Ring to Main Linac 7
5. Ring to Main Linac
Overview
The ILC Ring to Main Linac (RTML) is the collection of beamlines which transfer the beam from the damping ring to the main linac on each side of the collider. In addition to transporting the beam between these two regions, the RTML beamlines perform all the manipulations in the beam conditions which are required to match the parameters of the beam extracted from the damping ring to the parameters required of the beam injected into the main linac. The parameter matching includes:
· Completing the achromatic extraction of the beam from the damping ring
· Moving the beam from the axis of the damping ring to the axis of the main linac (geometry match)
· Collimation of beam halo generated in the damping ring
· Transformation of the beam polarization direction from the direction required in the damping ring (nominally vertical) to the direction required by experimenters at the IP
· compensation of the beam jitter introduced in the damping ring, during extraction from the damping ring, or during transport to the ends of the ILC site
· Compression of the bunch length from the equilibrium length of the damping ring to the shorter length required in the linac and at the IP.
In addition, the RTML must provide instrumentation and diagnostics sufficient to measure and control emittance growth, beam jitter amplification, spin dilution, and other beam quality reductions which would otherwise be introduced by the beamlines of the RTML; and the RTML must provide a set of dumps and stoppers which can stop the beam from entering downstream systems during beam tuning at upstream locations or during maintenance accesses at downstream locations. Last but not least, the RTML must be made as cost effective as possible within the constraints of achieving its technical goals.
Baseline
A baseline configuration for the RTML has been selected.
Description
Below is a description of the RTML, divided according to sub-beamlines in longitudinal (”S-position”) order. Figure 1 shows an approximate topology of the layout of RTML and its beamline composision.
Figure 1: Approximate topology of the RTML layout and its beamline composision.
For most beamlines, only a qualitative description can be presented at this time, as the detailed design has not yet begun. The exception is the bunch compressor, which is in a relatively more advanced state of development.
Extraction Geometry and Betatron Match
This beamline completes the extraction of the beam from the damping ring, including closing the dispersion introduced by the extraction kickers, septa, and bend magnets; transfers the beam from the damping ring elevation to the main linac elevation, if the two are different (for example, if the damping ring is in a shallow tunnel and the linac is deep) by means of achromatic bends; and matches the betatron functions from the damping ring extraction to those required by the downstream emittance measurement system. The beam which is extracted from the damping ring is initially travelling in the direction opposite to the nominal linac direction; its direction of travel is reversed by the turnaround.
Emittance and Trajectory Measurement
Skew Correction and Emittance Measurement
This beamline uses a set of four orthonormal skew quadrupole magnets to couple all four xy coupling terms in the beam matrix; this allows coupling introduced by the process of beam extraction from the damping ring to be corrected globally. The coupling correction section is followed by an emittance measurement station, which checks to make sure that the emittance correction is properly performed. The current baseline calls for a system which can measure only the projected horizontal and vertical emittances, and not the full beam matrix and extract the normal mode emittances and the coupling terms. The system for emittance measurement uses four laser wires driven by a common laser, with 45 degree phase advance between the wires in each plane. There is also a small chicane which separates the full-energy beam from the low-energy particles and photons produced by Compton scattering in the laser wire. The proposed system for emittance measurement at this location calls for use of either a profile monitor or a conventional (not laser) wirescanner, and uses quadrupole strength scans to perform the beam matrix reconstruction. The baseline contains both a wire scanner and an OTR screen at this location. The emittance measurement station must be capable of measuring emittances during multibunch operation, using many bunches to measure the emittance within 1 train, and must also be capable of measuring emittances during single-bunch operation, using many pulses (at 5 Hz) to complete one measurement. In the same beamline, a set of beam position monitors measures the bunch-by-bunch trajectory of the beam for feedforward correction. At the end of this beamline there is an insertable stopper pulsed extraction system and a full-power (220 kW) dump which can be used to stop the beam from passing into the collimation section and other downstream areas. This stopper can be used when the beam extracted from the damping ring is being tuned up, when downstream systems are unprepared to receive beam, or duing Machine Protection System interrupts which occur within a single bunch train. This stopper can probably only handle a fraction of the nominal beam power, and so can only be used for short trains, low repetition rates, or some combination of the two The exact specification for the beam-handling capability of the insertable stopper is pending further study..
Transverse Collimation
This beamline uses a spoiler/absorber scheme to collimate halo particles which are generated in the damping ring. Collimation is 2 phases x 2 planes x 1 iteration, and both planes are nominally collimated at the same depth. The spoiler positions and apertures are adjustable; the absorbers may also need to be adjustable. The beam is sufficiently enlarged at the spoiler locations to prevent damage to the spoilers in the event of a direct hit from the beam core by a small number of bunches (and assumingrequiring either that damping ring extraction is halted after that number of bunches); the spoilers, in turn, enlarge the beam core to prevent damage to the absorbers from a direct hit from several bunches of the beam.
Damping Ring Stretch
This is a simple coasting beamline, the purpose of which is to make up the distance between the damping ring extraction and the Escalator Beamline (see below); the damping rings are fixed to be at or near the center of the ILC site, while the Escalator’s longitudinal position is determined by the specific point at which the RTML beamline’s entry into the linac tunnel is desired. allow the position of the damping ring to be adjusted within the overall ILC footprint. The DR Stretch and/or the transverse collimation section also contain several BPMs which are used to measure the bunch-by-bunch position of each bunch within a train, to permit the trajectory to be corrected (in a feed-forward sense) by correctors in the trajectory correction section. The total length and composition of the DR Stretch is to be determined.
