Photocathode 1.5 (1, 3.5) cell Photocathode 1.5 (1, 3.5) cell superconducting RF gun with electric and superconducting RF gun with electric and

magnetic RF focusingmagnetic RF focusing Transversal normalized rms emittance (no thermal emittance) 0.62 π mm mrad

Bunch charge 1 nC

Laser pulse duration / Laser pulse rise time 20 ps / 1 ps

Axis peak induction of TE mode 0.3 Tesla

Surface peak induction of TE and TM modes 0.132

Tesla Acceleration frequency

1300 MHz

Axis peak field of acceleration mode 50

MV/m Electron bunch energy

4.62 MeV Energy spread (minimum, rms)

0.32%

B u d k e r I N P-FZR

V.N.Volkov@inp.nsk.su

The calculation results obtained by SuperLANS and ASTRA codes

RF gun geometry. RF gun geometry. What are the electric and magnetic RF What are the electric and magnetic RF

focusing?focusing?

1 – Heat sink2 – Choke cell3 – Photocathode Cu stalk4 – Cathode cell5 – Electric TM field pattern6 – Magnetic TE field pattern7 – Cavity full cell8 – TE mode coupler (90º routed)9 – TM mode coupler pipe

Scaled cathode region

Cu

T=78K

T=78K

Electric RF focusing region Magnetic RF focusing region

Other injector parameters Other injector parameters

Acceleration field frequency 1300 MHz

Acceleration peak field at the cavity axis 50 MV/m

Launch phase of bunch centre (optimized) 55º

Laser spot radius at the photocathode (optimized)

1.5 mm

Depth of photocathode Cu stem in back cavity wall (optimized to create optimal RF focusing)

2 mm

External quality factor of input coupler (Qext) 3.79·105

Input power

/ Average beam current assuming the Qext

320 kW

/ 70 mA

Dissipated 1300 MHz power at Cu pipe of input coupler assuming the Qext and TCu=78K

9.4 W

Dissipated 1300 MHz power at cavity Nb wall (assuming unloaded quality factor Qo=1010, 2K)

12.13 W

Dissipated 1300 MHz power at photocathode Cu stem assuming TCu=78K

5 W

Surface peak field at the photocathode 32.8 MV/m

Frequency of magnetic focusing TE mode 3788 MHz

Axis peak induction of TE mode (optimized) 0.3 Tesla

Maximum vector sum of surface induction of 1300 and 3788 MHz (the limit is 0.18 Tesla)

0.132 Tesla

Surface peak induction of TE mode 0.108 Tesla

Ratio of Peak Induction on the surface and on the axis (RPI)

0.358

Unloaded Quality factor of TE mode assuming the Qo for 1300 MHz

0.85·108

Dissipated RF power of TE mode at cavity Nb wall

13.43 W

Dissipated RF power of TE mode at Cu pipe of input coupler assuming TCu=78K

3.63 W

Transversal normalized emittance of bunch (thermal emittance is not taken into account)

0.62

π mm mrad

Full emittance: thermal emittance of Cs2Te photocathode (0.64 m) is taken into account

0.89

π mm mrad

Axis coordinate of emittance minimum disposition from the cathode

0.85 m

RF fields in the cavity RF fields in the cavity /SLANS cod/SLANS codThe vectors of TE and TM fields are ortogonalThe vectors of TE and TM fields are ortogonal

F=1300 MHz

F=3788 MHz

E 50 MV/m

axis

BTE 0.3 T axis

BTM 0.128 T surface

BTE 0.108 T surface

BTM+BTE 0.132 T surface

Peak fields

High order TE modes selectionHigh order TE modes selection for low Ratio of Peak Induction (RPI) at the surface and at the for low Ratio of Peak Induction (RPI) at the surface and at the

axisaxis

F, MHz RPI

2572.5 0.539

3787.8 0.358

3899.7 0.819

3947.2 0.863

F=2572.5 MHz F=3787.8 MHz

F=3899.7 MHz F=3947.2 MHz

TE021

Pipe cut offTE

frequency5226 MHz

TE011

Emittance dependence from TE field phaseEmittance dependence from TE field phase

oTEavn A 2sin

n – transversal normalized rms emittanceav- average emittanceA – emittance amplitudeφTE – TE mode phaseo - constant phaseBTE – TE mode peak induction at the axis, TR – laser spot radius at the photocathode, mmTM – launch phase (here TM=50º at maximum bunch

energy)

