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Interaction region layout of the VLEPP based photon linear collider with ultimate luminosity
Nuclear Instruments and Methods in Physics Research A 355 (1995) 154-156
NUCLEAR INSTRUMENTS &METHODS IN PHYSICS RESEARCH SectIonA
ELSEYIER
Interaction region layout of the VLEPP based photon linear collider with ultimate luminosity V.A. Alexandrov
*,
E.A. Kushnirenko,
A.A. Sery, N.A. Solyak
Branch of the Instirute ofNuclear Physics, 142284 Protvino, Moscow Region, Russian Federation
Abstract In the present paper, a possible interaction region layout of the VLEPP based Photon Linear Collider (PLC) with ultimate luminosity is discussed. In order to remove spent electron beams, the crab-crossing scheme is used. The detector is protected from produced pairs and secondary particles by means of the detector magnetic field and a shielding mask.
1. Introduction The PLC project considered in Ref. [S] is characterized by ultimate luminosity, which imposes certain requirements on the interaction region layout. Unlike in earlier PLC projects [l], where electron beams after conversion were, as a rule, deflected from the collision point by means of a magnetic field, in this PLC version the distance between the conversion and collision points is just on the order of the bunch length. That preserves an extremely small size of the photon bunch, making it impossible, however, to use an external field for deflection of electron beams after conversion from the collision point. In the considered PLC version, the parasitic ee and ey luminosities are proposed to be suppressed by means of strong beam-beam effects 151.This strong beam-beam disruption results also in large angles of spent beams and secondary particles. So, the interaction region configuration should be optimized from the point of view of background conditions.
2. Interaction region layout Fig. 1 represents the principal scheme of the PLC interaction region. To provide effective conversion of laser emission into hard y-quanta, a laser beam is focused at the electron beam. To achieve optimized focusing, the diffractive contribution to the focused laser beam size must be much smaller than its characteristic size: adifh
and
<<
&
or
F2 K a&,/A
I,<&,,
’ Corresponding author. Elsevier Science B.V. SSDI 0168-9002(94)01196-6
where A is the laser wavelength, F is the focus distance of the mirror, a, is the size of the laser beam on the focusing mirror, and I, and 1, are the longitudinal sizes of the laser and electron beams. To optimize the size of the photon beam on the last focusing mirror, a weakly defocusing mirror is applied to the FEL output. The distribution of spent beams should be taken into account to determine the structure of the interaction region. The largest deviation angles correspond, as a rule, to low-energy particles. In Ref. [S], the electron spectrum after conversion taking multiple Compton scattering into account has been analyzed. It is shown therein that the spectrum is sharply cut off at the energy of about 5% of the maximum. These low-energy particles will be strongly deviated by the field of an opposite bunch. Besides, the number of low-energy particles is also increased because of synchrotron radiation in the colliding beam field. Here, one should take into account that the distance, where the radiation effectively appears, is approximately equal to
2, = 2CTJfi. This is so because the particles, while passing through this distance, are shifted in the transverse direction by a distance approximately equal to the horizontal beam size and come afterwards into the declining field region, in which the radiation is already negligible. Taking into account this fact and using the formulas from Ref. [3], we obtain for our parameters an average number of radiated photons per electron of N, = 7 and a minimum electron energy after collision of E,, = 3%. According to these estimates, the maximum deviation angle of these particles by the beam field should be about 30 mrad. More accurate results were obtained by numerical simulation (see Fig. 2, which shows energy versus angle
VA. Alexandrov et al. /Nuci. instr. and Meih. in Phys. Res. A 355 (19951 154-156
Fig. 1. interaction
255
region layout of the Photon Linear Collider with ultimate luminosity.
distribution for electrons after collision), however, even rough estimates show that some specia1 measures should be used to take out the spent beams from the interaction region.
0
2 0.
Nevertheless, all particles of the spent primary beams can be removed beyond the detector if the beams will collide at an angle ( 13,)of 30 mrad. The luminosity fall due to incomplete overlapping of bunches should be compensated by “crab crossing”. For this purpose, before the collision, bunches should be turned in the horizontal plane by a special cavity. It is desirable that final lenses be of the minimum possible sizes. From this point of view, it seems quite reasonable to use a superconducting final quadrupole formed by four straight-line rods, which allows one to minimize the volume occupied by the lens. This lens, its fields, and its thermal conditions are described in Ref. [4] in detail. A usual iron lens of special design can be used for our purpose, too (see Fig. 3). Thus, an angle between colliding beams and lenses with small transverse dimensions can avoid collision of spent primary beams with the opposite collider elements.
