PSI contribution to the

ILC-EUROTeV collaboration

 

Work Package 6: Integrated Luminosity Performance Studies (ILPS)

Task: Bunch Compression Design (BCDES)

 

 

1     Task quick overview

 

1.1           Magnetic chicane.

1.2            

See as well:      ILC chicane design concepts

(http://www-project.slac.stanford.edu/ilc/acceldev/LET/BC/)

 

In a collider the bunch specification at the interaction point, such as transverse and longitudinal dimensions, are driven by the Luminosity requirement.

 

In order to focus the entire bunch at the collision point for maximum luminosity the rms bunch length has to be ≤ than the vertical beta function βy.

The luminosity needed for a 0.5-1 TeV Center of Mass collider requires a very strong focusing which in the present design studies corresponds to a βy of the order of only 400 μm. For this reason in the baseline design concept one assume today an rms pulse length of 300 μm, with a possible extension down to 150 μm.

 

In the present ILC design concepts, the linear collider injector complex (injector linac + damping ring) is generally providing a low emittance beam with a longitudinal rms extension σz of the order of 6 mm. In order to achieve the specified length the bunch must therefore be compressed by at least a factor of 20 before being injected in the main linac.

 

 

 

Figure 1: schematic view of the accelerator complex

 

As represented in figure 1, the needed compression can be carried out using a magnetic compression chicane. Two or more compression stages are more suitable for large compressor factors and probably the only alternative for bunch length below 300 μm (30 μm for CLIC).

 

Within the EUROTeV collaboration, the first task of PSI will be:

 

A conceptual design of a magnetic compression chicane suitable for multi-TeV colliders.

 

We will concentrate our efforts on the final compression stage sitting in front of the main linac, which is the more critical with respect to the Coherent Synchrotron Radiation (CSR) effects because of the shorter bunches and the highest energy of the beam. As first approach we will start from the existing ILC design concepts for a 500 GeV Center of Mass collider (Tesla and US-cold) and CLIC, assuming minor changes in the injector complex for a multi-TeV upgrade.

 

1.3           Path length tuning chicane

 

See as well:      D.Schulte, Proceedings of LINAC 2004, pp138,  Lübeck, Germany (2004)

 

            CLIC drive beam path length chicane layout

(http://clic-study.web.cern.ch/CLIC-Study/Report/34DriveFreq.html#342)

(from CLIC report: CERN 200-008)

                       

CLIC Layouts (http://clic-study.web.cern.ch/CLIC-Study/Layout/Overall.html)

 

In the two beams CLIC concept a drive beam is accelerated up to 2 GeV and then decelerated to produce the RF power needed to feed the main linac 30 GHZ accelerating structures. As shown on figure 2, in order to obtain the required time structure and energy, the bunch trains of the drive beam are first accelerated and then manipulated with a delay loop and in two combiner rings.

The RF voltage and phase jitter in the main linac accelerating structures depends directly on the drive beam stability (current and phase). The main process which can lead to a longitudinal jitter of the drive beam is connected to the small energy deviations induced by RF phase or amplitude jitters that are turned in longitudinal jitter by the bunch compressor. Temperature changes, ground motion and timing errors can as well contribute to this phenomenon.

The relative energy error after the main CLIC linac should not exceed 0.1, which correspond to a maximum phase error of 0.25o (at 30 GHz). To accommodate this tight requirements the jitter of the longitudinal drive beam must therefore be stabilized within 7 μm. For this reason a phase feedback system that should reduce all incoming phase errors of the drive beam as to be considered. The most efficient location for this system is immediately upstream of the decelerator units in the so called “turn around loop” where the bunch further compressed (figure 2).

Figure 2: Schematic view of the CLIC complex

 

Figure 3 shows the principle of the drive beam feedback system at the turn around loop. The correction range suited for such a system is ~100 μm, the compression factor ~10.

 

Figure 3: Schematic view of the drive beam phase feedback system.

 

Within the EUROTeV collaboration, the second task of PSI will be:

 

A conceptual design of a path length tuning chicane

 

The chicane will be used as well as bunch compressor in order to match the decelerating structures. The total requested compression corresponds to a reduction of the bunch length from 2mm down to 290-170 μm. In the early design reported in the CLIC study the correlated r.m.s. momentum-spread needed was approximately 1.5-1.2%, which correspond to and R56 ~0.15 m.

 

2       Beam parameter summary

 

2.1               Compression chicane

 

See as well:      Proposed ILC beam parameters (SLAC-ILC web server)

                        CLIC parameter Table (CERN CLIC web server)

                        CLIC report CERN 2000-008, 28 July 2000

                        ILC, Y. Kim proposal, 1st ILC workshop at KEK, 2004

 

Table 1: CLIC beam parameters (Multi-TeV collider example)

 

Table 1 gives an overview of the ambitious beam parameter at the chicane and at the interaction point for the CLIC project. Those values are continuously re-discussed, and should be used as baseline for the ultimate chicane design.