εav, m 0.805

0.712

Aε, m 0.212

0.08

BTE, T 0.28 0.3

R, mm 1.0 1.5

1 2

Set examples

Set examples

Parameter scanning for emittance minimizationParameter scanning for emittance minimization //ASTRA codASTRA cod

TE induction (BTE induction (BTETE), laser spot size (R), launch phase (), laser spot size (R), launch phase (TMTM))

+ +

1.0 1.25 1.5 1.75 R,mm

1.0 1.25 1.5 1.75 R, mm

0.32

0.30

0.28

0.26

0.32

0.30

0.28

0.26

BTE,T

BTE,

T

Average emittance, m Emittance amplitude, m

φTM=46.3ºBTE=0.29 TR=1.5 mmεmin=0.7m

Extreme values φTM

Average emittance, m 0.62 55º

Emittance amplitude, m 0! 60º

Energy, MeV 4.62 50º

Energy spread, KeV 15 42º

Launch phase scanning

+0.7 +

Optimum

for n=5%:BTE=0.03TDR=0.6 mm

TM=10º

Sensitivity

Bunch time evolutionBunch time evolution

Phase space, KeV/c

X, mm

Bunch cross section, mm

Bunch rotation is subtracted here

Bunch rotates by magnetic TE field

60 cm drift

Emittance compensation instances:Emittance compensation instances:without any RF focusing, with only electric RF focusing, with without any RF focusing, with only electric RF focusing, with only magnetic RF focusing, with sum - electric and magnetic only magnetic RF focusing, with sum - electric and magnetic

RF focusingRF focusing

Optimized settings &

performances

Without any RF focusing

Electric RF focusing only

Magnetic RF focusing only

Electric and magnetic RF

focusing

εn, mm mrad 3.66 1.49 1.28 0.62(εn

2+ εth

2)1/2 3.76 1.72 1.44 0.89

R (laser), mm 2 2 1.5 1.5

φTM, deg 49.4º 46.3º 49.4º 55º

Cathode depth, mm

0 2 0 2

BTE (axis, peak), T

0 0 0.3 0.3

B (surf., peak), T

0.128 0.128 0.132 0.132

Rth 43.0 mm mrad] - Cs2Te photocathode thermal normalized emittance [K.Floettmann studed]

1 cell superconducting RF gun (“DROSSEL”)1 cell superconducting RF gun (“DROSSEL”) with electric and magnetic RF focusing with electric and magnetic RF focusing

Optimized performancesOptimized performancesBunch transv. norm.emitt., m

0.51÷0.52

Emitt. minimum disposition, m 0.265

Average Energy, MeV 2.26

Launch phase of 1300 MHz 25.0º

Laser spot radius, mm 1.5

BTE (peak,, axis), T 0.300

BTE (peak, surface), T 0.168

RPI 0.56

BTM (surface), mT 0.123

| BTM+ BTE|, mT 0.173

Emittance TE compensationEmittance TE compensation

TE021

3.5 cell superconducting RF gun3.5 cell superconducting RF gun with electric and magnetic RF focusing with electric and magnetic RF focusing

Bunch transv. norm.emitt., m 0.78÷0.98

Emitt. minimum disposition, m 4.25

Average Energy, MeV 8.82

Launch phase of TM 1300 MHz 74.6º

Laser spot radius, mm 1.5

BTE (peak, axis), T 0.324

BTE (peak, surface), T 0.136

RPI 0.42

BTM (surface), mT 0.115

| BTM+ BTE|, mT 0.144

TE021

Conclusions• Emittance compensation by the electric and magnetic RF focusing

as well as a high accelerating gradient are the key factors in getting a small emittance with a large charge.

• Either electric or magnetic RF focusing diminish the emittance more than twice. And together – about 6 times.

• The peak induction of magnetic field on the axis is about 0.3 T. And sum of magnetic fields on cavity surface is less than the limit of 0.18 T.

• The induction of peak magnetic field on cavity surface proved to be small due to vector summation of orthogonal TE and TM fields. Also because of an unoverlapping of their peak fields on the surface.

• TE021 mode has a smallest ratio of magnetic peak induction on the surface to the peak induction on the axis.

• The dependence of emittance from TE phase has oscillatory view. There are RF gun parameter settins at which the oscillatory amplitude becomes zero.

• Transversal emittance remains small in wide range of RF gun settings.

Acknowledgments

The author would like to thank Dietmar Janssen (FZR),

Klauss Floettmann (DESY), Victor Petrov (BINP)

for helping in the work.