Crystal slow extraction of positrons from the Frascati DA$\mathrm{\Phi }$NE collider

The slow high-efficiency extraction from a ring positron accelerator (SHERPA) project’s aim is to develop an efficient technique to extract a positron beam from one of the accelerator rings composing the $\mathrm{DA}\mathrm{\Phi }\mathrm{NE}$ complex at the Frascati National Laboratory of INFN, setting up a new beam line able to deliver positron spills of O(ms) length, excellent beam energy spread and emittance. The most common approach to slowly extract from a ring is to increase betatron oscillations approaching the third order tune resonance to gradually eject particles from the circulating beam. SHERPA proposes a paradigm change for lepton machines using coherent processes in bent crystals to kick out positrons from the ring, a cheaper and less complex alternative. A description of this innovative nonresonant extraction technique is reported in this manuscript, including its performance preliminary estimation.

Figure 1

$\mathrm{DA}\mathrm{\Phi }\mathrm{NE}$ complex schematic layout: the beam from the LINAC (bottom left) can be driven either to the BTF-1 and BTF-2 lines (middle left) or to a damping ring (top left) and from there injected counterclockwise into the electron main ring or clockwise into the positron one (in red).

Figure 2

Accumulator ring, location of the bent crystal and extraction.

Figure 3

One possible configuration for the optics Twiss functions of the $\mathrm{DA}\mathrm{\Phi }\mathrm{NE}\mathrm{DR}$ to allow crystal slow extraction.

Figure 4

H phase space plot at the extraction septum of particles injected with a +10 mm offset from beam pipe center and tracked for 18 turns. The extraction septum is represented by a red region.

Figure 5

H phase space plot at the extraction septum of particles injected with a +10 mm offset from beam pipe center and tracked for 1 turn. Top: particles do not receive a kick by the crystal. Bottom: particles receive a 1 mrad kick by the crystal. Particles close to the septum are moved out of it, in the extraction channel.

Figure 6

H phase space of particles achieving $-20\mathrm{mm}$ of displacement, or more, at the extraction point plotted at the crystal. Top: turn 1. Bottom: turn 500. Particles migrate to negative horizontal offsets due to dispersion and energy loss up to go through the crystal (represented by the dashed red line in the plot).

Figure 7

Rate of particles kicked by the crystal achieving $-20\mathrm{mm}$ of displacement, or more, at the extraction point. The number of particles is shown on the vertical axis, while the number of turns after the first interaction with the crystal is shown on the horizontal axis. The initial tracked population is ${10}^3$ particles and each turn corresponds to $0.1\mu \mathrm{s}$. Each bin corresponds to 100 turns.

Figure 8

Nonlocal extraction scheme locations using the $\mathrm{DA}\mathrm{\Phi }\mathrm{NE}$ positron main ring; particles are injected in the long external arc (PL) and circulate clockwise: in option (1) the crystal and the septum are placed upstream and downstream the main experiment interaction point, in case (2) they are placed around the ring crossing region on the opposite side.

Figure 9

Turn by turn single particle tracking results seen at the crystal location (top) and the extraction septum (bottom). The particle trajectory is initially set to zero position and angle and at turn zero it loses 0.7% of its energy. At turn 6 it interacts with the crystal and it reaches 20 mm of displacement at the septum. If not extracted, the particle continues to oscillate with a large amplitude.