Probing half ${\beta }_y^{*}$ optics in the Accelerator Test Facility 2

A nanometer beam size at the interaction point (IP) is required for future linear colliders to achieve the desired rate of particle collisions. KEK Accelerator Test Facility 2 (ATF2), a scaled down implementation of the linear collider beam delivery system, serves for demonstrating the feasibility of the final focus system (FFS). An unprecedented low vertical beam size at the IP of about 40 nm has been already measured in ATF2 using the optics with a nominal ${\beta }_y^{*}$. In our study we decrease the ${\beta }_y^{*}$ value in order to investigate the performance of more chromatic optics and to study the limits of beam focusing at the IP. Stronger beam focusing amplifies the aberrations from the final focus imperfections which cause an increase of the beam size at the IP. Simulations show that the multipolar errors and final doublet fringe fields spoil the IP beam sizes for ultralow ${\beta }_y^{*}$ optics but can be mitigated either by increasing the value of the horizontal ${\beta }^{*}$ or installing a pair of octupole magnets. We report on our first experimental steps towards the ultralow ${\beta }_y^{*}$ in ATF2. New methods for the beam diagnostics at the IP were developed in order to precisely set the desired optics. ${\beta }_y^{*}$ value was half the nominal value. The beam tuning was performed and the measured beam size is compared with the simulation results.

Figure 1

${\beta }_y$ function along the ATF2 beam line in the case of nominal ${\beta }_y^{*}$, half ${\beta }_y^{*}$ and ultralow ${\beta }_y^{*}$ optics.

Figure 2

Expected vertical beam size in ATF2 for three considered ${\beta }_y^{*}$ values and proposed mitigation methods.

Figure 3

Example of QD0FF scan for vertical beam parameters evaluation at the IP from the last week of February 2016 operation. Lower cut for the beam size measurement was set to $3\mu \mathrm{m}$. Only the ratio ${ϵ}_y/{\beta }_y^{*}$ can be resolved. The effects of dispersion and wire properties are subtracted. Change of the ${\beta }_y^{*}$ value for the maximum waist offset is less than 5%.

Figure 4

Shintake monitor schematic design. The electron beam interacts with a transverse interference pattern generated by two crossing laser beams. The number of scattered photons varies with the fringe size and the particle beam size [6].

Figure 5

The relation between ${\alpha }_y$ knob amplitude and beam waist offset derived from the simulations. A is the proportionality coefficient.

Figure 6

Measured modulation during the ${\alpha }_y$ scan to resolve the vertical emittance at the IP and the ${\beta }_y^{*}$ value using the Shintake monitor in 30 degree mode.

Figure 7

Comparison of measured vertical emittance at three locations using different methods versus beam intensity. Emittance at the IP was measured only for one beam intensity but this study is ongoing.

Figure 8

IP vertical beam size (black) and the orbit jitter (red) versus the beam tuning in half ${\beta }_y^{*}$, $10{\beta }_x^{*}$ optics. Points representing the beam size correspond to the optimum knob setting and error bars account for the uncertainty of finding the optimum knob setting by fitting the knob scan data (as in Fig. 6) and for the systematic error of the Shintake monitor as described in Sec. 2c.

Figure 9

Intensity dependence of IP vertical beam size for half ${\beta }_y^{*}$, $10{\beta }_x^{*}$ optics. Black points stand for the measured beam size, red curve for the fit according to Eq. (11) and blue band for the bunch intensity restriction of the tuning and final beam size measurement.

Figure 10

IP vertical beam size (black) and the orbit jitter (red) versus the beam tuning in half ${\beta }_y^{*}$, $25{\beta }_x^{*}$ optics. Points representing the beam size correspond to the optimum knob setting and error bars account for the uncertainty of finding the optimum knob setting by fitting the knob scan data (as in Fig. 6) and for the systematic error of Shintake monitor as described in Sec. 2c.

Figure 11

The IP beam sizes measured in ATF2 (red) and obtained with simulations without the orbit correction (black) for half ${\beta }_y^{*}$, $10{\beta }_x^{*}$ and half ${\beta }_y^{*}$, $25{\beta }_x^{*}$ optics.