High Power Couplers for TTF - FEL 1. Requirements for High Power Couplers on superconducting Cavities 2. Characteristics of pulsed couplers 3. Standing wave pattern in the coaxial coupler line 4. Advantages of pulsed coupler operation 5. Design of the TTF Coupler 6. Coupler pumping 7. Technical coupler interlock 8. e- problem during operation 9. Conclusion 1
1. Requirements for Couplers on SC Cavities - transfer the power to the beam - strong mismatch in absence of beam between cavity and generator Q cavity, no beam = 10 10 = 10 4 Q cavity + beam = 10 6 Q extern = 10 6 = 1 - cold warm transition, low heat loads are necessary - it has to be cleaned to the standard of the sc cavity surfaces (usually by dustfree water) - clean assembly of coupler to the cavity in the class 10 clean room - protection of the clean cavity surface during assembly to the module - safety against window failures during operation - diagnostic 2
2. Characteristics of pulsed couplers Forward and reflected power at a cavity (long pulse, Q load = 3e6, >>1, no beam) P 1 SW P 2 TW SW: Pref 1 = Pfor TW: Pref 2 = 4 x Pfor U 2 = 2 x U 1 Denis Kostin 3
Forward and reflected power at a cavity (1.3 ms pulse, no beam, steady state not reached) SW: Pref 1 = Pfor P 1 SW P 2 TW TW: Pref 2 < 4 x Pfor U 2 < 2 x U 1 depends from the pulse length Denis Kostin 4
Forward and reflected power at a cavity without beam and reduced Pfor during Flat Top P 1 P 2 SW TW SW: Pref 1 = Pfor TW: Pref 2 << 4 x Pfor Flat top U 1 > U 2 Denis Kostin 5
Cavity with beam SW TW SW: Pref 1 = Pfor only filling time TW: Pref 2 = Pfor beam U 1 = 2 x U 2 6
3. Standing wave pattern in the coaxial coupler line End of RF pulse End of filling time Distance along the coupler [cm] Nikolay Sturm 7
What could go wrong during operation? Frequency of cavity Phase of cavity Beam current (beam loss) Lorenz force detuning (change of frequency due to the high field forces between the walls of the cells) Tuning of Q ext with a stub tuner in the wave guide causes additional field enhancement All this leads to a change of the standing wave pattern in the coupler. Areas with low electric fields during normal operation are then in the high fields. 8
Standing wave pattern without Lorenz Force detuning End of RF pulse End of filling time Distance along the coupler [cm] Nikolay Sturm 9
Standing wave pattern with Lorenz Force detuning at 25MV/m -The standing wave pattern is changing due to the Lorenz Force detuning - the high field areas are moved to areas with normally lower fields Distance along the coupler [cm] Nikolay Sturm 10
4. Advantages of pulsed coupler operation Low average power Easy handling of coupler events: Most dangerous events are gas discharge in the coupler (it can cause destruction of the coupler, has to be detected and interrupted). At cw a reduction of power is not sufficient -> power switch off is needed (the voltage to ignite the discharge is much bigger than the voltage needed to keep the discharge). At pulsed operation we get a power off for free (can start the next pulse at lower power and increase power slow). 11
A Possible interlock event handling during operation of a pulsed accelerator 12
5. Design of the TTF Coupler coaxial is easy for: two windows for: cold coax: warm coax: suppress multipacting : flexibility: copper plating: -variable coupling -fabrication -assembly -clean assembly of the high gradient cavity -save against window rupture -at 70 Ohm, 40 mm diameter -at 50 Ohm, 60 mm diameter -Al 2 O 3 with TiN coating - bias on inner conductor -bellows in the warm and cold coax -20 m -high thermal conductivity (RRR 30) -high purity, Hydrogen free 13
big pump port (CF35) not in use only the small one (CF16) beam pipe 14
Fermi Lab coupler 15
TTF 2 coupler 16
TTF 2 coupler 17
TESLA Coupler TTF 3 18
TTF 3 coupler 19
70 K shield coupling tuning 2 K beam pipe 4 K shield module vessel 20
6. Coupler pumping beam line Coupler Module Penning common pumping line Ti evaporation pump Ion getter pump 21
7. Technical Coupler Interlock 3 times e - (charged particles) light in coupler vacuum light in wave guide (air side) temperature cold ceramic temperature warm ceramic vacuum coupler vacuum cavity bias voltage cryogenic OK low level RF gate on klystron (connection to BIS foreseen) all thresholds are hardware and have to be set in hall 3 22
8. e - Problem during operation No beam, but Pfor not reduced by 6 db high reflected power peak at the end of the pulse this causes a gas discharge detected with the e- probe 23
Change of the forward power pulse form reduced the e- signal The shape of the power pulse can be optimized to reduce the reflected power peek 24
9. Conclusion Pulsed couplers have a standing wave pattern Standing wave pattern can change during operation by: Frequency change, e.g. Lorenz Force detuning Beam loss or wrong beam compensation Low and high field areas can change and cause gas discharge (interlock events) Therefore: a very good Lorenz Force detuning compensation at high cavity gradients is essential 25