T. Junquera

Neutron Irradiation Tests of Calibrated Cryogenic Sensors at Low Temperatures

This paper presents the advancement of a program being carried in view of selecting the cryogenic temperature sensors to be used in the LHC accelerator. About 10,000 sensors will be installed around the 26.6 km LHC ring, and most of them will be exposed to high radiation doses during the accelerator lifetime. The following thermometric sensors : carbon resistors, thin films, and Pt, are exposed to high neutron fluences (> 1015 n/cm2) at the ISN (Grenoble, France) Cryogenic Irradiation Test Facility in which a cryostat is placed in a shielded irradiation vault where a 20 MeV deuteron beam hits a Be target resulting in a well collimated and intense neutron beam. The cryostat, the on-line acquisition system, the temperature references and the main characteristics of the irradiation facility are described. The main interest of this set-up is its ability to monitor on-line the evolution of the sensors by comparing its readout with temperature references that are in principle insensitive to the neutron radiation (i.e. Argon gas bulbs when working at about 84 K, and below 4.5 K, either helium gas bulbs or the saturation pressure of the superfluid liquid helium bath). The resistance shifts of the different sensors at liquid helium temperatures are presented.

Comparative Study of Mechanical Properties and Residual Stress Distributions of Copper Coatings Obtained by Different Thermal Spray Processes

Abstract In this work, the influence of the thermal spraying processes, atmospheric plasma spraying (APS), high velocity oxygen fuel (HVOF), and vacuum plasma spraying (VPS), on the microstructure and properties of copper coatings is discussed. The differences in microstructure, microhardness, and residual stress for each type of coating are shown. The X-ray diffraction (XRD) method is used to evaluate the mechanical anisotropical characteristics of the materials and the residual stress distribution. The particularity of this study is that the thickness of the coating is of the millimetre scale; the massive coating specimens without a substrate were obtained for XRD microelastic modulus measurements and for macrotensile measurement. Microhardness distributions have been obtained in coatings and in substrates that pass through the interface zones. It was observed that dense coatings could be obtained by industrial processes such as APS and HVOF, but they induce oxidation in the copper coating. The copper oxidation is limited in the VPS process. The elastic modulus values measured by the in situ XRD method show that the HVOF coating has more rigidity than the APS coating. HVOF and VPS copper coatings have higher microhardness values than the APS copper coating. The residual stress values are in traction in the APS, VPS, and HVOF coatings: the residual stress level in the HVOF and APS coatings is less important than that in the VPS coating. The microstructure observations show that the VPS copper coating has a recrystallised structure whereas the APS and HVOF copper coatings have a splat type structure. As a consequence, the VPS copper coating has a high Young’s modulus, an important mechanical resistance, and a high elongation as compared with the other copper coatings.

A NOVEL ROTATING TEMPERATURE AND RADIATION MAPPING SYSTEM IN SUPERFLUID He AND ITS SUCCESSFUL DIAGNOSTICS

An novel rotating temperature and radiation mapping system in He II has been developed to investigate field emission (FE) & thermal breakdown (TB) in TESLA 9-cell SRF cavities. More than 10,000 spots on a cavity surface can be analyzed in one turn with 5° stepping. 116 special surface scanning thermometers have been developed to measure surface temperature in He II. 32 photodiodes are employed to study the X-rays induced by FE electrons. Each rotating arm holds 14 thermometers and 4 photodiodes. A unique driving and suspension system is designed to gently turn the 9 arms around the cavity and uniformly press the thermometers against cavity surfaces. A moving adapter device (pancakes) is designed for rotating a large number of electronic cables which become inflexible in superfluid He.

Superconducting RF cavity stiffening with thick plasma sprayed copper coating

High mechanical stability is needed for the Superconducting RF (SRF) cavities which are proposed for future electron and proton accelerators. The cavity structure is subjected to Lorentz forces and external mechanical vibrations which induce large resonance frequency changes. For pulsed beam accelerators, the required field stability results in an additional RF power and critical low-level RF control system. In order to improve the mechanical stability of SRF cavities, a stiffening method is proposed: it consists with the coating of the cavity external surface with a thick layer of copper using a plasma spraying technique. As mechanical and thermal transfer properties are the main critical parameters of this layer, a comparative study of samples obtained with different thermal spraying techniques was performed in the last two years. A coating technique based on Inert Gas Plasma Spray (IPS) allows us to obtain copper layers with low porosity and reduced oxidation meeting the required thermal and mechanical performances. The IPS technique is presented together with the more relevant results obtained on samples and prototype cavities. From the industrial feasibility point of view, this method is fully realistic and provides interesting performances at moderate cost.

