The conceptional design of the proposed linear electron-positron collider TESLA is based on 9-cell 1.3 GHz superconducting niobium cavities with an accelerating gradient of E acc $ 25 MV͞m at a quality factor Q 0 $ 5 3 10 9 . The design goal for the cavities of the TESLA Test Facility (TTF) linac was set to the more moderate value of E acc $ 15 MV͞m. In a first series of 27 industrially produced TTF cavities the average gradient at Q 0 5 3 10 9 was measured to be 20.1 6 6.2 MV͞m, excluding a few cavities suffering from serious fabrication or material defects. In the second production of 24 TTF cavities, additional quality control measures were introduced, in particular, an eddy-current scan to eliminate niobium sheets with foreign material inclusions and stringent prescriptions for carrying out the electronbeam welds. The average gradient of these cavities at Q 0 5 3 10 9 amounts to 25.0 6 3.2 MV͞m with the exception of one cavity suffering from a weld defect. Hence only a moderate improvement in production and preparation techniques will be needed to meet the ambitious TESLA goal with an adequate safety margin. In this paper we present a detailed description of the design, fabrication, and preparation of the TESLA Test Facility cavities and their associated components and report on cavity performance in test cryostats and with electron beam in the TTF linac. The ongoing research and development towards higher gradients is briefly addressed.
By measuring the pressure dependence of the velocity of sound, we have determined both the pressure dependence of the density and the Gruneisen constant u of liquid He. Measurements were made below 0.1 K and in the vicinity of 0.5 K. Our determinations of the pressure dependence of the density agree quite well with that determined by Boghosian and Meyer, who used a capacitance bridge. Since the latter results rely on the validity of the Clausius-Mossotti relation and a pressure-independent electric polarizability, the present work can be interpreted as supporting both of these assumptions.We found that u(pp) -= (p/c)dc/dp = 2.84 under the vapor pressure at 0.1 K. Using this value of u to calculate the attenuation of sound according to a three-phonon mechanism, we obtain an attenuation of less than half the measured value. Thus, the present theory of sound attenuation must be incomplete.
Measurements of the attenuation of sound in liquid 4 He down to 0.1 K have been performed at 12, 30, 36, 60, 84, 90, 108, 132, 150, and 208 MHz. Measurements of the temperature dependence of the velocity of sound were made at 12, 36, 60, and 84 MHz. These data are compared with recent theoretical work, particularly that of Khalatnikov and Chernikova. The attenuation data agree well with theory in the vicinity of the peak in the attenuation near 1 K but do not agree elsewhere, the observed attenuation being greater than that predicted by theory. The temperature dependence of the velocity at low temperatures is found to be less than that predicted by theory, while the frequency dependence of the velocity (at finite temperature) is opposite to that predicted by theory.
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