Conduction Cooled SRF Photogunfor Electron Microscopy

Roman Kostin, Chunguang Jing, Scott Ross, Euclid Beamlabs LLC, Bolingbrook, ILSam Posen, Timergali Habiboulline, Ivan Gonin, Oleg Pronitchev Fermilab, Batavia, IL

SRF photoguns become a promising candidate to produce highly stableelectrons for UEM/UED applications because of the ultrahigh shot-to-shotstability compared to room temperature RF photoguns. SRF technology wasprohibitively expensive for industrial use until two recent advancements:Nb3Sn and conduction cooling. Euclid is developing a continuous wave(CW), 1.5-cell, MeV-scale SRF conduction cooled photogun operating at 1.3GHz. Technical details of the gun are presented together with the currentstatus. The beam generated by the back wall UV laser illumination.

Introduction

Stress Analysis and Fixtures Design

RF tuning

SRF gun parameters

Standard feedthroughs were used: SMA pick-up and N-type FPC. The position of the FPC is thesame as in Tesla cavity. Initial simulations revealed only 10dB signal over the noise level. Thepick-up was moved closer to the cavity and the signal raised to 20dB. Mathematical noisefiltering can significantly increase signal to noise ratio (see two bottom pictures).

Summary

• SRF gun was developed for UED/UEM applications• The RF properties were optimized• Beam parameters satisfy the requirements for UED/UEM• The gun is designed for conduction cooling by only one cryocooler• Universal fixtures were designed for VTS and RF tuning• The gun was tuned and BCPed and is ready for 2K test early JULY!• Supported by DOE SBIR DE-SC0018621

MOPTEV001

RF Parameter ValueFrequency, 1.3 GHzLength 1.45cell

(160mm)Q0 at 4°K (Rs = 20 nΩ ) 1.16×1010

R/Q (critical coupling) 176.9 ΩGeometry factor 232 ΩWall Power dissipation 0.9 WEon axis 20 MV/mEmax_surface 23.5 MV/mBmax_surface 43.3 mTEacc 10MV/mBeam energy 1.6MeV

0

0.2

0.4

0.6

0.8

1

75 125 175 225 275

F=1301.273925 MHz F=1300.499044 MHz

Beam Parameter ValueApplication UED UEMBeam energy 1.655

MeV1.655

MeVCharge 5fC 0.5pCLaser pulse length, rms 6.4fs 6.4fsLaser spot size 36um 180umBeam bunch length, rms 167fs 741fsBeam emittance 6.6nm 39nmEnergy spread (relative) 1.3e-5 6.4e-5

-150dB

-130dB

-130dB

-140dB

10dB

20dB

300K

300K

-132dB

-165dB

-30dB

-156dB

300K

300K 4K

Couplers cross-talk

Couplers-crosstalk

Couplers-crosstalk

w/o couplers-crosstalk

w/o couplers-crosstalk

Qext=E-11

Qext=E-10

Electric field on axis at Eacc=10MV/m

Magnetic field on surface at Eacc=10MV/m

Thermal Analysis

Thermal analysis was conducted which included temperature dependent material properties, thermalcontact resistance and RF power dissipation (BCS resistance). The results can be found on figure on theleft. Stable operation can be achieved for Rbcs+10nOhm and 20nOhm with only one cryocooler.

Fixtures for vertical test were designed from non magnetic materials compatible with Fermilab VerticalTest Stand (VTS) set-up. The design was analyzed and is safe for operation: stress level is below 17MPa.

Power dissipation at Eacc=10MV/mTemperature distribution at Eacc=10MV/m,

Rbcs=20nOhm

Electric field distribution at Eacc=10MV/m

Deformations at 1.5bar pressure in m. Stresses in MPa along the gun surface.

BCP

Gun surface as manufactured

Gun surface after Rotational 140um BCPField balance tuned

VTS fixtures were used for RF tuning as well

ΔT=0.1K