6
0,.
rod
Fig. 2. Distributions of BY-E and f?,- f$ for electrons after collision.
Fig. 3. The beamline cross section near the face of the tinal quad.
VI. SYSTEM CONSIDERATIONS
156
VA. Alexandrou et al. /Nucl.
Instr. and Meth. in Phys. Res. A 355 (1995) 154-156
But numerous secondary e+e- pairs created in the collision may have much larger angles [6]. The longitudinal field of the detector will help to remove the low-energy portion of them. The special shielding mask can be used to prevent detection of the secondary particles created in collisions of particles with collider elements [2]. Half of the outward mask angle should be about 300 mrad at our parameters.
3. Conclusion In this work, the possible layout of the interaction region of the VLEPP-based Photon Linear Collider with ultimate luminosity has been considered. The beams are supposed to collide with some angle between them in order to simplify removing spent beams. Detector protection against pairs and secondary particles will be realized because of the magnetic field of the detector and the shielding masks.
Final superconducting lenses with small transverse dimensions could be used to decrease the number of particles that hit collider elements.
References V.I. Telnov, Proc. Physics and Experiments with Linear Colliders Workshop, Saariselka, Finland, September 1991, and references therein. Dl T. Tauchi, Final Focus and Interaction Region Workshop, SLAC, March 2-6, 1992. 131 K. Yokoya and P. Chen, Electron Energy Spectrum and Maximum Disruption Angle under Multiphoton Beamstrahlung, SLAC-PUB-4935, March 1989. [41 E.A. Kushnirenko, A.A. Mikhailichenko and A.A. Sery, Superconducting Final Focusing Quad for Linear Collider, Preprint INP 92-35, Novosibirsk (1992). [51 V.E. Balakin and A.A. Sery, these Proceedings (Workshop on Gamma-Gamma Colliders, Berkeley, CA, USA, 1994) Nucl. Instr. and Meth. A 355 (1995) 157. E. Kushnirenko, A. Likhoded and A. Sery, ibid., p. 111. ['31
[ll
NUCLEAR INSTRUMENTS &METHODS IN PHYSICS RESEARCH SectIonA
ELSEYIER
Interaction region layout of the VLEPP based photon linear collider with ultimate luminosity V.A. Alexandrov
*,
E.A. Kushnirenko,
A.A. Sery, N.A. Solyak
Branch of the Instirute ofNuclear Physics, 142284 Protvino, Moscow Region, Russian Federation
Abstract In the present paper, a possible interaction region layout of the VLEPP based Photon Linear Collider (PLC) with ultimate luminosity is discussed. In order to remove spent electron beams, the crab-crossing scheme is used. The detector is protected from produced pairs and secondary particles by means of the detector magnetic field and a shielding mask.
1. Introduction The PLC project considered in Ref. [S] is characterized by ultimate luminosity, which imposes certain requirements on the interaction region layout. Unlike in earlier PLC projects [l], where electron beams after conversion were, as a rule, deflected from the collision point by means of a magnetic field, in this PLC version the distance between the conversion and collision points is just on the order of the bunch length. That preserves an extremely small size of the photon bunch, making it impossible, however, to use an external field for deflection of electron beams after conversion from the collision point. In the considered PLC version, the parasitic ee and ey luminosities are proposed to be suppressed by means of strong beam-beam effects 151.This strong beam-beam disruption results also in large angles of spent beams and secondary particles. So, the interaction region configuration should be optimized from the point of view of background conditions.
