The Quantum Efficiency and Thermal Emittance of Metal Cathodes

Transcription

1 T Quantum fficincy and Trmal mittanc of Mtal Catods David H. Dowll Tory Sminar Jun, 6 I. Introduction II. Q and Trmal mittanc Tory III. Ral World Issus Surfac Rougnss Masurmnts Diamond Turning vs. Polising Ar-Ions vs. H-Ions Cratrs of t Moon IV. Conclusions Collaborators:. Colby, B. Kirby, J. Scmrg, J. Smdly Q and Trmal mittanc of Mtal Catods Jun, 6 Tory Sminar D. David H. Dowll H. Dowll dowll@slac.stanford.du

2 Som gnral introductory commnts T Q is obviously important to rduc t siz and cost of t driv lasr T connction btwn Q and t trmal mittanc is not always apprciatd T bam quality from futur lctron sourcs for LCLS-lik applications will b dominatd by t trmal mittanc Trfor improvmnts in bam brigtnss will rquir a dtaild undrstanding of pysics nar t catod. Q and Trmal mittanc of Mtal Catods Jun, 6 Tory Sminar D. David H. Dowll H. Dowll dowll@slac.stanford.du

3 W can now produc atomically clan surfacs wic can b usd for tortical studis Q Wavlngt nm initial.63mc.7mc 3.3mC.3mC Work unction V Intgratd H-ion Carg mc Q can b computd using t tr-stp poto-mission modl Spicr:. Poton is absorbd by lctron rflctivity and absorption dpt. lctron travls to surfac lctron-lctron, lctron-ponon scattring 3. lctron scaps to vacuum nrgy and barrir pntration Q and Trmal mittanc of Mtal Catods Jun, 6 Tory Sminar 3 D. David H. Dowll H. Dowll dowll@slac.stanford.du

4 Drivation of t Q assuming t fr lctron gas modl of a mtal Stp : Absorption of poton rmi-dirac distribution at 3dgK f D / kbt Bound lctrons ff - ff ff scottky ff ff mittd lctrons Stp : Transport to surfac lctrons los nrgy by scattring, assum - scattring dominats, - is t probability t lctron maks it to t surfac witout scattring Stp 3: scap ovr barrir p scap critrion: > m p total cosθ m θ max p p total p ff ff Q R ff d N f D d N f N f D D N f cosθ D max dcosθ dcosθ,, θ π dφ π dφ Q and Trmal mittanc of Mtal Catods Jun, 6 Tory Sminar 4 D. David H. Dowll H. Dowll dowll@slac.stanford.du

5 David H. Dowll Q and Trmal mittanc of Mtal Catods Jun, 6 Tory Sminar D. H. Dowll dowll@slac.stanford.du 5 ff ff R Q k π 4 ik n η Wavlngt nm Rlctivity Wavlngt nm Optical Absorption Lngt angstroms Φ Φ π π θ θ cos cos d d d d d d R Q ff f ff ff ff R Q Probability lctron at dpt s, absorbs a poton and scaps witout scattring: s s f ds s f Assuming t tr-stp modl t prvious rlation can b sparatd into tr factors and intgrating ovr angls and initial lctron nrgy givs:

6 David H. Dowll Q and Trmal mittanc of Mtal Catods Jun, 6 Tory Sminar D. H. Dowll dowll@slac.stanford.du 6 ff ff m m d d ff ff 3/ ff ff ff m ff m R Q 3 / ff ff R Q ff scottky Assum t lctron-lctron scattring lngt can b avragd ovr nrgy: nrgy abov rmi Lvl V - Scattring Lngt angstroms 3/ ' ' m m T Q is tn givn by:

7 inal xprssion for Q and comparison wit xprimnt R ff Q ff ff 3 / m m ff ff scottky.. 3 scottky rmi nrgy, Work unction, 5 V / m 7V 4.3V V Quantum fficincy scattring angstroms Wavlngt nm Tory at MV/m Tory at 5 MV/m xprimnt ttp:// submittd to PRST-AB Q and Trmal mittanc of Mtal Catods Jun, 6 Tory Sminar 7 D. David H. Dowll H. Dowll dowll@slac.stanford.du

8 So for mtals tr is xcllnt agrmnt btwn t fr lctron gas modl and xprimnt, but wat about t trmal mittanc? T problm is bot tortical and xprimntal: No trmal mittanc masurmnts at low fild wit clan catod No Q vs. wavlngt at ig fild to vrify t Q tory Q and Trmal mittanc of Mtal Catods Jun, 6 Tory Sminar 8 D. David H. Dowll H. Dowll dowll@slac.stanford.du

