Kirk, M.

Ondreka, D.

Spiller, Peter

GSI Repository

GSITemplate2007 Status - SIS-18 Slow Extraction M. Kirk, D. Ondreka, and P. J. Spiller GSI, Darmstadt, GermanyWe report here recent investigations with the aim to re-duce the ripple in the spill coming out of SIS-18. Tracking studies were undertaken in which the RF Knock-Out modulation was switched from the existing BPSK based method to the dual FM method. Instead of the conventional BPSK carrying a pseudo-random bit train from a 16-bit Linear Feedback Shift Register, the dual FM consists of two anti-phased saw-tooth FM sig-nals [1]. In both cases the excitation spectrum is at the lowest frequency betatron sideband. Similar techniques have been evaluated by simulation for single FM as well as for band-limited white noise [2] however without the aim to assess the spill’s microstructure. Dual FM requires a larger peak voltage; however im-provement to the microstructure has been demonstrated. Figure 1 shows a few ms of spill at 10 μs resolution with the Hardt condition set. The Hardt condition [3] is par-ticularly important for high current operation. It remains to be seen if it would be simpler to increase the bandwidth and peak voltage of the BPSK modulation while observ-ing that the voltage is limited by the size of the deflection angle of the ions from the RF exciter which should be small enough to maintain the beam’s diffusion. The in-creased smoothness is also evident from figure 2 for the same spill at ms resolution for which the smoother curve results from dual FM. The BPSK’s peak voltage was ad-justed to closely match the curves for valid comparison. 0123456780 1 2 3 4 5 6 7Time [ms]Spill current normalized to mean BPSK, Spill 60-67 ms (Fig. 1), 290 V peakDual FM, Spill 60-67 ms (Fig. 1), 1400 V peak Figure 1: Simulated spills from a DC circulating beam. 00,10,20,30,40,50,60,70,80,910 0,05 0,1 0,15 0,2 0,25 0,3Time [s]Circulating beam intensity (normalized)BPSK, 290 V peakDual FM, 1400 V peak Figure 2: Simulated circulating beam intensity. Experimental confirmation of the above prediction re-quires the Hardt condition to be fulfilled through a lower-ing of the horizontal chromaticity from its natural value. Accurate determination of chromaticity is necessary as the chromaticities are known to differ a lot from their set values according to analysis of head tail oscillations [4]. The histogram in figure 3 demonstrates the marginal im-provement in spill quality by increasing the bandwidth of the BPSK excitation defined as the full width of the cen-ter lobe (i.e. twice the bit-train’s frequency) in the spectral power density, normalized here to the revolution fre-quency. The spill quality factor (abscissa) is the spill cur-rent normalized to its local moving average in a window 100 bins wide. Each bin is as usual, 10 μs in size. As be-fore, the circulating DC beam intensity curves were made to coincide by adjusting the peak voltage. 012345670 2 4 6 8 10 12 14 16Spill current normalized to meanCounts / 104Normalized bandwidth: 0.025Normalized bandwidth: 0.1 Figure 3: Spill quality measured for BPSK knock-out. Dual FM is foreseen as a temporary solution; Demon-strated in [1], a third mono-frequency may serve to ex-tract the ions close to the separatrix while the dual FM diffuses ions towards this region. Diode white noise is yet another alternative. It should be noted that power supply ripple to magnets, neglected in these simulations, may considerably add to the spill ripple making experimental confirmation more difficult but not impossible. References [1] K. Noda et al., “Advanced RF-KO slow-extraction method for the reduction of spill ripple”, NIM A 492, (2002) p. 253. [2] V. Nagaslaev et al., IPAC’11, “Third Integer Reso-nance Slow Extraction Using RFKO at High Space Charge”, San Sebastián, September 2011, p. 3559. [3] W. Hardt, “Ultraslow Extraction out of Lear”, CERN/PS/DL/LEAR, Note 81-6. [4] R. Singh, “Chromaticity Measurements using the Head-Tail modes at SIS-18”, GSI 2012-Report.  PHN-SIS18-ACC-24 GSI SCIENTIFIC REPORT 2012280 FAIR@GSI

Status - SIS-18 Slow Extraction

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Status - SIS-18 Slow Extraction

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