Shrikant Pattalwar

The Advantages and Challenges of Helical Coils for Small Accelerators—A Case Study

Most of todays particle accelerators are used in industry or for medical applications, for example, in radioisotope production and cancer therapy. One important factor for these applications is the size of the accelerator, which ideally should be as small as possible. In this respect, fixed-field alternating-gradient accelerators (FFAGs) can be an attractive alternative, which combine the best features of conventional synchrotrons and cyclotrons: FFAGs deliver better performance than synchrotrons while retaining flexibility. Of particular interest are accelerators for protons of moderate energy (0.25-1 GeV) and light ions such as carbon (up to 400 MeV per nucleon), for example, for proton/carbon-ion charged particle therapy or potential future applications such as accelerator-driven subcritical reactors. Due to high magnetic rigidity, a compact machine can be only achieved by using high field superconducting magnets. A disadvantage of FFAGs is that the magnetic elements can be very challenging. Quite often, complicated multipole fields are required, in combination with stringent geometric constraints. In this paper, we demonstrate the advantages of helical coil technology by means of an accelerator for proton therapy.

Micromechanical components with novel properties

This paper describes the fabrication techniques and characterization of silicon dioxide micromechanical components with a layer of porous aluminum oxide which results in novel properties. An aqueous SOL process has been developed to obtain a layer of porous aluminum oxide on the silicon dioxide. The micro-porous surface, so realized, can be used as sensitive moisture and gas detectors. Various parts fabricated in silicon dioxide are cantilever, cross beam, spiral spring and resonator, coated with porous aluminum oxide, and micro-probes, coated with chromium-gold for electrical contacts. This paper also demonstrates the use of the trapezoidal pit etched in silicon during micromachining as a radiation concentrator. The results of derivation of the concentrator efficiency clearly shows the advantage of the reflections from the trapezoidal cavity.

Specific heat measurements in small samples at low temperature

We describe a calorimeter which has been used to measure the heat capacity of small samples weighing 50 to 200 mg in the temperature range of 1.5 K to 20 K. We have used the thermal relaxation method as well the heat pulse method for our measurements with the same experimental set-up. An accuracy of better than 5% is obtained with this system.

Heat capacity and AC susceptibility measurements of HoBa2Cu3O7−δ between 77 and 300 K

The specific heat Cp and AC susceptibility χ of the high temperature superconductor HoBa2Cu3O7−δ have been measured between 77 and 300 K, in order to evaluate the stability of the system. From the whole specific heat curve approximate values of the Debye temperature ϑD and the electronic specific heat constant γ of the material have been determined. The plot of Cp/T vs. T shows a jump near the superconducting transition temperature Tc. The maximum jump was observed in the freshly prepared sample. After some time, the transition width, seen in both measurements, broadens and the specific heat jump ΔCp/Tc decreases. The specific heat curve broadens towards higher temperatures while the AC susceptibility curve broadens towards lower temperatures. This observation can be qualitatively explained if we assume that, at least, two different types of superconducting microstructures develop with time.

Specific heat anomaly in the 90 K superconductor HoBa2Cu3O7−y

We report here the results of our heat capacityCp measurements on a monophasic material HoBa2Cu3O7−y. ΔCp/Tc, the jump inCp at the superconducting transition temperature (=91 K) of the material is measured to be 31 mJ/mol-K2.

Low temperature heat capacity measurements on HoBa2Cu3O7−δ

Specific heats were measured on two samples of HoBa2Cu3O7−δ in the temperature range 1·7 K to 10 K. In addition to the known Schottky behaviour, a peak in the specific heat curve was observed near 7·9 K in both the samples. This peak is probably due to impurity contribution and the specific heat measurements were undertaken in holmium oxide (Ho2O3), which was the suspected impurity. However, no peak was observed in the specific heat curve of holmium oxide

Design approach for the development of a cryomodule for compact crab cavities for Hi-Lumi LHC

A prototype Superconducting RF (SRF) cryomodule, comprising multiple compact crab cavities is foreseen to realise a local crab crossing scheme for the “Hi-Lumi LHC”, a project launched by CERN to increase the luminosity performance of LHC. A cryomodule with two cavities will be initially installed and tested on the SPS drive accelerator at CERN to evaluate performance with high-intensity proton beams. A series of boundary conditions influence the design of the cryomodule prototype, arising from; the complexity of the cavity design, the requirement for multiple RF couplers, the close proximity to the second LHC beam pipe and the tight space constraints in the SPS and LHC tunnels. As a result, the design of the helium vessel and the cryomodule has become extremely challenging. This paper assesses some of the critical cryogenic and engineering design requirements and describes an optimised cryomodule solution for the evaluation tests on SPS.

Intermediate quality control tests in the development of a superconducting RF cryomodule for CW operation

Through an international cryomodule collaboration, ASTeC at Daresbury Laboratory has taken the primary responsibility in leading the development of an optimised Superconducting RF (SRF) cryomodule, operating in CW mode for energy recovery facilities and other high duty cycle accelerators. For high beam current operation, Higher Order Mode (HOM) absorbers are critical components of the SRF Cryomodule, ensuring excessive heating of the accelerating structures and beam instabilities are effectively managed. This paper describes some of the cold tests conducted on the HOM absorbers and other critical components during the construction phase, to ensure that the quality and reliable cryomodule performance is maintained.

COOL-IT: A HEAT EXCHANGER SYSTEM TO PROVIDE GASEOUS HELIUM AT INTERMEDIATE TEMPERATURES FOR SRF LINAC

ALICE, a prototype accelerator developed at the Daresbury laboratory UK, has successfully demonstrated the energy‐recovery technique by circulating the electron beam to more than 20 MeV. At the heart of ALICE is a superconducting linac operating at 2 K. At high average‐current operation the performance of Superconducting RF (SRF) cavities suffer from instabilities due to the generation of higher‐order modes (HOM) as well as microphonics. HOMs are extracted out of the cavities using HOM absorbers operating at 80 K. This, however, increases the demand for cooling power at intermediate temperatures, i.e. at 80 K and 5 K, by more than an order of magnitude.In order to provide this extra cooling capacity with gaseous helium a new cryogenic system, ‘COOL‐IT,’ (System for cooling to intermediate temperatures) is being developed. It will provide two streams of helium gases at 80 K and 5 K. COOL‐IT uses a set of heat exchangers cooled by liquid helium and liquid nitrogen to generate two cold streams. It will be in...

Physical vapour deposition of NbTiN thin films for superconducting RF cavities.

The production of superconducting coatings for radio frequency (RF) cavities is a rapidly developing field that should ultimately lead to acceleration gradients greater than those obtained by bulk Nb RF cavities. The use of thin films made from superconductors with thermodynamic critical field, Hc > HC(Nb), allows the possibility of multilayer superconductor – insulator – superconductor (SIS) films and accelerators that could operate at temperatures above 2 K. SIS films theoretically allow increased acceleration gradient due to magnetic shielding of underlying superconducting layers [1] and higher operating temperature can reduce cost [2]. High impulse magnetron sputtering (HiPIMS) and pulsed DC magnetron sputtering processes were used to deposit NbTiN thin films onto Si(100) substrate. The films were characterised using scanning electron microscopy (SEM), x-ray diffraction (XRD), Rutherford back-scattering spectroscopy (RBS) and a four-point probe.