GRP-Cryostat for a Cryogenic Current Comparator

Large-volume glass fibre liquid-helium-cryostat with beam tube developed for a Cryogenic Current Comparator.

Cryogenic Current Comparators are used for beam diagnostics in accelerators and storage rings.

Purpose of a Cryogenic Current Comparator

Cryogenic Current Comparators (CCC) have been developed for electrical metrology to compare ratios of two electrical currents with the highest precision [1]. This is needed, for example, for high precision measurements of resistances, non-contact measuring of tiny currents or the amplification of small currents.

Intensities of beam currents in particle accelerators or storage rings need to be measurable. A major problem arises if the following requirements must be met:

• the beam has to be (energetically) unaffected
• very small beam currents have to be measured
• reduction of measurements to national standards must be achieved.

In accelerator research, the monitoring and non-destructive measurement of very small beam intensities is a major challenge. The beam currents to be measured are generated by charged particles such as ions, protons or antiprotons. The production of anti-particles is particularly complex and the yield is low.

A solution to this problem is the detection of the magnetic field generated by the moving charged particles using a non-destructive beam monitoring system based on the CCC-principle [2].

The CCC consists of a superconducting low-temperature SQUID (Superconducting Quantum Interference Device), a superconducting ring-shaped pickup coil and a highly effective meandering superconducting shield. This device enables the measurement of continuous (DC) and pulsed beam currents.

A current resolution of 6 - 65 pA Hz^-1/2 depending on the frequency range should be achieved, allowing measurement of ion beams with intensities down to 10⁷ particles per second with high accuracy.

To measure such tiny currents, the CCCs are using dc-SQUID recording techniques [3], which require cooling with liquid helium (4.2K), i.e. cryostats.

Up to now these cryostats for CCC were made from metallic material. To prevent disturbing currents due to electromagnetical noise they used an essential circuit breaker located in the 4.2K area. Supracon AG innovative design transfers the circuit breaker to an ambient temperature area of the cryostat. An obvious solution is a cryostat made from glass-fibre enforced epoxy. A new approach allows Supracon AG to manufacture the first cryostat for a CCC from epoxy enforced materials such as glass-fibre.

Specifications of the cryostat

Height: 900mm

Diameter: 488mm

Overall diameter: 540mm

Neck diameter: 120mm

Beam tube diameter: 50mm

Helium reservoir: 70l

After a standard refilling process, we proved the thermal performance data of the cryostat (including coil, SQUIDs and cables) to be:

Boil-off rate: about 5 l/d

Holding time: up to 14 d

Next steps

The GSI Helmholtz Centre for Heavy Ion Research in Darmstadt is now subjecting the CCC built in Jena to a series of tests to prepare the device for a future continuous running.

References

[1] I. K. Harvey, “A precise low temperature dc ratio transformer”, Rev. Sci. Instrum. Vol. 43, 1972.

[2] W. Vodel, R. Geithner, P. Seidel: SQUID-Based Cryogenic Current Comperator. in P. Seidel, Applied Superconductivity, Handbook on Devices and Applications, Vol. 2, Weinheim, Wiley-VCH, 2015, 1096-1110.

[3] P. Seidel, Applied Superconductivity, Handbook on Devices and Applications, Vol. 2, Weinheim, Wiley-VCH, 2015.