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Register a new userAccelerating fields up to 49 MV/m in TESLA-shape superconducting RF niobium cavities via 75C vacuum bake
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In this paper we present the discovery of a new surface treatment applied to superconducting radio frequency (SRF) niobium cavities, leading to unprecedented accelerating fields of 49 MV/m in TESLA-shaped cavities, in continuous wave (CW); the corresponding peak magnetic fields are the highest ever measured in CW, about 210 mT. For TESLA-shape cavities the maximum quench field ever achieved was ~45 MV/m - reached very rarely- with most typical values being below 40 MV/m. These values are reached for niobium surfaces treated with electropolishing followed by the so called mild bake, a 120C vacuum bake (for 48 hours for fine grain and 24 hours for large grain surfaces). We discover that the addition during the mild bake of a step at 75C for few hours, before the 120C, increases systematically the quench fields up to unprecedented values of 49 MV/m. The significance of the result lays not only in the relative improvement, but in the proof that niobium surfaces can sustain and exceed CW radio frequency magnetic fields much larger than Hc1, pointing to an extrinsic nature of the current field limitations, and therefore to the potential to reach accelerating fields well beyond the current state of the art.
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We report the finding of new surface treatments that permit to manipulate the niobium resonator nitrogen content in the first few nanometers in a controlled way, and the resonator fundamental Mattis-Bardeen surface resistance and residual resistance accordingly. In particular, we find surface infusion conditions that systematically a) increase the quality factor of these 1.3 GHz superconducting radio frequency (SRF) bulk niobium resonators, up to very high gradients; b) increase the achievable accelerating gradient of the cavity compared to its own baseline with state-of-the-art surface processing. Cavities subject to the new surface process have larger than two times the state of the art Q at 2K for accelerating fields > 35 MV/m. Moreover, very high accelerating gradients ~ 45 MV/m are repeatedly reached, which correspond to peak magnetic surface fields of 190 mT, among the highest measured for bulk niobium cavities. These findings open the opportunity to tailor the surface impurity content distribution to maximize performance in Q and gradients, and have therefore very important implications on future performance and cost of SRF based accelerators. They also help deepen the understanding of the physics of the RF niobium cavity surface.
The Tera Electronvolt Superconducting Linear Accelerator TESLA is the only linear electron-positron collider project based on superconductor technology for particle acceleration. In the first stage with 500 GeV center-of-mass energy an accelerating field of 23.4 MV/m is needed in the superconducting niobium cavities which are operated at a temperature of 2 K and a quality factor Q0 of 10 to the 10. This performance has been reliably achieved in the cavities of the TESLA Test Facility (TTF) accelerator. The upgrade of TESLA to 800 GeV requires accelerating gradients of 35 MV/m. Using an improved cavity treatment by electrolytic polishing it has been possible to raise the gradient to 35 - 43 MV/m in single cell resonators. Here we report on the successful transfer of the electropolishing technique to multi-cell cavities. Presently four nine-cell cavities have achieved 35 MV/m, and a fifth cavity could be excited to 39 MV/m. In two high-power tests it could be verified that EP-cavities preserve their excellent performance after welding into the helium cryostat and assembly of the high-power coupler. One cavity has been operated for 1100 hours at the TESLA-800 gradient of 35 MV/m and 57 hours at 36 MV/m without loss in performance.
A systematic study is presented on the superconductivity (sc) parameters of the ultrapure niobium used for the fabrication of the nine-cell 1.3 GHz cavities for the linear collider project TESLA. Cylindrical Nb samples have been subjected to the same surface treatments that are applied to the TESLA cavities: buffered chemical polishing (BCP), electrolytic polishing (EP), low-temperature bakeout (LTB). The magnetization curves and the complex magnetic susceptibility have been measured over a wide range of temperatures and dc magnetic fields, and also for di erent frequencies of the applied ac magnetic field. The bulk superconductivity parameters such as the critical temperature Tc = 9.26 K and the upper critical field Bc2(0) = 410 mT are found to be in good agreement with previous data. Evidence for surface superconductivity at fields above Bc2 is found in all samples. The critical surface field exceeds the Ginzburg-Landau field Bc3 = 1.695Bc2 by about 10% in BCP-treated samples and increases even further if EP or LTB are applied. From the field dependence of the susceptibility and a power-law analysis of the complex ac conductivity and resistivity the existence of two different phases of surface superconductivity can be established which resemble the Meissner and Abrikosov phases in the bulk: (1) coherent surface superconductivity, allowing sc shielding currents flowing around the entire cylindrical sample, for external fields B in the range between Bc2 and Bcohc3, and (2) incoherent surface superconductivity with disconnected sc domains between Bcohc3 and Bc3. The coherent critical surface field separating the two phases is found to be Bcoh c3 = 0.81Bc3 for all samples. The exponents in the power law analysis are different for BCP and EP samples, pointing to different surface topologies.
Ambient magnetic field, if trapped in the penetration depth, leads to the residual resistance and therefore sets the limit for the achievable quality factors in superconducting niobium resonators for particle accelerators. Here we show that a complete expulsion of the magnetic flux can be performed and leads to: 1) record quality factors $Q > 2times10^{11}$ up to accelerating gradient of 22 MV/m; 2) $Qsim3times10^{10}$ at 2 K and 16 MV/m in up to 190 mG magnetic fields. This is achieved by large thermal gradients at the normal/superconducting phase front during the cooldown. Our findings open up a way to ultra-high quality factors at low temperatures and show an alternative to the sophisticated magnetic shielding implemented in modern superconducting accelerators.
We report the direct observations of sub-macropulse beam centroid oscillations correlated with higher order modes (HOMs) which were generated by off-axis electron beam steering in TESLA-type superconducting RF cavities. The experiments were performed at the Fermilab Accelerator Science and Technology (FAST) facility using its unique configuration of a photocathode rf gun injecting beam into two separated 9-cell cavities in series with corrector magnets and beam position monitors (BPMs) located before, between, and after them. Oscillations of ~100 kHz in the vertical plane and ~380 kHz in the horizontal plane with up to 600-{mu}m amplitudes were observed in a 3-MHz micropulse repetition rate beam with charges of 100, 300, 500, and 1000 pC/b. However, the effects were much reduced at 100 pC/b. The measurements were based on HOM detector circuitry targeting the first and second dipole passbands, rf BPM bunch-by-bunch array data, imaging cameras, and a framing camera. Calculations reproduced the oscillation frequencies of the phenomena in the vertical case. In principle, these fundamental results may be scaled to cryomodule configurations of major accelerator facilities.
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