Cryogenic Current Comparator

for FAIR Transfer linesFebin KurianGSI Helmholtzzentrum für Schwerionenforschung GmbH, Darmstadt

and Helmholtz Institute Jena

Febin Kurian, GSI Darmstadt 1

ContentsIntroduction to Beam Current Measurements

Beam Intensity Measurements with Cryogenic Current

Comparator (CCC) at FAIR

CCC Working Principle, SQUID and Electronics

Superconducting Magnetic shield

Upgrade of GSI CCC and Beam Measurements

Status and Conclusion

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Beam Current Measurements

Ref. P. Forck, Lectures on Beam Instrumentation and Diagnostics

Standard Beam current measurement techniques

Current TransformersMeasurement of beam’s magnetic fieldNo dependence on beam energy Detection threshold ~1µA

Faraday cupsMeasurement of beam’s electric chargesDestructive techniquesFor low energy onlyDetection down to 10pA

Particle detectors (Scint., IC, SEM)Detection of particles energy loss in matterUsed for lower currents at higher energiesPlastic Scintillators, Ionization chamber, SEM etc...

160 nA16

Particles per second

Nuc

lear

Cha

rge

Z

Working ranges of the spill-intensity monitors used for the slow extraction GSI

(SCL: space charge limit)

Non destructive device is preferred,Beam is not influenced, allowing for online measurement of high intensity beams

SIS

FRS

ESR

SIS 100/300

HESR SuperFRS

NESR

CR

RESR

CBMHADES

FLAIR

CRYRING

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Cryogenic Current Comparator for FAIRForeseen installation locations:

Slow extraction from SIS18/100/300Super Fragment SeparatorIn front of beam dumpsCollector Ring (CR)In CryRing

For these beam lines,Min. beam intensity of 104 pps (with spill time 1 sec.)Max. intensity of 1012 pps => Beam current of 160 nA (protons) - 4.5µA (U28+)

CCC realizes non-destructive online monitoring of slowly extracted beams (very low (nA) mean beam current).

Along with high intensity beams,Slowly extracted beams also have to be transported to experiments up to long extraction times

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Working Principle - CCC (1/2)Principle• Measure the beam’s magnetic field

• Ceramic gap to prevent shielding by

mirror currents

• Magnetic alloy toroid – Flux concentrator

• Enhance the field coupling to the pick up coil

• Superconducting pick up coil

• Detects the magnetic field of the beam

• Superconducting Quantum Interference Device

(SQUID), a high sensitivity magnetic flux sensor

• Efficient magnetic shield to screen any magnetic

noise field components

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beam

DC SQUID

Temperature /Pressure Readout

GM refrigerator

Thermal insulation + Radiation shield

SQUID control + Readout

50cm

Oscilloscope & FFT

LHe Dewar

He gas exhaustAmplifier

Working Principle- CCC (2/2)

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SQUID - PrincipleSQUID – Superconducting QUantum Interference Device

Figure courtesy: hyperphysics webpage(http://hyperphysics.phy-astr.gsu.edu/hbase/solids/squid.html)

Flux quantizationEven in the presence of a steadily increased magnetic field, the magnetic flux associated with a superconducting ring is quantized by the elementary flux quantum

ɸ0 = h/2.e = 2.06x10-15 T.m2

Josephson JunctionTwo superconductors separated by a thin insulating layer allows tunneling of electrons proportional to the phase difference of the wave functions in the absence of a voltage

DC SQUIDTwo identical Josephson junctions form a

superconducting ring. For a magnetic field applied to the ring, the voltage across the junction oscillates with a period of one flux quantum.

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Current Measurement Using SQUID(2/2)FLL mode of operation

The output signal of the SQUID is linearized by flux locked loop mode of operation, fed to an amplification stage and read out in terms of equivalent current.

Typical schematic connection scheme of CCC

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Superconducting Magnetic shield

• Meissner effect ensures that all magnetic field components have to pass through the meander shaped plates before detected by the pick up coil

• Meander shape of the shield plates causes strong attenuation for all magnetic field components except the azimuthal magnetic field which has information about the beam.