Escalator
This beamline consists of a vertical arc, a coasting FODO lattice at a shallow angle, and an additional vertical arc. The purpose of the Escalator is to transport the damped beam from the beamlines listed above, which are at the elevation of the DR housing, to the beamlines listed below, which share the main linac tunnel. Depending on the exact layout adopted for the ILC, there may also be some horizontal bending sections in the Escalator; however, all bend magnets will be either pure-vertical or pure-horizontal bends. The Escalator section also contains 3 stoppers with burn-through monitors which are part of the Personnel Protection System (PPS), and are used to ensure the safety of workers in the main linac when beam is present in the Skew Correction section.
Return Line
This beamline is a coasting FODO lattice which runs anti-parallel to the main linac in the same tunnel. Its purpose is to transport the beam from the escalator to the extreme upstream end of the ILC site, where it is turned around and transported forward again to the IP.
Transverse Collimation
This is an additional collimation system, which is needed to collimate beam halo particles generated in the Damping Ring Stretch, Escalator, and Return beamlines. Unlike the Transverse Collimation section near the DR extraction point, the Transverse collimation system which follows the Return line cannot be protected by the simple expedient of stopping damping ring extraction since it is too far away from the damping rings and therefore too many bunches are already in the RTML before extraction can be switched off. The second Tranverse Collimation system relies on the general ILC MPS, which uses pilot bunches and polling of correctors and other devices to ensure the safe passage of the beam through the ILC. The second Collimation section also contains the beam position monitors which are used for trajectory correction via feed-forward across the Turnaround.
Turnaround
This beamline reverses the direction of travel of the beam so that it is headed in the direction of the main linac. The purpose of the turnaround is to allow the bunch-by-bunch trajectory measurements to be fed forward over a shorter path length to the Trajectory Correction section. The exact configuration of the turnaround has not been selected and is likely at least partially site-dependent, but it is probably on the order of 170 meters in length and includes bending magnets, quadrupole magnets configured in a FODO lattice, and possibly sextupole magnets for matching and suppression of high-order dispersion. The turnaround will introduce emittance growth from synchrotron radiation: it is estimated that athe baseline “cat-bone” style turnaround (with a 90 degree bend followed by a 262 degree reverse bend) with 170 meter length can limits emittance growth to about 0.16 um, or about 92% of the emittance at extraction from the damping ring. The turnaround also contains spoilers and absorbers for collimation of off-energy particles generated in the damping ringReturn line or via multiple coulomb scattering from the collimators in the second collimation section.
Spin Rotator
This beamline uses 4 strong solenoid magnets to allow the beam polarization vector to be set to any orientation desired by the experimenters. The first half of the system contains two solenoids which are powered in series and separated by an Emma rotator (a beamline which performs a +I transformation in the horizontal plane and a -I in the vertical), to allow the polarization to be adjusted without introducing coupling from the solenoids. This is followed by an achromatic arc of approximately 8 degrees, which completes the turnaround of the beam trajectory; the arc is followed by another pair of solenoids separated by an Emma reflector. The combination of the two solenoid pairs and the bending system allows the polarization to be pointed in any direction required by the experimenters.
Trajectory Correction
This beamline is a simple FODO array with 2 horizontal intra-train dipole correctors separated by 90 degrees in betatron phase, and 2 vertical intra-train correctors with the same phase separation. Bunch-by-bunch trajectory information is measured in the upstream emittance measurementcollimation and/or DR stretch sections, and fed forward to this location to correct the beam jitter generated in the damping ring and during extraction (i.e., by jitter in the extraction kicker amplitude or driven by quad vibrations in the Return line).
Emittance Measurement and Coupling Correctionand Coupling Correction
This beamline uses a set of 64 laser wire profile monitors to make sure that the emittance correction is properly performed. The current baseline calls for a system which can measure only the projected vertical and horizontal emittances, but cannot measure the full beam matrix, and extract the normal mode emittances, or the and coupling terms. The emittance measurement station must be capable of measuring emittances during multibunch operation, using many bunches to measure the emittance within 1 train, and must also be capable of measuring emittances during single-bunch operation, using many pulses (at 5 Hz) to complete one measurement. Although it would be possible to include a second set of orthonormal skew quads at this location, and therefore to separately correct the coupling from the damping ring septum and from the spin rotator, studies have shown that the system as described, with a single skew correction section at the upstream end, performs comparably to two skew correction systems in the limit of small xy coupling.This section also contains a system with 4 skew quads, similar to the Skew Correction section described above, immediately upstream of the emittance measurement station. This is necessary due to the large number of betatron wavelengths between the initial Skew Correction Section and the Emittance station.
First Stage Bunch Compressor
The first stage bunch compressor is divided into the following subregions:
· An RF section which generates the necessary correlation between longitudinal position and energy. This section contains 32 24 9-cell RF cavities arranged in 4 3 cryomodules of 8 cavities each, based on the assumption that this is the cryomodule configuration which will be used for the ILC main linac. Because the bunch is long in this section, relatively strong focusing is used to limit the emittance growth from transverse wakefields: quad spacing is 1 quad per cryomodule, with 90 degree phase advance per cell in x and y. The cavities are phased near the zero-crossing (-100 degrees is typical), and require gradients of up to 18.4 MV/m. There are no spare modules in this section, but there is a spare klystron and modulator which can be connected to the cryomodules via an RF switch in the event that the BC1 klystron or modulator should fail.