2. Interaction region layout Fig. 1 represents the principal scheme of the PLC interaction region. To provide effective conversion of laser emission into hard y-quanta, a laser beam is focused at the electron beam. To achieve optimized focusing, the diffractive contribution to the focused laser beam size must be much smaller than its characteristic size: adifh
and
<<
&
or
F2 K a&,/A
I,<&,,
’ Corresponding author. Elsevier Science B.V. SSDI 0168-9002(94)01196-6
where A is the laser wavelength, F is the focus distance of the mirror, a, is the size of the laser beam on the focusing mirror, and I, and 1, are the longitudinal sizes of the laser and electron beams. To optimize the size of the photon beam on the last focusing mirror, a weakly defocusing mirror is applied to the FEL output. The distribution of spent beams should be taken into account to determine the structure of the interaction region. The largest deviation angles correspond, as a rule, to low-energy particles. In Ref. [S], the electron spectrum after conversion taking multiple Compton scattering into account has been analyzed. It is shown therein that the spectrum is sharply cut off at the energy of about 5% of the maximum. These low-energy particles will be strongly deviated by the field of an opposite bunch. Besides, the number of low-energy particles is also increased because of synchrotron radiation in the colliding beam field. Here, one should take into account that the distance, where the radiation effectively appears, is approximately equal to
2, = 2CTJfi. This is so because the particles, while passing through this distance, are shifted in the transverse direction by a distance approximately equal to the horizontal beam size and come afterwards into the declining field region, in which the radiation is already negligible. Taking into account this fact and using the formulas from Ref. [3], we obtain for our parameters an average number of radiated photons per electron of N, = 7 and a minimum electron energy after collision of E,, = 3%. According to these estimates, the maximum deviation angle of these particles by the beam field should be about 30 mrad. More accurate results were obtained by numerical simulation (see Fig. 2, which shows energy versus angle
VA. Alexandrov et al. /Nuci. instr. and Meih. in Phys. Res. A 355 (19951 154-156
Fig. 1. interaction
255
region layout of the Photon Linear Collider with ultimate luminosity.
distribution for electrons after collision), however, even rough estimates show that some specia1 measures should be used to take out the spent beams from the interaction region.
0
2 0.
Nevertheless, all particles of the spent primary beams can be removed beyond the detector if the beams will collide at an angle ( 13,)of 30 mrad. The luminosity fall due to incomplete overlapping of bunches should be compensated by “crab crossing”. For this purpose, before the collision, bunches should be turned in the horizontal plane by a special cavity. It is desirable that final lenses be of the minimum possible sizes. From this point of view, it seems quite reasonable to use a superconducting final quadrupole formed by four straight-line rods, which allows one to minimize the volume occupied by the lens. This lens, its fields, and its thermal conditions are described in Ref. [4] in detail. A usual iron lens of special design can be used for our purpose, too (see Fig. 3). Thus, an angle between colliding beams and lenses with small transverse dimensions can avoid collision of spent primary beams with the opposite collider elements.
6
0,.
rod
Fig. 2. Distributions of BY-E and f?,- f$ for electrons after collision.
Fig. 3. The beamline cross section near the face of the tinal quad.
VI. SYSTEM CONSIDERATIONS
156
VA. Alexandrou et al. /Nucl.
Instr. and Meth. in Phys. Res. A 355 (1995) 154-156
But numerous secondary e+e- pairs created in the collision may have much larger angles [6]. The longitudinal field of the detector will help to remove the low-energy portion of them. The special shielding mask can be used to prevent detection of the secondary particles created in collisions of particles with collider elements [2]. Half of the outward mask angle should be about 300 mrad at our parameters.
3. Conclusion In this work, the possible layout of the interaction region of the VLEPP-based Photon Linear Collider with ultimate luminosity has been considered. The beams are supposed to collide with some angle between them in order to simplify removing spent beams. Detector protection against pairs and secondary particles will be realized because of the magnetic field of the detector and the shielding masks.
Final superconducting lenses with small transverse dimensions could be used to decrease the number of particles that hit collider elements.
References V.I. Telnov, Proc. Physics and Experiments with Linear Colliders Workshop, Saariselka, Finland, September 1991, and references therein. Dl T. Tauchi, Final Focus and Interaction Region Workshop, SLAC, March 2-6, 1992. 131 K. Yokoya and P. Chen, Electron Energy Spectrum and Maximum Disruption Angle under Multiphoton Beamstrahlung, SLAC-PUB-4935, March 1989. [41 E.A. Kushnirenko, A.A. Mikhailichenko and A.A. Sery, Superconducting Final Focusing Quad for Linear Collider, Preprint INP 92-35, Novosibirsk (1992). [51 V.E. Balakin and A.A. Sery, these Proceedings (Workshop on Gamma-Gamma Colliders, Berkeley, CA, USA, 1994) Nucl. Instr. and Meth. A 355 (1995) 157. E. Kushnirenko, A. Likhoded and A. Sery, ibid., p. 111. ['31
[ll