1. 10/4/06Workshop on High QE Milan Diamond Amplified Photocathodes Preparation and Capsule Formation John Smedley

2. 10/4/06Workshop on High QE Milan Collaborators I. Ben-Zvi A. Burrill X. Chang J. Grimes T. Rao Z. Segelov Q. Wu R. Hemley, CI, Washington Z. Liu, CI, Washington S. Krasnicki CI, Washington K. Jenson, NRL, Washington J. Yater, NRL, Washington R. Stone, Rutgers U J. Bohon, Case Western U P. Stephens, SUNY SB

3. 10/4/06Workshop on High QE Milan Overview The DAP concept & capsule cathodes Diamond options Diamond preparation Secondary Yield Measurements Transparent cathode preparation Capsule fabrication Thinning diamond

4. Diamond Amplified Photocathode 3 – 10 kV Transparent Conductor (Sapphire w/ITO or thin metal ) Photocathode (K 2 CsSb) Laser Primary electrons Secondary electrons Diamond Thin Metal Layer (10-30 nm) Diamond (30 μm) Hydrogen Termination Primary electrons Secondary electrons

5. 10/4/06Workshop on High QE Milan Advantages Secondary current can be >300x primary current –Lower laser power –Higher average currents Diamond acts as vacuum barrier –Protects cathode from cavity vacuum and ion bombardment –Protects cavity from cathode (prevents Cs migration) –Should improve cathode lifetime e - thermalize to near conduction-band minimum –Minimize thermal emittance

6. 10/4/06Workshop on High QE Milan Challenges Electrons must escape diamond –Diamond must be 15-30 microns for 700 MHz RF –Surface must have Negative Electron Affinity (NEA) Diamond must not accumulate charge –Material must have a minimum of bulk traps –Metal layer required to neutralize holes –Metal layer must avoid surface traps Field in the diamond is a critical parameter –Field should be high enough for v e to saturate –Field should be low enough to minimize e - energy –Modeling suggests 3 MV/m – good for SRF injector

7. 10/4/06Workshop on High QE Milan Jewelry Store Natural Type IIa –Single crystal – few grain boundaries –Nitrogen content, variable, expensive Polycrystalline Chemical Vapor Deposition –Available in large sizes, low impurity content, cheap, can be grown to promote desired crystal orientation –Grain boundaries (weaken structure & charge traps) Single-crystal CVD –Single crystal, low impurities –Unavailable in large sizes, few options for crystalline orientation

8. 10/4/06Workshop on High QE Milan Diamond Preparation Acid etch –Remove graphite and surface contaminants –Strip metal layer –Provide C-O bond Hydrogenation –Bake to >450C in nTorr vacuum to break C-O bonds –Expose sample to ~1 μTorr of atomic H for 20 min –Hydrogen cracked by 1800C Tungsten filament –Hydrogen termination is robust versus atmospheric exposure

9. 10/4/06Workshop on High QE Milan Diamond Preparation - Metallization Initial layer must bond well to diamond –Titanium forms Ti-C layer Subsequent layers –Protect titanium from oxidation –High conductivity –Low electron stopping power (Low Z) Ti-Pt sputtering is a common choice for diamond detectors; we have had some success with gold We may use Ti-Pt-Al, with 5nm of Ti, 5 nm Pt and 20 nm Al Possibly thicker layer near the perimeter

10. 10/4/06Workshop on High QE Milan Experimental Arrangement Two types of measurements Transmission – both sides metallized Emission – one side metallized, one side hydrogenated Primary Secondary Kapton Diamond A V2V2 I2I2 A V1V1 I1I1

11. 10/4/06Workshop on High QE Milan Gain for CVD Detector Grade Transmission Mode

12. 10/4/06Workshop on High QE Milan Gain for CVD Detector Grade Emission Mode

13. 10/4/06Workshop on High QE Milan Gain for CVD Electronic Grade, Emission Mode Large emission threshold implies charge trapping Change process order Threshold field reduced dramatically

14. 10/4/06Workshop on High QE Milan Transmission Photocathode Metal coating and refractive index of diamond make standard reflection illumination less attractive Concept – K 2 CsSb on a transparent conductor –Model suggests optimal thickness is 10-30 nm –Resistivity is ~20Ωm (10 9 Ω/sq for t=20 nm) Fisher, McDonie and Sommer, J. Appl. Phys. 45 487 (1974) –For I>100μA, a conducting substrate is required 10 nm Copper Indium Tin Oxide Resistivity (Ω/sq)1.78-12 Trans. (532 nm)55%>85% Trans. (355 nm)35%>75% Is ITO compatible with K 2 CsSb?

