Advanced Energy Systems Inc. P.O. Box 7455, Princeton, NJ Phone:(609) Fax:(609) Jangho Park Ultrafast High-brightness Electron Source

ATF USER MEETING April 26-27, 2012 Ultrafast High-Brightness Electron Source 2 Putting Accelerator Technology to Work Introduction Generation and preservation of ultrafast high-brightness electron beams is one of the major challenges in accelerator R&D. In order to generate and preserve the ultrafast high- brightness electron beam, transverse and longitudinal space charge effects have to be considered. Several approaches to achieving ultra-short bunches have been explored such as velocity bunching and magnetic compression. However, each option suffers drawbacks (extra RF and bulky) in achieving a compact ultrafast high-brightness source. A compact ultrafast high-brightness electron source is demanded for Ultrafast/Femtosecond Electron Diffraction (UED/FED) and advanced accelerator experiment.

ATF USER MEETING April 26-27, 2012 Ultrafast High-Brightness Electron Source 3 Putting Accelerator Technology to Work Bunch Lengthening For a high-aspect-ratio (short bunch length) electron bunch, the asymptotic bunch length due to the space charge forces of a uniformly accelerated bunch, ignoring the drive laser duration and assuming prompt response from a copper cathode where the laser spot radius is kept constant, is expressed by: Bunch lengthening due to the space charge is inversely proportion to the square of the bunch radius and the square of the accelerating field. Bunch length stretches due to the longer path lengths of the outer particles compared with electrons closer to the axis. Bunch lengthening due to the geometrical effects is proportional to the square of the beam radius.

ATF USER MEETING April 26-27, 2012 Ultrafast High-Brightness Electron Source 4 Putting Accelerator Technology to Work R=2.55mm R=3.35mm R=4.25mm R c =30mm Gun Cavity Design Gun Cavity Design Aspects – Higher Acceleration Field with x-band RF – Utilizing a curved cathode – Axisymmetry coupling – Copper cathode

ATF USER MEETING April 26-27, 2012 Ultrafast High-Brightness Electron Source 5 Putting Accelerator Technology to Work Cavity Field Configuration Axial FieldRadial Field Cathode A: Curved, Rc=30mm Cathode C: Flat cathode

ATF USER MEETING April 26-27, 2012 Ultrafast High-Brightness Electron Source 6 Putting Accelerator Technology to Work Transverse Bunch Size Evolution Cathode A: Curved, Rc=30mm Cathode C: Flat cathode

ATF USER MEETING April 26-27, 2012 Ultrafast High-Brightness Electron Source 7 Putting Accelerator Technology to Work Beam Dynamics Results Cathode A: Curved, Rc=30mm Cathode B: Curved, Rc=40 mm, laser pulse shaping Cathode C: Flat cathode 25fs UV

ATF USER MEETING April 26-27, 2012 Ultrafast High-Brightness Electron Source 8 Putting Accelerator Technology to Work Beam Dynamics Results Beam dynamics simulation results at the z = 50 cm target location. ParameterUnit Value by Cathode Type ABCAB Peak Accelerating Gradient[MV/m]230 Beam Energy[MeV]2.7 Bunch Charge[pC] Bunch Length[femtosec] Beam Current[A] Energy Spread[keV] Target Beam Size[mm] Transverse rms Emittance[mm-mrad] Brightness[10 14 A/m 2 ] Cathode A: Curved, Rc=30mm Cathode B: Curved, Rc=40 mm, laser pulse shaping Cathode C: Flat cathode

ATF USER MEETING April 26-27, 2012 Ultrafast High-Brightness Electron Source 9 Putting Accelerator Technology to Work Temperature Results Temperature K Mean Temperature of Gun  C 3-D ANSYS 4.0x10 -5 duty factor Cooling channels 20  C cooling water

ATF USER MEETING April 26-27, 2012 Ultrafast High-Brightness Electron Source 10 Putting Accelerator Technology to Work Displacement Results

ATF USER MEETING April 26-27, 2012 Ultrafast High-Brightness Electron Source 11 Putting Accelerator Technology to Work Freq Shift Frequency Shift determined from 3-D ANSYS original geometry eigenvalue solution and new geometry from displacements resulting from RF/Thermal loads. Frequency Shift from Displacements: -159 kHz Frequency shift from rf losses can be offset with cooling temperature

ATF USER MEETING April 26-27, 2012 Ultrafast High-Brightness Electron Source 12 Putting Accelerator Technology to Work Gun Assembly Ding Holes For Tuning (4 per Cell) Cathode Cell Full Cell Vacuum Cross 2.75 CF Flanges Vacuum Grille RF Input Waveguide to Coax Transition Beamline Flange 1.33 Conflat

ATF USER MEETING April 26-27, 2012 Ultrafast High-Brightness Electron Source 13 Putting Accelerator Technology to Work FEED (2) RETURN INTERNAL PLENUMS Cooling Arrangement

ATF USER MEETING April 26-27, 2012 Ultrafast High-Brightness Electron Source 14 Putting Accelerator Technology to Work 12.5 RF In Z [cm] Laser Bucking Coil Solenoid RF Gun Laser Mirror 1.Beam Viewer with Faraday Cup 2.Pin-hole 3.CTR Target E-beam Ion Pump RF Window Linear motion Ion Pump UV View Port Linear motion Camera Interferometer IR View port Ion pump Beam Viewer Camera Steerer Gate Valve Proposed Experimental Setup

ATF USER MEETING April 26-27, 2012 Ultrafast High-Brightness Electron Source 15 Putting Accelerator Technology to Work Proposed Experimental Setup

ATF USER MEETING April 26-27, 2012 Ultrafast High-Brightness Electron Source 16 Putting Accelerator Technology to Work Conclusions AES is developing a ultrafast high-brightness electron source for FED experiments and advanced accelerator applications. The ultrafast high-brightness electron beam from X-band curved cathode RF gun is feasible. Numerical calculations are improved 5 to 10 times brighter than S-band PC gun. Thermal-structural and RF frequency shift analysis shows that the design of this electron gun running at 4.0x10 -5 duty factor is robust. Planning installation and tests at ATF would start July The ATF beam tests will include; – validate the source, – provide a useful data point on the bunch length and emittance. This work was supported by DOE SBIR Phase I grant No. DE-SC The Phase II grant is anticipating in July Thank You!!!