Optimization c athode design with gun5

D. Lipka, V. Vogel, DESY Hamburg, Germany, Oct. 2012

Optimization cathode design with gun5

D. Lipka, V. Vogel, DESY Hamburg, Germany


Tilted backplaneTo decrease field on cathode edge and keep high performance for beam dynamics: backplane to cathode are tilted, here 10°,

On blue line field strength can be monitored


Tilted backplaneThe cathode holder and spring for this simulation is used like present FLASH case


1D Results > S-ParametersLength of first cell is adapted to reach 1299.9 MHz resonance frequency of p-mode


E-field on axisWith this adaption the balance is almost perfect, here the maximum field strength is scaled to 60 MV/m, butThe field strength on cathode is reduced due to the tilted backplane


E-field on cathodeE-field strength on cathode edge is maximum with 63.11 MV/m. This is a factor of 1.37 higher compared to cathode center field strength, Compared with present case: factor is 1.39, only slight reduction


E-field on surfaceDiagram shows E-field strength on surface. Compared to present case: • Lower field on cathode• Relatively high field on

iris• Surface is longer due to

tilt and adaption to get balance and resonance frequency


ASTRA simulation To check the beam performance with the tilted

backplane ASTRA simulations are performed Laser beam settings:

Q=1nC, longitudinal flat-top with 20 ps time duration and 2 ps rise and fall time

Transverse size sx,y=0.75 mm radial symmetry, no thermal emittance


ASTRA simulation: fields

E-field tilted backplane

E-field gun5 designSolenoid field

Fields as a function of distance to cathode

E-field maximum scaled to 60 MV/m, will be compared with gun5 design

Solenoid field taken from standard FLASH type For optimization of emittance rf-phase of E-field

and solenoid strength will be varied


For optimization of emittance following optimization results are found: Tilted backplane: rf-phase between laser and

highest energy gain has to be +20°, at 0° electrons are lost due to backwards acceleration, solenoid maximum strength has to be 0.23 T

Design gun5: rf-phase between laser and highest energy gain has to be +10°, solenoid maximum strength has to be 0.25 T

ASTRA simulation results


ASTRA results: tilted backplane

A small emittance observed, no electron loss, energy 6.3 MeV still with 20° rf phase due to longer first cell


ASTRA result: default gun5 design

A small emittance observed too, energy 5.9 MeV with 10° rf phase


ASTRA results: emittance

tilted backplane

Emittance as a function of distance to cathode

Design gun5

Tilted backplane minimum emittance of 0.8 p mm mrad at z=1.54 m

Design gun5 minimum emittance of 1.05 p mm mrad at z=1.25 m

→ still smaller emittance with tilted backplane and reduced field at cathode


To reduce field on cathode edge the holder rounding are reduced from bending radius 3 mm to 1 mm

Change design: smaller rounding holder


Similar field distribution along z-axis like before because of only small change

Resonance frequency still at 1299.9 MHz

E-field on z-axis


E-field on cathode Field maximum strength on edge

is 57.9 MV/m, a factor of 1.31 higher compared to center

The factor is reduced from 1.39 (present case)

Summary: field strength on edge is reduced from 84.6 to 63.1 (10°) and to 57.9 MV/m for smaller rounding


Field strength in units of E/m

higher field on iris visible in both optimized design visible

Smaller field strength due to smaller bend radius

Even smaller bend increases field again

E-field strength on resonator surface


H-field strength on cathode due to modification is reduced: Default: 31.5 kA/m 10°: 28.7 kA/m Smaller rounding: 21.1

kA/m

H-field strength on cathode

Field strength maximum at connection between spring and cathode

Default case


Similar settings for optimal emittance: Bmax=0.23 T, phase +20°

Emittance 0.89 p mm mrad at 1.57 m distance to cathode

ASTRA simulation results


Compare shunt impedance

The shunt impedance for each case is:

• Default case: 7.2 MΩ• 10° case: 9.0 MΩ• 10° and smaller rounding: 9.1 MΩ


Emittance reduced from 1.05 to 0.8 (10°) and 0.89 p mm mrad (last case)

Maximum E-field strength on cathode surface reduced from 84.6 to 63.1 (10°) and to 57.9 MV/m (last case)

Maximum H-field strength on cathode surface reduced from 31.5 to 28.7 (10°) and 21.1 kA/m (last case)

Conclusion


Slice emittances

Default case at z=1.25 m Tilted backplane at z=1.54 m Tilted backplane and smaller rounding at z=1.57 m

Slice emittances are shown as a function of longitudinal slices at the z-position with best projected emittance

Best slice emittances reached for default case in the center (about 0.2 p mm mrad), only front and end contributes to the larger projected emittance

Other both slice emittances at the center higher, at the front and end are smaller compared to default case

Optimization c athode design with gun5

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