1 Roman Barday Plans for field emission studies at HZB for BERLinPro Unwanted Beam Workshop BERLinPro

2 Electron source for BERLinPro Beam energy50 MeV Average current100 mA Bunch charge77 pC Normalized emittance<1 mm mrad Repetition rate1.3 GHz 6 m Main linac SRF Gun Booster Merger Beam dump final beam parameters are determined by the performance of the electron source: SRF gun demonstration of the feasibility to use ERL technology for future synchrotron light sources BERLinPro=Berlin Energy Recovery Linac Project

3 Electron source for BERLinPro Development of an electron source with I=100 mA and  ~0.6 mm·mrad High average current I ave =100 mA Photomaterials with high QE in the green part of the light spectrum  cathode work function <2 eV. Semiconductor CsK 2 Sb (  ~1.9 eV) is a baseline photocathode for BERLinPro.  (Mo)~4.6 eV,  (Cs 2 Te)~3.6 eV. Low beam emittance  ~0.6 charge (current) Semiconductor CsK 2 Sb  thermal ~0.4 eV High launch field E launch =E peak ·sin  to minimize emittance growth due to space charge in the gun cavity  high peak field on the cathode surface Field emission grows exponentially with the field amplitude. Limiting factors: Field emission (unwanted beam), extracted from the cavity can limit the operation of the SRF gun: particles loss in the booster, pressure rise (ESD), damage of the machine components,… Field emission is relevant for understanding of multipacting, which can also limit the performance of the SRF gun

4 Sources of field emission „Foreign particles“ introduced into the system with the plug or occur in the gun (friction). Typical size: µm Cathode 100 mA<one week  cathode change Plug/unactivated cathode surface (GaAs) roughness „as received“ after cleaning (thermal, ion beam,…) Photocathode (oxidations, carbon contaminations,…) Role of the work function? According to FN equation field emission current increases by several orders of magnitude, while decreasing the work function from 4 eV to 2 eV. What is a dominant effect: enhancement factor or work function? Field emission current I=I( ,  ) morphology work function

5 Sources of field emission: metalic particles „Foreign particles“ on the surface like copper or iron (conducting!) introduced into to the system with the plug (friction). gun Transport chamber Transfer chamber CuBe spring Mo plug Preparation chamber Transport chamber „particles free“ plug beam produce particles ø 10 mm 6 mm

6 Sources of field emission: metalic particles Sources of dust particles: Bellows, valves Friction plug/spring Friction plug holder/filter Coating, which reduce particles production Tests with coatings were not successful SRF gun transfer system transport system NC cathode cathode filter LN 2 reservoir based on HZDR design optical polishing: january 2013  test particle counter

7 Sources of field emission: cathode roughness Surface roughness S. Karkare, I. Bazarov, Appl. Phys. Lett. 98, (2011) AFM images of GaAs surfases. Surface roughness before activation less than 0.5 nm Surface roughness of heat and activated GaAs ~6 nm V. Shutthanandan, Phys. Rev. ST Accel. Beams 15, (2012) Helium ion miscoscope images of an unactivated and activated GaAs. The surface undergoes roughening. FE from cleaned GaAs Test at HZDR?

8 Sources of field emission: Photocathode (work function) J.D. Jarvis, FEL2010 Work function of CsK 2 Sb ~1.9 eV NEA GaAs Does low cathode work function automatically mean higher field emission? Oxidations modify the potential and can produce additionally surface states Increase current by more than one order of magnitude J.D. Jarvis, FEL2010 Energy diagram of the emitter-vacuum interface in the presence of an adsorbate Field emission current according to FN equation for an emitter with A= m 2,  =100 FN for metals

9 Sources of field emission: Photocathode (work function) The Los Alamos Free-Electron Laser facility 26 MV/m at CsK 2 Sb cathode 1.3 GHz, 22 MHz micropulse repetition rate micropulse length 15 ps macropulse length 100 µs macropulse repetition rate 1 Hz 2.2 mA dark current A. H. Lumpkin, LANL Photomaterials with high QE in the green part of the light spectrum  cathode work function <2 eV. Semiconductor CsK 2 Sb (  ~1.9 eV) is a baseline photocathode for BERLinPro

10 Sources of field emission FE study in the (S)RF gun is possible, but expensive Not easy to interpret the results More convenient, to study FE in a separate DC setup Fit of the F-N equation for a SRF gun with Pb cathode  A [nm 2 ]I/A [A/cm 2 ] Before cleaning After cleaning Gun0.1 after cleaning Gun0.2 before cleaning Images of the dark current on the YAG:Ce view screen E local =200*18MV/m  3.6GV/m (OK) A=0.03nm 2 (not physical)

11 DC-setup for field emission study d power supply 10mm plug view screen as anode d~0.4 mm U~10 kV Current measurement (integral signal) Characterisation of the emitter in terms of  Image of the emitters on the view screen (spatially resolved) E~10-25 MV/m U cathode anode D E d<<D homogeneous field gradients ~10-25 MV/m are realistic at a small gap and a low bias voltage Emitter view screen cathode molybdenum plug

12 DC-setup for field emission study: plugs (metals), field emitter d d~0.4 mm U~10 kV Current measurement (integral signal) Characterisation of the emitter in terms of  Image of the emitters on the view screen (spatially resolved) E~10-25 MV/m U cathode anode E d<<D homogeneous field gradients ~10-25 MV/m are realistic at a small gap and a low bias voltage power supply 10 mm plug view screen D

13 DC-setup for field emission study: plugs (metals), field emitter d U cathode anode D E Front and back sides of the YAG:Ce crystal coated with ITO View screen (E e ~keV) YAG:Ce crystal ITO coating (thickness ~10-20 nm) to avoid accumulation of the absorbed electric charge in the crystal power supply 10 mm plug view screen S q =0.46nm S a =0.3nm 235.2X235.2µm 2 S q =0.29nm S a =0.23nm 235.2X235.2µm 2 F. Siewert

14 DC-setup for field emission study: plugs (metals), field emitter Chamber for plugs transportation S q =7.3 nm S a =5.9 nm 235.2X235.2µm 2 F. Siewert Surface of polished Mo substrate. 200 mm

15 DC-setup for field emission study: divergence d U FEA pepperpot-mask E view screen Emittance measurement Diamond Field Emitter Arrays L drift CCD-Camera Pepper-pot mask made of Tungsten with a thickness of 50 µm has holes with a diameter of 50 µm and spaced by 1 mm. Size of the beamlet at the screen  angular divergence  N =  x  x‘ Bo Choi, Vanderbilt University xx  x‘  x /L drift +f(R,L,D)

16 DC-setup for field emission study: cathodes Field emission from semiconductors Improved Fowler-Nordheim equation for semiconductors? Band bending, field penetration into the crystal Temperature dependence, T cath ~80 K V BB ~0.7 eV