15. 10/4/06Workshop on High QE Milan Diamond brazed on to Nb Successfully vacuum tested Polished from 200 to 70 micron thickness Chemically treated brazed Nb Ceramic brazed to Nb Cold weld Cu to Cu w/ indium Diamond Capsule Fabrication

16. 10/4/06Workshop on High QE Milan Thinning Diamond Diamonds will be brazed to support structure prior to final polish Mechanical polishing is extremely slow, and prone to breakage Reactive Ion Etching may also be an option –Etch rates of 40 nm/min have been demonstrated with O 2 plasmas Sandhu & Chu, Appl. Phys. Lett. 55 437 (1989) –Surface finish and effect on polycrystalline surfaces yet to be determined –Graphite formation may be a problem

17. 10/4/06Workshop on High QE Milan Conclusions A functioning DAP injector will provide a path forward for ampere-class linacs and ERLs For sufficiently pure diamonds, transmission gains of >300 have been demonstrated. ~14 eV of primary energy = 1 e-hole With proper preparation, emission gain of >300 with small emission threshold. H termination can provide a robust, NEA surface for electron emission. e - affinity is affected by surface crystalline structure. Synthetic diamonds have desirable properties, although some development is still needed. The energy spread of the emitted beam is expected to be <1 eV With proper choice of the electric field in the diamond, v = v sat while energy spread is nearly unchanged Still a lot to do!

18. 10/4/06Workshop on High QE Milan Still to do Test other diamonds, including some 30μm thick Understand differences between transmission gain and emission gain Demonstrate gain with I p = 10mA (larger area) Temporal and energy profile of emitted electrons Characterize impurities and crystalline orientation Determine the proper method of metallization Develop transparent photocathode Capsule fabrication and thinning diamond to 30μm are significant engineering challenges

19. 10/4/06Workshop on High QE Milan Secondary Electron Yield Measurements Ongoing work a NRL –J. Yater and A. Shih, J. Applied Physics 87 (2000) 8103, 97 (2005) 093717 Primarily reflection mode Boron-doped diamonds, Natural and CVD H or Cs termination Measured Secondary Electron Yield (SEY) and Energy Distribution Curves (EDC)

20. 10/4/06Workshop on High QE Milan Deposition System Laser access from both sides of cathode

21. 10/4/06Workshop on High QE Milan Natural Diamond, 100 orientationNatural Diamond, 111 orientation SEY EDC SEY EDC

22. 10/4/06Workshop on High QE Milan CVD Diamond, polycrystalline SEY EDC Implications Crystalline orientation affects NEA surface Diamond quality affects Secondary Electron Yield Electron energy at emission is <1 eV SEY of 100+, with 3 keV primaries

23. 10/4/06Workshop on High QE Milan Modification of 1.3 GHz gun for testing diamond capsules

24. 10/4/06Workshop on High QE Milan SRF Gun modified to accept Choke joint with diamond

25. 10/4/06Workshop on High QE Milan Measurements with Type II a Natural diamond

26. 10/4/06Workshop on High QE Milan High Average Current Linacs Applications: –Linac-based light sources –Free-electron lasers –Electron-cooling for RHIC Challenges –Energy recovery –Large laser power required, even for high QE Cathodes:

27. 10/4/06Workshop on High QE Milan Photoinjector

28. 10/4/06Workshop on High QE Milan BNL SEEP Project Goals –Identify suitable diamond- tested 4 types of diamond: natural, synthetic optical, electronic, single crystal Best so far- natural and electronic grade –Determine optimal metallic coating –Measure gain in transmission mode >200 –Measure Temperature dependence of gain No effect –Measure gain in emission >30 –Measure time behavior and EDCs –Capsule fabrication –Test in RF injector