Geithner, R.

Huelsmann, P.

Kowina, P.

Kurian, F.

Neubert, R.

Reeg, H.

Schwickert, M.

Sieber, T.

Singh, R.

Stöhlker, T.

GSI Repository

Beam test with the Cryogenic Current Comparator* F. Kurian1#, 2, 3, R. Geithner3, P. Huelsmann2, P. Kowina1, R.Neubert4, H. Reeg1, M. Schwickert1, R. Singh1, T. Sieber1 and T. Stoehlker1, 3 1GSI, Darmstadt, Germany; 2Goethe University Frankfurt, Germany; 3 Helmholtz Institute, Jena, Germany; 4 Friedrich-Schiller-University, Jena, GermanyCryogenic Current Comparator (CCC) units are fore-seen to be installed at various locations of the FAIR fa-cility, for the online measurement of ion beam current down to nA range. In this contribution, successful test measurements of ion beam current using the upgraded CCC at GSI are reported. The CCC data show excellent match of the measured spill structure with the SEM measurements.  For the development of the improved version, the exist-ing CCC system at GSI was refurbished as a prototype for FAIR, to test new sensor components [1]. Based on the studies done on different sensor combinations by R. Geithner et al. [2], a new dc-SQUID and new control electronics were selected and installed in the refurbished system. After successful test measurements in lab envi-ronment, the CCC was installed in the beam diagnostic test bench HTP at the extraction line of the GSI synchro-tron. The signals measured by the CCC were amplified and read out at the equipment room located about 70 m from the beam line.  A built-in single-turn wire loop was used to calibrate the CCC output voltage to equivalent current. Response to known dc currents from a precision source (Keithley 261) in the range of 2 nA to 5 μA was measured. From this, a current sensitivity of 71.5 nA/ɸ0 was calculated, which is equivalent to 74.2 nA/V output to the oscilloscope.  Figure 1: Comparison of the beam current signal measured by (a) CCC, (b) SEM, and the difference between the two signals (c). See text for details.  Slowly extracted Ni26+ ion beams at 600MeV/u in the intensity range from 2.8 ൈ 10଼	particles per spill to 5 ൈ 10ଽ	particles per spill were used to test the CCC re-sponse to the beam signal. A set of beam signals with the extraction time in the range of 64 ms to 5 seconds at dif-ferent beam intensities was measured by the CCC. With an anti-aliasing filter (10 kHz cut-off frequency) at the output, the CCC measured the beam current down to 2 nA with a resolution of 1.2 nA rms. To compare the current measured by the CCC with a conventional technique, a Secondary Electron Monitor (SEM) - installed about 1 m downstream the CCC’s installation point - was used in parallel. The secondary electron current produced by the SEM was amplified by a trans-impedance amplifier (Femto DHPCA-100). Figure 1 shows a typical spill structure of an ion beam (about 4 ൈ 10ଽ particles) extracted over 64 ms measured by CCC (a) and SEM (b). The SEM signal was normal-ized to the CCC signal (normalized to total charge) for comparison. The difference between the spill structures measured by CCC and SEM as plotted in Figure 1 (c) shows excellent agreement. The spikes in the extracted beam, originating from the power converters of the mag-nets for the resonant extraction, are visible in the CCC measurement as shown in Figure 1. These ripples corre-spond to 50 Hz and its harmonics in the frequency spectra as expected [3].   Figure 2: Comparison of the current measurement using CCC and SEM within a bandwidth of 200 Hz.  In Figure 2 the current signals as measured with the CCC are plotted vs. SEM data in the range of 2 nA-105 nA. Whereas the relation is precisely linear (standard error: 0.7%) as expected, the 19% deviation of the meas-ured average current is subject of ongoing examinations. References [1] F. Kurian et al. “Measurements with the upgraded cryogenic current comparator”, IBIC 2013. [2] R. Geithner et al. A Cryogenic Current Comparator for FAIR with Improved Resolution, IBIC 2013. [3] M. Kirk et al. “SIS-18 rf knock-out optimisation studies”, IPAC2013, Shanghai, China.  ___________________________________________  * Work supported by Helmholtz Institute, Jena. #f.kurian@gsi.de GSI SCIENTIFIC REPORT 2014 FG-GENERAL-17DOI:10.15120/GR-2015-1-FG-GENERAL-17 435

Beam Test with the Cryogenic Current Comparator

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Beam Test with the Cryogenic Current Comparator

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