Radiation Source ELBE
Mitglied der Leibniz-GemeinschaftDietmar Janssen
Axial Power Input in Photocathode Electron Guns
D. Janssen, FZR, Germany, V. Volkov, BINP, Novosibirsk, Russia,
Radiation Source ELBE
Mitglied der Leibniz-GemeinschaftDietmar Janssen
Cathode
RF cavity
Beam tube
In normal and superconductingRF guns the cathode can be separeted by avacuum gap from the surrounding cavity
Idea: This gap can be used for coaxial input of RF power
Advantages: - No additional RF input coupler necessary- No emittance growth by nonsymmetric RF fields in the coupler plane- No RF losses by the coaxial line around the cathode- Optimal arrangement of magnetic coils in the normal conducting case is possible- Easy to make the RF input variable by a quarter wave filter in the superconducting case
Vacuum gap
Power input in RF guns
Radiation Source ELBE
Mitglied der Leibniz-GemeinschaftDietmar Janssen
(1) Niobium Cavity(2) Choke Flange Filter(3) Cooling Insert (4) Liquid Nitrogen Tube
(5) Ceramic Insulation(6) Thermal Insulation(7) 3 Stage Coaxial Filter(8) Cathode Stem
Beamline Preparation Chamber
68 7
1 2 3 4
5
First operation of a SRF gun
D.Janssen et al., NIM-A, Vol. 507(2003)p314-317
T = 4.2K, Q = 2.5*108, Ezmax= 22MV/mE = 900keV, I = 520μA
RF powerinput
Radiation Source ELBE
Mitglied der Leibniz-GemeinschaftDietmar Janssen
New SRF Gun – Cavity design
He-vessel
power coupler
HOM filter
gun half-cell
choke filter
cathode
cathode cooler
LN2 reservoir
Cathode transfer rod
PosterWPAP007
Radiation Source ELBE
Mitglied der Leibniz-GemeinschaftDietmar Janssen
Main coupler flange
Pic-up flange
HOM coupler
HOM coupler
Coupler plane of the cavity
Radiation Source ELBE
Mitglied der Leibniz-GemeinschaftDietmar Janssen
Beam tube with higher order mode – and main coupler
HOM coupler(TESLA design)
Main couplerMax. power 10kWDesign M.ChampionDevelopment of HEPL, Stanford
Radiation Source ELBE
Mitglied der Leibniz-GemeinschaftDietmar Janssen
RF power input around the cathode
Power input
Power input
Warm RF window
LN2 cooling
Tube for cathode exchange
Cold RF window
cavity
cathode
Bellows for adjustment of Qext and heat isolation
End of the cryostat
Quarter wave choke
L
Δz
Radiation Source ELBE
Mitglied der Leibniz-GemeinschaftDietmar Janssen
External quality factor and heat load of a cathode-RF coupler
3.6 3.8 4.0 4.2 4.4
0
10
20
30
40
50
60
70
80
90
100
1103.6 3.8 4.0 4.2 4.4
106
107
108
109 Heat load of the cathode Beam power
Hea
t loa
d of
the
cath
ode
[W],
beam
pow
er [k
W]
Width z of the choke cell [mm]
Ext
erna
l qua
lity
fact
or
External quality factor
Field parameters for Eacc = 25MV/mEzmax(r=0) = 50MV/m, Urmax= 6.5kV
Esmax= 43.6MV/m, Bsmax= 0.11T
Radiation Source ELBE
Mitglied der Leibniz-GemeinschaftDietmar Janssen
Summary
In RF electron guns a coaxial gap between cathodeand cavity can be used for RF power input.
This special input coupler-is easy adjustable,-create no additional emittance growth and-produce in the superconducting case a small heat load even at large RF power