004• 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 theHAPPEx experience with superlattice this June; we may want to askfor 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)

1I02 14.01 4.14 1I04 33.45 20.31 0I02A 9.67 15.65 0L03 4.58 1.67 0L04 12.92 6.83 0L06 21.17 12.74 0R05 13.23 7.99

C20A 3.57 0.50 H00 2.97 0.58 H00A 2.97 0.44 H00B 2.85 0.62

2-pass R_PZTx (um/V) R_PZTy (um/V)

1I02 13.59 3.90 1I04 32.93 19.90 0I02A 10.32 15.86 0L03 4.44 1.65 0L04 14.14 7.44 0L06 24.64 14.11 0R05 14.81 8.44 C20A 1.13 2.85 H00 0.83 1.72 H00A 0.83 1.36 H00B 0.74 1.16

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

50 100 150 200 250 300

-6

-4

-2

2

4 Y component of X pzt

50 100 150 200 250 300

-10

-5

5

X component of X pzt

50 100 150 200 250 300

-10

-5

5

10

15 X component of Y pzt

50 100 150 200 250 300

-4

-2

2

4

6

8

Y 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

Nov DecOctSepJulJunMayAprMar Aug

HAPPEx resumes

Long Shutdown

HAPPEx concludes

94 days of beamHypernuclear

Laser TableOpportunity#1May 28 - 30

Laser TableOpportunity#2

July 5, 6

Laser TableOpportunity#3

Aug 8, 9

Facility Development

Improve Gun3 Lifetime

Laser Room Studies

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