Needle cathodes for high-brightness beams. Chase Boulware Jonathan Jarvis Heather Andrews Charlie Brau

Transcription

1 Needle cathodes for high-brightness beams Chase Boulware Jonathan Jarvis Heather Andrews Charlie Brau

2 Outline of the talk What is brightness? Definition Sources Why is brightness important? Light sources FELs How do we get high brightness? Photoemission Field emission Photofield emission

3 Definition of brightness Emittance is! -1 x area in phase space (old definition) Or, weighted average over beam (rms emittance) Brightness is Density in transverse phase space Local property of beam () NNdIBddA

4 Electron sources span many orders of magnitude in brightness and current Brightness (A/m2-steradian) 1.E+19 1.E+18 1.E+17 1.E+16 1.E+15 1.E+14 1.E+13 1.E+12 1.E+11 1.E+10 1.E+09 1.E+08 1.E+07 Nanotubes Needle photo emission Photo-field emission Thermionic emission 1.E-07 1.E-05 1.E-03 1.E-01 1.E+01 1.E+03 Current (A) Field emission DC photo gun RF photoinjectors Storage rings

5 Why brightness is more important than current Brightness is a useful figure of merit Normalized brightness is roughly invariant with respect to beam current, electron energy Can be used to compare different devices Often it s the most important parameter When brightness is the most important parameter, lower current may be possible Lower current reduces other problems, including radiation, halo, CSR, space charge

6 Spectral brilliance of Compton x-rays depends on brightness, not current For small emittance spectral brilliance is

7 Gain of Thompson FEL depends on brightness, not current In strong optical guiding regime 22eLNIB!"> power gain is independent of current

8 Claudio s microwave-undulator x-ray FEL (FEL 2005) Parameters " W = 1.45 cm a W = 0.4 # = 2150 " L = 2 nm M. Xie formulas I e = 2000 A B N = 2.5 x A/m 2 -sr µ = 0.7 /m Gain in strong-guiding regime (my formula) µ = 0.8 /m Minimum current for strong guiding I e > 400 A Beam diameter d = 70 µm

9 High electric fields at the surface enhance cathode performance High electric fields: Conventional DC guns ~ 10 6 V/m Conventional RF guns ~ V/m Needle cathodes ~ V/m Enhanced performance due to Schottky effect on photoemission Field emission Photo-field emission Reduced space-charge effects

10 Electron emission at the surface of a metal in vacuum occurs by four mechanisms Photoelectric Emission $ Thermionic Emission Energy Fermi Level Photo-field Emission Field Emission Metal Vacuum

11 Schottky effect reduces surface barrier at high electric field Field is enhanced at tip of needle Schottky effect lowers barrier at surface

12 Schottky effect enhances quantum efficiency of needle cathodes SQRT(Quantum Efficiency) photocurrent/laser intensity (Am^2/W) 3.0E E E E E E E E E E E E E-17 E=0 E=0 E=10 9 E= nm photons Tungsten 213-nm photons (Space charge) Energy above barrier excited energy above barrier (ev) 266 nm,.5e11 W/m^2 337 nm, 5.7e11 W/m^2 337 nm, 1.5e11 W/m^2 355 nm, 8e11 W/m^2 355 nm, 4.8e11W/m^2 266 nm, calculation 337 nm, calculation Threshold law for quantum efficiency Holds over wide range Increases QE by orders of magnitude

13 Laser damage to cathode limits achievable photocurrent density Figure of merit for cathodes is % QE x P damage Tungsten cathodes P damage ~ W/m 2 for 5-ns pulses P damage ~ W/m 2 for 1-ps pulses Limiting current density for QE ~ 10-4 J max ~ 10 7 A/m 2 for 5-ns pulses J max ~ 10 9 A/m 2 for 1-ps pulses Highest current densities will be space-charge limited

14 Space-charge limits current density Planar geometry Spherical geometry* For small spot size (<< tip radius)** For E tip ~ 10 9 V/m, r tip ~ 1 mm, J ~10 8 A/m 2 * a 2 = O(5-10) **Y. Y. Lau

15 Needle cathodes produce high brightness in RF guns* Field at cathode enhanced by Example: 1 mm diameter, 1 cm long E 0 = 50 MV/m E tip = O(500 MV/m) * Lewellen, Sardegna Space-charge limit ~ 10 8 A/m 2 Brightness ~ A/m 2 -str before pulse compression!

16 Field emission involves electrons in levels near the Fermi level Tunneling through barrier causes characteristic steep voltage dependence (Fowler-Nordheim) $ Energy Fermi Level Field emission Metal Vacuum

17 The characteristic steep voltage dependence of photo-field emission at 514 nm strongly indicates tunneling from just above the Fermi level Laser Power=0.2 W Laser Power=0.3 W log(i/v 2 ) Laser off T=1125 K!=4.10 ev T=950 K!=4.22 ev Fowler-Nordheim Theory T=300 K!=4.5 ev /V

18 Photoelectrons excited from d-conduction band into s-conduction band where they reside just above Fermi level for ~ microsecond Energy Optical excitation Relaxation fast Fermi Level slow Metal Vacuum Photo-Field Emission No effect observed in ZrC (only one conduction band)

19 Conclusions High brightness is often more important than high current Needle cathodes operate at high electric fields ( V/m) Enhanced emission from cathode Reduced space-charge effects Interesting physical effects are found at high electric fields Field-enhanced photoemission (Schottky) Photo-enhanced field emission (tunneling)