Abstract
The powering of octupoles during third-integer resonant slow extraction has been studied and recently tested with the beam at the CERN Super Proton Synchrotron (SPS) in order to increase the extraction efficiency and reduce the induced radioactivity along the extraction straight. The octupoles distort the particle trajectories in phase space in such a way that the extracted separatrix is folded, which decreases the particle density impinging the wires of the extraction septum at the expense of increasing the extracted beam emittance. During experimental SPS machine studies a reduction of over 40% in the specific (per extracted proton) beam loss measured at the extraction septum was achieved. In this paper, the prerequisite studies needed to safely deploy the new extraction scheme in a limited time-frame are described, the experimental results are presented and an outlook given toward the next steps to bring slow extraction with octupoles into routine operation.
Highlights
Slow extraction using the third-integer resonance and thin electrostatic septa is a process with inherent beam loss, inducing radioactivation of the machine, reduced component lifetime, and severe limitations on personnel access and maintenance
In view of tightening restrictions on doseto-personnel for the necessary hands-on maintenance of accelerator equipment, and ever-increasing experimental requests for higher slow-extracted proton flux, the Super Proton Synchrotron (SPS) Losses and Activation Working Group [1,2] was established to investigate, implement, and follow-up various methods to reduce the induced radioactivity in the extraction straight located in Long Straight Section 2 (LSS2) and the SPS in general
The simulation results indicate that a loss reduction of up to approximately 40% is possible at the extraction septum
Summary
Slow extraction using the third-integer resonance and thin electrostatic septa is a process with inherent beam loss, inducing radioactivation of the machine, reduced component lifetime, and severe limitations on personnel access and maintenance. The application of higher-order multipole fields to manipulate the spatial density of the beam presented to the extraction septum is not a new concept and has been reported various times in literature [5,6,7] It is one of the main slow extraction beam loss reduction techniques pursued at CERN in recent years [8,9,10,11]. In the present operational scenario four extraction sextupoles (LSE) are used to drive the third-integer resonance and increase the amplitude of particles on outward spiraling separatrices, which closely resemble straight lines in phase space In this scenario, the spatial density of the beam at the septum drops off quadratically with amplitude, i.e., ∝ 1=x2. Spill length Momentum (p0) Relativistic factor (βrγr) Magnetic rigidity (Bρ) Momentum spread (Δp=p0) Horizontal emittance (rms, norm.) (εx;n) Horizontal emittance (rms, geom.) (εx;g) Vertical emittance (rms, norm.) (εy;n) Vertical emittance (rms, geom.) (εy;g) Horizontal tune (on resonance) (Qx) Vertical tune (Qy) Horizontal chromaticity (ξx 1⁄4 Q0x=Qx) Vertical chromaticity (ξy 1⁄4 Q0y=Qy)
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