Cryogenic Thermometer Calibration Facility for the LHC

The “Institut de Physique Nucleaire” (IPN) and the European organization for Particle Research (CERN) collaborate on the design and the construction of a cryogenic thermometer calibration facility for the Large Hadron Collider (LHC). The thermometers are calibrated within the temperature range 1.6 K to 300 K, with an accuracy of ± 5 mK from 1.6 K to 4.2 K and about 1 % for higher temperatures. The calibration system has a capacity of 94 thermometers including 4 RhFe working temperature references. All thermometers are mounted on an isothermal copper block hung in a vacuum vessel and thermally insulated. A circulating helium heat exchanger cools down the copper block. The temperature is stabilized by means of a Proportional Integral regulator which controls a resistive heater mounted near the heat exchanger. For each step of temperature, calibration data are obtained by comparison of the thermometer resistance with the temperature deduced from the working references. The thermometers can be calibrated either in vacuum or immersed in saturated liquid helium. The particularities of this apparatus are its active cooling loop and controlled heating which enable to stabilize the temperature at previously fixed values with good dynamic characteristics. This paper presents the cryogenic thermometer calibration facility and the results of the first calibrations.

Thermal Stability Analysis of Superconducting RF Cavities

Future linear accelerators using superconducting RF cavities (TESLA proposal), requires accelerating gradients Eacc=25 MV/m to be achieved reliably at large scale. This high gradient level is mainly limited by electron emission and thermal instabilities (quench). Impressive improvements have been recently accomplished by pushing further the onset threshold of electron emision using careful surface cleaning techniques. On the other side micron size resistive defects embedded in the niobium walls of the cavity continue to induce thermal breakdowns for Eacc in the range 15 to 25 MV/m. In this paper the thermal stability of SRF cavities is analysed using analytical and numerical simulation models. The effects of the most relevant parameters (i.e. defect size, RF frequency, thermal conductivity, cooling conditions, etc.) having an incidence on the cavity quench are studied. The steady-state and the transient solutions are presented in two cases : the defect free surface, and the micron size defect on the cavity surface. Experimental observations of the thermal events occuring during the quench have been obtained with the help of two diagnostic systems: surface thermometers working in superfluid helium and RF measurements. All the proposed experimental and modelling methods could contribute to get a more complete insight on the thermal effects taking place in the cavity wall.

Surface scanning thermometers for diagnosing the TESLA SRF cavities

In order to investigate the field emission and the thermal breakdown of 9-cell TESLA SRF cavities, 150 specially developed surface scanning thermometers have been built. The description of the thermometers and their calibration in superfluid helium are presented. A special test chamber equipped with a heated niobium plate is used to study the thermometer thermal response versus the heater power at different bath temperatures. The comparison of thermometer response with numerical simulations results and experimental data obtained with reference thermometers mounted on the Nb plate using a thermal bonding agent, allows to get an estimation of the measurement efficiency of scanning thermometers. Experimental data obtained with cavities are analysed with the help of the calibration results and numerical simulations.

X-ray diffraction residual stress analyses on a copper coating realized by inert plasma spray

The copper coating has been proposed to stiffen the niobium superconducting cavity in order to reduce its deformations under Lorentz forces. The environment of thermal spray process is very important to get the wanted copper properties, as a consequence, the Inert Plasma Spray (IPS) process was used to realize the copper coating onto niobium. With the use,of X-ray diffraction method, the radiocrystallographic elastic constants (REC) of the IPS copper coating have been measured firstly, and then, in using the measured REC, the residual stress distribution has been determined secondly as a function of the depth in the copper coating and in the substrate.

Design and test of superconducting RF cavity prototypes for high intensity proton accelerators

High intensity proton beams, in the multi-MW range (typically 1 GeV and a few mA) are considered today for different applications: neutron sources, nuclear waste transmutation, radioactive ion beams and neutrino factories. All the foreseen projects are based on superconducting RF cavities for the high energy part of the linac accelerator between 100 MeV and 1 GeV. In this paper we present conceptual and experimental work made by the French group in the R&D preliminary phase. The aim of this study was to design an optimized cavity prototype integrating the more recent progress on RF superconductivity in terms of fabrication and preparation techniques. To reach high accelerating gradients while keeping safety margins and good reliability imposes careful cavity geometry optimization and detailed study of some important technological issues. The most relevant results obtained with several cavity prototypes (accelerating gradient, multipactor,...) are presented. Some other important components of the cavity (helium tank and cold tuner) are also discussed.