Abstract

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 Eacc >= 25 MV/m at a quality factor Q0 > 5E+9. The design goal for the cavities of the TESLA Test Facility (TTF) linac was set to the more moderate value of Eacc >= 15 MV/m. In a first series of 27 industrially produced TTF cavities the average gradient at Q0 = 5E+9 was measured to be 20.1 +- 6.2 MV/m, 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 electron-beam welds. The average gradient of these cavities at Q0 = 5E+9 amounts to 25.0 +- 3.2 MV/m 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 R&D towards higher gradients is briefly addressed.

Highlights

  • In the past 30 years, electron-positron collisions have played a central role in the discovery and detailed investigation of new elementary particles and their interactions

  • 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

  • Important questions still remain to be answered, in particular, the origin of the masses of field quanta and particles— within the standard model explained in terms of the so-called Higgs mechanism — and the existence or nonexistence of supersymmetric particles which appear to be a necessary ingredient of any quantum field theory attempting to unify all four forces known in nature: the gravitational, weak, electromagnetic, and strong forces

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Summary

INTRODUCTION

In the past 30 years, electron-positron collisions have played a central role in the discovery and detailed investigation of new elementary particles and their interactions. While the CEBAF cavity fabrication methods were adopted for TTF without major modifications, important new steps were introduced in the cavity preparation: (i) chemical removal of a thicker surface layer, (ii) a 1400 ±C annealing with titanium getter to improve the Nb heat conductivity and to homogenize the material, (iii) rinsing with ultrapure water at high pressure (100 bar) to remove surface contaminants, and (iv) destruction of field emitters by a technique called high power processing The application of these techniques, combined with extremely careful handling of the cavities in a clean room environment, has led to a significant increase in accelerating field. The low frequency of 1.3 GHz permits the acceleration of long trains of particle bunches with very low emittance making a superconducting linac an ideal driver of a free electron laser (FEL) in the vacuum ultraviolet and x-ray regimes For this reason, the TTF linac has recently been equipped with undulator magnets, and its energy will be upgraded to 1 GeV in the coming years to provide an FEL user facility in the nanometer wavelength range. VII, where the ongoing research towards higher gradients is briefly addressed

Basic principles of rf superconductivity and choice of superconductor
Surface resistance
Heat conduction in niobium
Influence of magnetic fields
Advantages and limitations of superconducting cavities
Overview
Choice of frequency
Cavity geometry
1.97 V pC21 m21
Lorentz-force detuning and cavity stiffening
Magnetic shielding
Helium vessel and tuning system
Design requirements
Input coupler A
Electrical properties
K thermal intercept
Higher-order modes
Niobium properties
Deep drawing and electron-beam welding
Cavity treatment
RESULTS
Results from the first series of TTF cavities
C18 C21 C28
Electron microscopy
X-ray fluorescence
Neutron activation analysis
Improvements in cavity production
Test results in vertical cryostat
Tests with main power coupler in horizontal cryostat
Cavity improvement by heat treatment
General demands on the rf control system
Sources of field perturbations
GHz LO
Operational experience
CAVITIES OF HIGHER GRADIENTS
Quality improvement of niobium
Improvement in cavity fabrication and preparation
Mechanical stability of the cavities
Seamless cavities
Niobium sputtered cavities
The superstructure concept
Full Text
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