Measurements and simulations of wakefields at the Accelerator Test Facility 2

Wakefields are an important factor in accelerator design, and are a real concern when preserving the low beam emittance in modern machines. Charge dependent beam size growth has been observed at the Accelerator Test Facility (ATF2), a test accelerator for future linear collider beam delivery systems. Part of the explanation of this beam size growth is wakefields. In this paper we present numerical calculations of the wakefields produced by several types of geometrical discontinuities in the beam line as well as tracking simulations to estimate the induced effects. We also discuss precision beam kick measurements performed with the ATF2 cavity beam position monitor system for a test wakefield source in a movable section of the vacuum chamber. Using an improved model independent method we measured a wakefield kick for this movable section of about $0.49\mathrm{V}/\mathrm{pC}/\mathrm{mm}$, which, compared to the calculated value from electromagnetic simulations of $0.41\mathrm{V}/\mathrm{pC}/\mathrm{mm}$, is within the systematic error.

Figure 1

Schematic diagram of the ATF2 beam line, taken from [7].

Figure 2

Twiss parameters for the nominal ATF2 lattice as a function of the longitudinal coordinate $s$.

Figure 3

gdfidl C-band reference cavity model (sliced at the symmetry plane).

Figure 4

The wake potential produced by a 7 mm long bunch traveling with different offsets in the vertical direction in the C-band reference cavity. The centered Gaussian shape shows the charge distribution used in calculations, where $z<0$ corresponds to the head of the bunch. The solid lines are calculated with gdfidl and the dashed lines with t3p. The dots indicate the position of the peak wake potential.

Figure 5

Weighted average wake potential of the C-band reference cavity as a function of beam offset fitted to a third order polynomial.

Figure 6

Simulated vertical orbit response for a 1 mm move of the movable section before quadrupole magnet QD10BFF (described in Sec. 3b).

Figure 7

Wakefield experimental installation with two C-band reference cavities, vacuum flanges and two bellows. The full 3D gdfidl model of the installation is also shown.

Figure 8

The transverse wake potential of the movable section and its components for a 1 mm translation and its individual components produced by a 7 mm long bunch, where $z<0$ corresponds to the head of the bunch.

Figure 9

Standard deviation of the residuals for all downstream BPMs.

Figure 10

The orbit change for an average bunch charge of $0.75\times 10^{10}$ particles with respect to the wakefield section position after pulse averaging and jitter subtraction of the CBPM readings in the vertical direction near quadrupole QD2BFF.

Figure 11

The vertical orbit response for each CBPM with respect to the movable section position for different bunch intensities after pulse averaging and jitter subtraction.

Figure 12

The normalized weighted average wake potential of the vertical orbit response with respect to the movable section position for different bunch intensities after pulse averaging and jitter subtraction. A linear fit is shown as well.

Figure 13

The vertical orbit response for each CBPM with respect to the movable section position for the movable section compared to the simulation for an average bunch charge of $0.75\times 10^{10}$ particles. The measurement and simulation are compared after the pulse averaging and jitter subtraction. A linear fit is shown as well.