Conceptual design of a beam line for post-collision extraction and diagnostics at the multi-TeV Compact Linear Collider

Strong beam-beam effects at the interaction point of a high-energy $e^{+}{e}^{-}$ linear collider such as the Compact Linear Collider (CLIC) lead to an emittance growth for the outgoing beams, as well as to the production of beamstrahlung photons and $e^{+}{e}^{-}$ coherent pairs. In this paper, we present a conceptual design of a 150 m long post-collision extraction line for the CLIC machine at 3 TeV, which separates the various components of the outgoing beam using a vertical magnetic chicane, before transporting them to their respective dump. In addition, detailed studies are performed in order to compute the power losses along the CLIC post-collision line. For the vacuum window at the exit of the post-collision line, we propose a thick (1.5 cm) layer of carbon-carbon composite, with a thin (0.2 mm) aluminum leak-tight foil. The stress levels in this exit window are estimated. Finally, we discuss the use of diagnostics along the post-collision line for monitoring and improving the quality of the $e^{+}{e}^{-}$ collisions and, in turn, the luminosity of the CLIC machine.

Figure 1

Horizontal and vertical angular distributions of the CLIC disrupted beam, obtained with GUINEA-PIG using Gaussian incoming beams and normalized to one bunch.

Figure 2

Relative energy spectrum of the CLIC disrupted beam, obtained with GUINEA-PIG using Gaussian incoming beams and normalized to one bunch.

Figure 3

Horizontal and vertical angular distributions of the beamstrahlung photons, obtained with GUINEA-PIG using Gaussian incoming beams and normalized to one bunch.

Figure 4

Horizontal and vertical angular distributions of the particles of the $e^{+}{e}^{-}$ coherent pairs with the same charge as the CLIC disrupted beam (Cohplus) and with the opposite charge (Cohminus), obtained with GUINEA-PIG using Gaussian incoming beams and normalized to one bunch.

Figure 5

Relative energy spectrum of the particles of the $e^{+}{e}^{-}$ coherent pairs with the same charge as the disrupted beam (Cohplus) and with the opposite charge (Cohminus), obtained with GUINEA-PIG using Gaussian incoming beams and normalized to one bunch.

Figure 6

Relative energy spectrum of the electrons and positrons coming from the radiative Bhabha scattering (full line) and from the incoherent pair production (dashed line). This plot shows the amount of particles per bunch crossing.

Figure 7

Schematic layout of the CLIC post-collision line.

Figure 8

Cross section of a window-frame magnet, with the relevant parameters to be considered for its design.

Figure 9

Schematic layout of the first 67 m of the CLIC post-collision line, where the separation of the various components of the outgoing beam is performed.

Figure 10

Transverse beam profiles obtained in the separation region, 67 m downstream of the interaction point.

Figure 11

Reconstructing the energy spectrum of the coherent pairs from the vertical beam profile of the wrong-sign charged particles just after the separation region.

Figure 12

Schematic layout of the transport line between the separation region and the C-type magnets, which passes through the instrumented dump of the wrong-sign charged particles of the coherent pairs.

Figure 13

Cross section of a C-type magnet, with the relevant parameters to be considered for its design. The disrupted beam passes between the poles, while beamstrahlung photons travel between the upper coils.

Figure 14

Dispersion function along the first 100 m of the CLIC post-collision line, obtained with MAD-X [12].

Figure 15

$y$ and $y^{\prime }$ distributions as a function of the energy for the disrupted beam, after the post-collision chicane.

Figure 16

Schematic drawing of the vacuum chamber for the disrupted beam and the beamstrahlung photons (here 75 m after the interaction point): two half ellipses with the same horizontal semiaxis but different vertical semiaxis ($Y_{\mathrm{down}}$ and $Y_{\mathrm{up}}$) are separated by a distance equal to $D_y$.

Figure 17

Variations of the dimensions of the vacuum pipe for the disrupted beam and the beamstrahlung photons, from the separation region to the fourth C-type magnet exit.

Figure 18

Vertical profiles for the charged beam (full line), including the particles of the coherent pairs with the right-sign charge (dashed line), and for the beamstrahlung photons (dotted line), as obtained at the end of the post-collision line, 150 m downstream of the interaction point.

Figure 19

Vertical apertures seen by the disrupted beam and the same-sign charged particles of the coherent pairs.

Figure 20

Relative energy spread of the lost particles as a function of the position of loss in the CLIC post-collision line, obtained when tracking the charged beams. The wrong-sign charged particles absorbed in their dump are not shown.

Figure 21

Energy deposited by a 1.5 TeV undisrupted beam (in $\mathrm{MeV}/\mathrm{g}{\mathrm{cm}}^{-2}$) along the exit window: the last two points correspond to the thin aluminum foil.

Figure 22

Displacement and distribution of the mechanical stress intensity over the CLIC exit window.

Figure 23

Influence of a (common) transverse size difference between the incoming bunches on the luminosity (top) and on the amount of beamstrahlung photons and coherent pairs (bottom).

Figure 24

Vertical profiles for the charged beams and the beamstrahlung photons at the end of each post-collision line, when the incoming beam 1 has twice the transverse dimensions of the incoming beam 2 at the interaction point. The distributions shown here correspond to ${10}^5$ $e^{+}{e}^{-}$ collisions, and not one bunch crossing.

Figure 25

Influence of a vertical position beam-beam offset on the luminosity (top) and on the amount of beamstrahlung photons and coherent pairs (bottom).

Figure 26

Influence of a vertical position beam-beam offset on the position of the high-energy peak of the disrupted beam (left) and on the vertical size of the beamstrahlung photon cone (right).

Figure 27

Variation of the amount of beamstrahlung photons (top) and coherent pairs (bottom) when scanning one beam through the other at the interaction point, assuming that the incoming beam (1) has twice larger transverse dimensions than the incoming beam (2).

Figure 28

Cerenkov beamstrahlung detector based on the conversion of high-energy beamstrahlung photons into muon pairs, that are the only charged particles penetrating the beam dump and can be detected by their emission of Cerenkov light in a gas detector. Note that the high-energy muons generated by the primary beam are predominantly forward peaked and miss the Cerenkov detector.

Figure 29

Schematic layout of the interferometric thermometer proposed to measure temperature variations following energy deposition by the disrupted beam in the water dump.