Defects in YBCO relevant for RF superconductivity: T-, f- and H-dependencies

Power dependencies of the surface impedance of Y-Ba-Cu-O

“…Despite those similarities in the resonance curves the TEoll-Nb-cavities yield convex Rs(Hd) dependencies shown in Fig. 5 or [3], [7], whereas the TEol I-dielectric cavity yields typically concave Rs(H,f> dependencies. This difference has been obtained even for one identical film, shown in Fig.…”

“…(4) with Hc(0) L-1 T. As shown above and in [1], [3], [5], [7] nonlinearities occur at H* = 0.1 mT. This can be explained by the low Hell = H* fields typical for Josephson fluxon penetration [3] , [7] fitting to R,,(T,f) [ll]. Extripic no?linearities are either due to defects with lowered T, and H, (T) or due to new degrees of freedom in the superconductor being active at GHz.…”

“…Prad depends on the field distribution in the coupling region, especially on a relative phase 0, in addition to 9. The coupling Prad =IAG12 by magnetic dipoles is given by [6] IA") = w r 3 2 , 4 , ( f i G ( 0 ) ,~i ) / 6 (3) where the coupling area radius r has been considered to be small, so that H'(r ) = const holds., Eq. (3) shows that the magnetic coupling is strongest for H' parallel to the field HG yielding a coupling factor increasing with r6.…”

“…In the nonlinear case &H) is still defined by 9 = 7d2 or W, , , but QL(H) has to be used explicitly to fit Eq.(2). The nonlinearities are modeled [3], [5] by dXs = dRs =Hn (Eq. (4)) and by an r-value Qeing independent of geometry [2], [7] r = 6Xs/6Rs = (-T26f/fo)/dRs (5) In the time domain, a pulse of duration tp, the rise W(tct,)…”

“…the increase of the surface impedance Z = R + iX with rf field and therewith the generation of harmonics, limit the application of superconducting rf cavities, not only in HTS communication devices [ 11 but also in rf accelerators [2]. These nonlinearities may occur on slow time scale, such as heating TEomn modes are used to study YBCO films as end-plates in Nb cavities [3] and in A1203 dielectric resonators [4], [5]. With ideal boundary conditions Z = 0 at the metallic walls, the TEom,, field distributions are obtained analytically or by MAFIA yielding eigenfrequency f0=9.…”