• The larger is the effective meander path- the stronger is the field attenuation

Important component in defining the current sensitivity of the CCCNeeds to attenuate unavoidable magnetic noise components (e.g.: Earth’s magnetic field (~50µT), bipolar and quadrupole magnets in the beam line)

Caution... Nothing is drawn to scale here!!!

beam

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Field Attenuation by S.C. Magnetic Shield

B

Attenuation factor, A=20 log(Bout/Bin)

For an applied transverse magnetic field Attenuation factor AT =148dB

For a Longitudinal Magnetic field, Attenuation factor AL =176dB

To put this into perspective, Earth’s magnetic field of ~40µT will attenuate to 30 fT due to the magnetic shield geometry.

Known external magnetic field applied through“Helmholtz coil” and measure the field inside the shield by SQUID sensor

Given the inductance L and the area A of the ring core at low temperature, the Magnetic flux can be calculated from the SQUID as,

The magnetic field measured inside the shield geometry using the SQUID across externally applied known magnetic field

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Re-Commission of GSI CCCGoal : Use the existing CCC as a prototype for the new CCC systemGSI CCC consists of :

•DC SQUID (UJ111) and SQUID controller developed at University of Jena

•Superconducting magnetic shield made of Lead (10 meander plates)

•Ring core- Vitrovac 6025-F

• Current sensitivity of the SQUID- 175nA/ɸ0

• Noise limited current sensitivity at low frequency

range (<100Hz) – 70 pA/√Hz

For the production of multiple CCC system, several SQUID and electronics were investigated.

Next step: Install the newly selected sensor unit and measure beam current

10 nA signal measured by the existing CCC at GSI

Time (1 S/div)

Cur

rent

(10

nA

/div

)

0

10

20

30

40

-20

-10

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CCC Installation in GSI

CCC

The CCC is installed in the high energy transport section of the Synchrotron SIS18, HTP which is used as the beam diagnostic test bench.

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Goal : Measure the beam current with the newly installed SQUID sensor and ElectronicsNew SQUID (from Supracon™) and SQUID electronics (from Magnicon™) were introduced into CCC.

Noise spectrum of the CCC installed in the extraction line of SIS18 The SQUID output signal of A 50 nA test pulse

(2s) applied to the calibration winding of CCC

Measurement of Beam current using CCC (1/4)

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Measurement of Beam current using CCC (2/4)Plot of an 8 nA current signals produced by unbunched beam of 600 MeV particles of

Ni26+ extracted from the synchrotron, SIS18 with an extraction time of 500mS.

1. DC Current Transformer signal from the synchrotron shows the full cycle of the beam operation2&3. The differential output signals of the extracted current beam by CCC4. Current measured by a Secondary Electron Monitor

12&3

4

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Measurement of Beam current using CCC (3/4)

Full spill of a measured ion current extracted from SIS18 with an extraction time of 500 msThe average beam current is 8 nA

Zoomed in view of the spill structure of the extracted beam

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Measurement of Beam current using CCC (4/4)Plot of a 3.5 nA current signals which are produced by bunched beam of 109 particles of

Ni26+ at an energy of 600 MeV extracted from SIS18. The beam is extracted with an extraction time of 1 second.

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Status and Conclusions1. The CCC system at GSI has been recommissioned for prototype

development of an improved CCC unit for FAIR

2. CCC has been upgraded with novel SQUID unit and first beam tests at GSI were very successful.

3. Sensitivity is found to be much higher than the previous installations- could accurately measure beam current below 1 nA

4. Parallel measurement of the beam current using a Secondary Electron Emission Monitor matches perfect the CCC signal.

5. A lot more to explore from the beam time measurement results...

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SummaryWhat a Cryogenic Current Comparator offer

• Non intercepting technique

• High Resolution (<100pA /√Hz )

• Measurement of the absolute value of current

• Exact calibration by using an additional wire loop

• Measurement is independent of ion energy and the trajectories in the beam

• Useful investigations of the beam structures

• Useful calibration tool for other techniques

Our collaborations:

W. Vodel, R. Geithner, R. Neubert, HIJ, Jena & Friedrich-Schiller University, JenaR. v. Hahn, MPI-Kernphysik, HeidelbergA. Peters, HIT, Heidelberg

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AcknowledgementsAcknowledgements

Helmholtz Institute, Jena for the support in this project.

Part of this PhD work was supported by DITANET, a Marie Curie Initial Training Network and also Frankfurt Institute Advanced of Studies (FIAS)

Each and everyone in the LOBI group at GSI, all put their own share of contribution into this work in one way or the other...