Spoke and other TEM-class superconducting cavities J.L. Muñoz, ESS-Bilbao Academy-Industry Matching Event CIEMAT, Madrid, 27.May.2013
Outline Introduction Basic design of TEM cavities Cavity design issues Beam dynamics, electromagnetic, mechanic Fabrication Testing Cavities in linacs Conclusions
Introduction TEM-class cavities are a family of SC cavities used for proton and ion acceleration in low-mid energy range TEM refers to Transverse ElectroMagnetic, in opposition to TM (eliptical,...) or TE (H) cavities. Spoke resonator Half wave resonator Quarter wave resonator
Introduction These cavities share some common characteristics: Small cavities, individually powered, large aperture Large β acceptance Efficient for accelerating ions β~0.03 to β~0.6 Cavities mainly made in bulk Nb (some sputtered) Accelerating gradients around 6-8 MV/m Design and fabrication procedures are similar
Introduction For CW linacs, an obvious advantage over RT cavities is the lower power consumption (in total about 100 times less, includind cryo) For pulsed and high intensity linacs they offer advantages (acceptance, fault tolerance, species versatility,...) For low-β acceleration, TEM cavities can provided the required Low frequency Short gap lengths
Basic design For TM cavities (elliptical,...) frequency depends on transverse dimension, increasing for lowering frequency TEM design comes from coaxial line resonator, and frequency depends on longitudinal dimension L= λ 2 L= λ 4 HWR QWR
Basic design From coaxial resonator to spoke cavity
Beam dynamics design Accelerating gaps ωz V acc = E z exp ( j )dz βc β λ /2 βgeom
Electromagnetic design Targets for the optimized structure: Resonant frequency value Quality factor (Q0) Surface electric and magnetic fields below maximum values (Emax/E0 and Bmax/E0) Set of parameters Modify 3D geometry Force characteristics Optimizer algorithm Compute figures of merit
Electromagnetic design Quality factor is related to energy stored and power losses: ωu Q0 = P Power losses: 1 2 P= R s S H ds 2 R s= R0 + R BCS Residual resistance, depends on surface quality, temperature and Nb quality. R s 10 20 nω BCS resistance, depends on frequency (as f2) and temperature
Electromagnetic design Surface fields are kept below some limits: Emax/Eacc < 5 Bmax/Eacc < 9 mt/(mv/m) Maximum Esurf: Achievable: 60 MV/m Reasonable specs: 30 MV/m Maximum Bsurf: Achievable: 120 mt Reasonable specs: 60 mt
Tuning, coupling Power couplers Tuning systems Power 10-20 kw Normally capacitive couplers Two or three temperature regions Integration in cryomodule Slow tuner: Correcting fabrication Slow He fluctuations Static Lorentz forces Fast tuner: Microphonics (vibrations, fast He fluctuations) Pulsed Lorentz forces
Multipacting Resonant field emission of electrons under the action of the EM field Conditions: 1. stable trajectories ending on cavity walls (cavity geometry) + 2. secondary emission coefficient >1 (surface preparation) + 3. initial electron impinging the right surface at the right field and phase to start the process (presence of free electrons) Initial electrons can be originated and captured far from the resonant trajectory (cavity geometry) (Slide taken from A. Facco, Low beta cavity design tutorial)
Mechanical design Targets for mechanical design are: Defining Nb thickness Placement of stiffeners Tuners Helium vessel and ports characteristics Considerations: Static analysis (He pressure,...) Dynamical analysis (mechanical modes) Thermal analysis
Mechanical design Atmospheric and He pressure on the cavity. Important due to small yield strengh of Nb at room T (about 50 MPa) One end fixed, naked cavity Two ends fixed, naked cavity
Mechanical design As a result of this analysis... Nb sheet thickness necesary is estimated Design of stiffening components Allowed He pressure fluctuations
Mechanical design Mechanical resonant modes Can be excited by mechanical componentes (pumps...). High deformations Example for double spoke resonator with stiffeners. Modes frequency: 276.9 Hz 277.0 Hz 338.6 Hz 341.0 Hz
RF-Mechanical Lorentz force detunning 1 2 2 (ε0 E μ0 H ) 4 Caused by radiation pressure: Proportional to -Ea2 For CW can be compensated as a static P=
Mechanical design He vessel Made in Nb, SS, Ti...
Fabrication Technology: Bulk Nb: From Nb sheets, rod, plates... EBW in vacuum Surface treatment (chemical-, electro-polishing, HWPR... Sputtered Nb on Cu: (QWR) 1-2 um of Nb on OFCE copper Better thermal properties, costs Simpler geometries Pb plated on Cu Older technique, results not comparable to Nb
Fabrication Niobium specifications: RRR > 250 / 300 Metallurgical characeristics: At least 95% recrystallization Average size of ASTM #5 (0.065 mm), grain larger than ASTM #4 (0.090 mm) Good surface quality
Fabrication (Taken from EURISOL DS Project Task 8: SC cavity development S. Bousson, et al.)
Fabrication and preparation Clean room and chemical facilities are needed Surface polishing and cleaning (electropolishing, chemical, high pressure water rinsing...) is of paramount importance for cavity performance.
Cavity testing Testing in vertial or horizontal cryostats
Cavity testing Main figure of characterization: Q slope
TEM cavities examples QWR and HWR are very succesful and used (or ready to) in several projects worldwide (TRIUMF, ANL ATLAS, SPIRAL-2,FRIB...) QWR linac for ISAC-II (TRIUMF) (Taken from G. Devanz 'Progress on the SRF developments for the high intensity projects in Europe', TTC 2012) LRF (Huelva) linac proposal
TEM cavities examples Spoke resonators are all prototypes, some ready to use (SSR for Project-X) (Project-X SSR-1 cavities, Fermilab) (TSR ready for test in Horizontal cryostat at IPN-Orsay)
TEM cavities More information on TEM cavities... Tesla Technology Collaboration Meetings (http://tesla-new.desy.de/meetings SRF conference proceedings (http://accelconf.web.cern.ch/accelconf) Any question... jlmunoz@essbilbao.org