9 ε trmal An xprssion for t trmal mittanc can also b drivd using t fr lctron gas modl R c 3 trmal mittanc microns/mm r r 3 r mc r r / Q and Trmal mittanc of Mtal Catods Jun, 6 Tory Sminar / r poton nrgy V Mv/m 5Mv/m MV/m xprimnts 3/ / wr R C is t catod lasr radius and D. David H. Dowll H. Dowll dowll@slac.stanford.du r ff xprimntal mittanc is largr by factor of.5 vn aftr taking into account t Scottky rduction of t barrir Nd to undrstand t ffcts of ig fild mission

10 T ical skin dpt is muc lss tan t R skin dpt, tis mans t lctrons ar acclrating insid t mtal bfor bing mittd mtal vacuum angstroms UV poton rf angstroms Applid R fild Tory and simulation cods ignor tis ffct, but it can b significant. Q and Trmal mittanc of Mtal Catods Jun, 6 Tory Sminar D. David H. Dowll H. Dowll dowll@slac.stanford.du

11 A crud stimat of t ffct can b mad by computing t nrgy sprad of t lctrons wn ty rac t surfac, s / mtal vacuum s s / s / s / ff s probability an lctron at dpt s, scaps ac lctron coming from a dpt s, will gain xt s as it movs to t surfac. T avrag nrgy gain and rms nrgy sprad ar, μ xt s s / s / ff ff ds ds xt ff σ s μ xt s / ff ds s / ff ds σ xt ff xt Q and Trmal mittanc of Mtal Catods Jun, 6 Tory Sminar D. David H. Dowll H. Dowll dowll@slac.stanford.du

12 σ xt ff xt stimat magnitud of nrgy sprad at 55 nm, 7 angstroms; 5angstroms; xt MV / m σ. 36V Us tis for crud stimat of mittanc: ε βγσ x ' σ x mσ σ x' βγmc βγ σ mc or a uniform radial distribution: σ x R And t trmal mittanc is: ε βγσ σ R x' x σ mc or ε / R.6microns / mm wic rougly agrs wit t ig fild masurmnts Q and Trmal mittanc of Mtal Catods Jun, 6 Tory Sminar D. David H. Dowll H. Dowll dowll@slac.stanford.du

13 A bttr calculation of t mittanc bgins wit t transvrs momntum: p T d dcosθ cosθmax d cosθ dcosθ max ds π dφp ds π T dφ s / ff p T 4π m d π s / ff 3 3 d s s / ff xt xt ds s xt ds s Numrically intgrat to obtain t mittanc: ε trmal βγσ σ x x' Rc p T mc / trmal mittanc microns/mm Appl id ild V/angstrom 36 angstroms 6 angstroms Q and Trmal mittanc of Mtal Catods Jun, 6 Tory Sminar 3 D. David H. Dowll H. Dowll dowll@slac.stanford.du

14 Including acclration insid t mtal incrass t mittanc about 5% for MV/m.V/angstrom. But to obtain agrmnt wit xprimnt rquirs a muc longr ffctiv man fr pat. Or igr fild nancmnt. trmal mittanc microns/mm Applid il d V/angstrom 36 angstroms 6 angstroms 6A man fr pat 36A man fr pat Clarly ig filds will significantly incras t trmal mittanc Q and Trmal mittanc of Mtal Catods Jun, 6 Tory Sminar 4 D. David H. Dowll H. Dowll dowll@slac.stanford.du

15 Surfac Pysics of Ral Catods Can b Vry Complx Vndor vs. klystron dpt. diamond-turnd sampls ly cut wit singl-crystal diamond tool by Diamond Turning, Inc. ly cut wit polycrystallin diamond tool in Klystron Sop Q and Trmal mittanc of Mtal Catods Jun, 6 Tory Sminar 5 D. David H. Dowll H. Dowll dowll@slac.stanford.du

16 Ar-Ion claning prfrntially mills out particular grains singl crystals, producing ig-contrast canyons in t surfac Q and Trmal mittanc of Mtal Catods Jun, 6 Tory Sminar 6 D. David H. Dowll H. Dowll dowll@slac.stanford.du

17 Claning of a damagd catod ARDB#B- Rplacmnt ARDB#3 rom Gun O4% C7% ARDB#3 H-cland O% C3% Q and Trmal mittanc of Mtal Catods Jun, 6 Tory Sminar 7 D. David H. Dowll H. Dowll dowll@slac.stanford.du

18 Summary and Conclusions Tory dvlopd for Q and trmal mittanc Grat* agrmnt btwn Q tory and xpt. Trmal mittanc tory rquirs inclusion of acclration insid t mtal to improv agrmnt wit tory But not all t possibl ffcts of surfac rougnss and matrial proprtis av bn includd T intraction of t lctrons wit t nar catod filds nds study Dynamical tim-dpndnt imag carg filds tc. Ts pnomna nd to b includd in simulation cods *B. Kirby quot Q and Trmal mittanc of Mtal Catods Jun, 6 Tory Sminar 8 D. David H. Dowll H. Dowll dowll@slac.stanford.du