Misalignment measurement of femtosecond electron bunches with THz repetition rate

Generation of a well-aligned bunch train is of vital importance for preserving the beam quality in a plasma wake-field accelerator. The simultaneous measurements of the relative misalignments in both planes, of the individual femtosecond electron bunches with THz repetition rate in a train, are reported and analyzed with simulations. The new method proposed in this paper demonstrates that the measurements can be done quickly by applying a single quadrupole scan together with a radio-frequency deflector (RFD), while the RFD strength and the RFD phase for each bunch do not need to be known.

Figure 1

Layout of the SPARC_LAB diagnostic section beam line. The beam is coming from the left side. The drawing is not to scale.

Figure 2

Typical fits for a single bunch in both planes without any jitter.

Figure 3

Comparisons between the input and simulated misalignments in both planes without any jitter. The bunch train head is on the left. The inputs ${\widehat{c}}_{y^{\prime }}$ are also shown in the plot since they affect the accuracies of ${\widehat{c}}_y$.

Figure 4

Comparison between the input and simulated displacements in the vertical plane without any jitter. The input displacements are ten times smaller than those in Fig. 3. Q1 is used for scanning.

Figure 5

Same as Fig. 4 but Q3 is used for scanning.

Figure 6

Sensitivities of the measurement error to the energy used in the transfer matrix calculation. Top: Q1 is used for scanning. Bottom: Q3 is used for scanning.

Figure 7

Errors of the simulated vertical displacements with 300 random RFD voltages (rms jitter 0.1%) and phases (rms jitter 0.1°), respectively. Q1 is used for scanning.

Figure 8

Fits in the vertical plane for the first (top) and the third (bottom) bunches in the train. Only the RFD voltage jitter is included. The data points without any jitter are also plotted together for comparison.

Figure 9

Same as Fig. 8 except that only the RFD phase jitter is included.

Figure 10

Residuals for the fits with only the RFD phase jitter. The residual is defined as the difference between the predicted value and the data.

Figure 11

Comparisons between the input and simulated misalignments in the vertical plane with both RFD phase and amplitude jitter included. Q1 is used for scanning.

Figure 12

Typical fits in the horizontal plane with an initial rms position jitter of $10\mu \mathrm{m}$ (top) and rms pointing jitter of $1\mu \mathrm{rad}$ (bottom).

Figure 13

Images of the bunch train on the screen (Flag U3) during the quadrupole scan from the No. 170652 experiment.

Figure 14

Fits for each bunch in the train in the horizontal and vertical planes for the No. 170652 experiment.

Figure 15

Residuals for the fits in the vertical plane shown in Fig. 14.

Figure 16

Measured misalignments of each bunch in the train for the No. 170652, No. 172023, and No. 172659 experiments. The bunch train head is on the left.