Experience with the ferrite tuner for LEB ( ) Slava Yakovlev 15/04/2016

Outline 1. Physics of the perpendicular bias. - frequency dependence for permeability - mechanisms and frequency dependence of magnetic losses; - non-linear effects; - permittivity and dielectric losses. 2. Design approaches: - loss minimizations: a. operation range for permeability, b. the tuner size, c. thermal effects - electric strength optimization a. triple points in the tuner; b. optimization of the window - bias optimization (slots) - HOM dampers 3. Ferrite ring characterization - magnetic losses - dielectric losses - test cavity. 4/15/2016Slava Yakovlev | Experience with the ferrite tuner for LEB,

Why perpendicular bias? 4/15/2016Slava Yakovlev | Experience with the ferrite tuner for LEB, R.L. Poirier, 1993 Longitudinal bias: -operation well below saturation -high losses (sizable hysteresis loops) -NiZn (3200 Gs, very low Q) Perpendicular bias: -operation in deep saturation -operation above gyromagnetic resonance -Yttrium garnet ferrites (810 Gs, high Q) L.M. Earley, 1983

Perpendicular bias: 4/15/2016Slava Yakovlev | Experience with the ferrite tuner for LEB, Ferrites at microwave frequencies, A.J. Baden Fuller

LEB cavity: 4/15/2016Slava Yakovlev | Experience with the ferrite tuner for LEB, R.L. Poirier, 1993

LEB cavity: 4/15/2016Slava Yakovlev | Experience with the ferrite tuner for LEB, – ferrite rings; 2 – yoke; 3- coils; 4 – windows; 5 - amplifier Contradictive approaches: 1.Wide range of the cavity frequency tuning, from MHz to MHz; 2.Minimization of the losses in the ferrite in order -to minimize of the max temperature in ferrite in order to operate well below Curie temperature (173 ∘ C) and -prevent the rings damage because of overheating Average temperature rise and pulse heating are important. 3.Optimization of the permeability range in order to minimize the losses; 4.Optimization of the ferrite volume; 5.Improvement of the tuner electric strength – gap voltage is 127 kV! 6.Operation of ferrite in linear regime 7.Bias magnet optimization Gap: C Coax: Z, l Tuner: L, ΔL l

LEB cavity 4/15/2016Slava Yakovlev | Experience with the ferrite tuner for LEB, Issues: Poor thermal conductivity of the garnet ferrite 0.05 W/(cm*K)– cooling problem; -Using BeO disks for cooling: thermal conductivity is 2.5 W/(cm*K); Prevention of arcing in the air-filled part of the tuner: -Elastoseal glue is used to fill the gaps between the rings and the cavity wall; Optimization of the window in order to minimize the surface fields in the air part and the field in ceramics below 6 kV/cm. Optimization of the vacuum coaxial and the acceleration gap geometry – electric strength, MP. Self-consistent optimization of the ferrite resonator and the bias magnet Ferrite ring characterization (dielectric losses are important also) 5 yttrium garnet rings, External diameter 590 mm Internal diameter 310 mm Thickness 27.5 mm

LEB cavity: electric strength optimization Tuner: -Tuner voltage optimization; -Gaps filling by elastoseal; -Prevention of the air bubbles in the glue (tuner wall pre-deformation) -Triple point protection. Window: -Optimization of the window shape 4/15/2016Slava Yakovlev | Experience with the ferrite tuner for LEB,

TRIUMF cavity: Non-linear effects measurements and analysis 4/15/2016Slava Yakovlev | Experience with the ferrite tuner for LEB, G.L. Hulsey, V.M. Petrov, V.P. Yakovlev, Tech Note SSCL (Sept ).

LEB cavity: Ferrite ring characterization “Domen”, S-Pb, Russia Test cavity: 4/15/2016Slava Yakovlev | Experience with the ferrite tuner for LEB, The ring W e /W M ~1%. Q e ~0.5e4 Error analysis: 2 Δω =5 MHz for all the rings except N3 Bias magnet provides δH 0 /H 0 < 2.5% 3 Hall probes Temperature control μ'μ' Q fm

Summary 4/15/2016Slava Yakovlev | Experience with the ferrite tuner for LEB, “The prototype cavity has been assembled with a conduction cooled tuner and tested. The tuner fault limited some of the high voltage and power testing, however much valuable information was obtained. The tuning range achieved was more than adequate to cover the LEB requirements. The high frequency tuner response starts to roll off at - 30 Hz. However the roll off appears to be slow and extrapolates to a non negligible response at 1 kHz. The overall cavity Q was somewhat lower than expected with the extra losses being located in the tuner. The source of these losses is not yet clear, however it is obvious that they limit the high power operation of the tuner. The cavity was successfully run at 80 kV and at moderate powers. However, the tuner was critically damaged when pushed to higher powers. Explanations of the fault are tied to breakdown of air pockets due to electric field stress and the high thermal stresses present in the tuner.” The reason of the tuner failure was analyzed and understood, the tuner has been repaired, cold-tested and shipped to SSC, but never tested at high power. The reason: arc caused by triple point excitation near the tuner wall. Special means for triple point protection were developed and implemented.