 

The chicane designs proposed for the ILC beam parameters (0.5 – 1 TeV Center of Mass) summarized in table 2 could be considered as the starting point towards the ultimate chicane design for a Multi-TeV collider. The optimum energies at which the compression should take place and the compression factors should be nevertheless adapted in order to minimize the strong collective effects due to the shorter bunch and the highest peak current.

Table 2: ILC proposed beam parameters

 

2.2   Tuning chicane

 

See as well: CLIC study (chapter 3)

 

Table 3: CLIC drive beam parameters

 

 

3     Chicane principle and difficulties – qualitative short overview.

 

See as well:

Andy Wolsky, Introduction to Bunch Compression

Working group B, ICFA Advanced accelerator and beam dynamics workshop, Sardinia, Chia Laguna, July 2002

 

 

The longitudinal size of a relativistic beam is not changed by the acceleration along the linac, and the collective effects with short bunches in the damping rings prevent us to reach the specified length already at the linac injection. A strong compression can nevertheless be achieved by a rotation of the longitudinal phase space in a magnetic chicane, which reduces the bunch length to the expense of the energy spread. The principle of the compression chicane is shown at figure 4. The incoming beam travels of crest trough an accelerating structure, which introduces a correlated energy spread along the bunch (less energy at the head, more energy at the tail). The beam is then bended using few dipole magnets and since the bending angle depends on the energy the head and the tail of the bunch experience a different path along the chicane.

 

Figure 4: Compression process in a magnetic chicane. (courtesy of Paul Emma).

 

The compression chicane principle is relatively simple, but it is necessary to find a compromise between the following aspects:

 

1)      The final energy dispersion ΔE/E introduced by the energy chirp needed for the compression process, can’t be too large in order to avoid a possible reduction in the luminosity (usually ΔE/E ≤1%).

2)    The bending angle at each dipole and the optimum beam energy must to be selected in order to prevent a too large emittance dilution due to Coherent Synchrotron Radiation (CSR) and Incoherent Synchrotron Radiation (ISR). Since the last compression stage take place at high energy (5-9 GeV), both effects have to be taken into account.

3)    A compromise between costs/size and performances has to be found.

4)    The chicane should accommodate some small fluctuations of the incoming beam (energy, alignment).

 

CSR and ISR effects are the main unwanted effects driving the chicane design. Figure 5 describe schematically the process. While bending the beam, each bunch radiate coherently at wavelength longer than the bunch length and incoherently at longer wave length. Since the bunch get compress the amount of radiation a short wavelengths increase along the chicane and overtake together with the ISR the bunch itself increasing the incoherent energy dispersion. Because of the different bending angle at the chicane dipole magnets, the additional incoherent energy spread causes a transversal size growth (emittance) of the beam on the bend plane. Moreover recent numerical observations show that small current modulations along the bunch can be strongly amplified by CSR. This phenomenon can be reduced by long bends, and introducing some additional un-coherent energy spread in front of the chicane.

 

Figure 5: Synchrotron radiation .in a bending magnet interacts with the bunch generating an uncorrelated energy spread which break the achromatic system (courtesy of Paul Emma).

 

 

4             Work plan - strategy

 

4.1           First order design

For the first design step we assume a two stage compression scheme and we concentrate on the second stage. Longitudinally we will use a perfect Gaussian distribution and a bi-Gaussian distribution for the transverse plane, the energy chirp will be linear. The CLIC beam parameters described here above, which are the most difficult to achieve, represent our goal for a multi TeV collider. While the ISR effects can be estimated analytically, the CSR contribution to the emittance dilution will be here evaluated using 1-D CSR numerical models.

 

In a similar way the path length chicane will be designed assuming the beam parameters described above, and optimized with respect to the CSR effects.

 

Few tracking codes include a CSR module capable to models the interaction between the radiation and the electron bunch. The simplest and faster models are one-dimensional and can be used as first approximation of the CSR effects before to go for more complicated and cpu consuming approaches. The program ELEGANT by M. Borland is a general multi purpose tracking code widely used for storage rings or linac optic designs, it include a one-dimensional model for CSR. One-dimensional computations can be made as well using the 1D solver option of the CSR-track code developed at DESY. As benchmark the simulation results made by those two codes will be compared.

 

4.2           Second order design

The chicane optimizations made here above will be investigated and re-optimized using a two dimensional and three-dimensional CSR models. Comparisons between CSR-Track and Trafic4 results are suitable. Sensitivity to micro-bunching instabilities should be investigated as well.

 

4.3           Third order design

A more realistic phase space for the incoming beam has to be used in order to evaluate the impact of non-linearity of the longitudinal phase space on the compression process. For this purpose we expect an input from the start-to-end simulations carried out within the EURO-TeV program.

 

4.4           Fourth order design

The chicane performances, the matching optic and the turn around lattice need to be characterized and optimized with respect to the possible dynamic and static imperfections (energy jitter, alignment errors).