Beam profile monitoring at the test beam line at the Compact Linear Collider test facility

The Compact Linear Collider (CLIC) is a study for a future linear electron-positron collider based on a two-beam acceleration scheme in which a high-intensity drive beam is decelerated in order to provide the power to accelerate the main beam for collision in the TeV range. The power extracted from the drive beam deteriorates the beam quality and increases the energy spread significantly. Monitoring of the beam properties is therefore challenging but essential. These challenges are being addressed experimentally at the CLIC test facility where up to 55% of the power is extracted from the beam in the test beam line, a small-scale version of the CLIC drive-beam decelerator, leaving the beam with a very wide energy profile. For monitoring of the transverse beam profile and Twiss parameters we use optical transition radiation screens and quadrupole scans. The intra-pulse-train energy spectrum before and after deceleration is measured with segmented beam dumps. In this paper we discuss the performance of these diagnostic devices with a particular emphasis on the large energy spread and its effect on the beam imaging techniques, and with a final outlook to the CLIC drive-beam diagnostics.

Figure 1

The layout of TBL showing the locations of the two diagnostic sectors including the TL2 and the TBL spectrometer lines. In its final configuration TBL will contain 16 PETS structures, of which only 4 are shown in the sketch.

Figure 2

A Placet simulation showing the beam energy distribution in TBL during the first 10 ns of a 28 A beam pulse train, initially at 150 MeV, decelerated in 16 PETS. The 3-ns long high-energy transient is followed by a 137 ns steady state with an unusually large energy spread; see Fig. 3.

Figure 3

Placet simulation of the TBL beam. Histogram of the energy distribution of a 28 A beam decelerated in 16 PETS. The large energy spread is clearly visible, as well as the asymmetric profile, resulting from the high-energy transient.

Figure 4

A schematic layout of a diagnostic sector.

Figure 5

Vertical cross section of the PSF irradiance at different wavelengths and for $E=150\mathrm{MeV}$.

Figure 6

Vertical cross section of the PSF irradiance (normalized intensity) at beam energies relevant to TBL and for $\lambda =500\mathrm{nm}$.

Figure 7

The energy distribution of a decelerated drive beam: the initial particle distribution compared to the intensity distribution of the collected OTR. The difference is visible only in the high-energy tail.

Figure 8

The beam size changes differently during a quadrupole scan depending on the energy spread in the beam.

Figure 9

FLUKA simulations: width of reconstructed distribution as a function of the width of the input beam.

Figure 10

FLUKA simulations of the detector performance on profile reconstruction. Expected beam distributions at the position of the segmented dump for 4 and 8 PETS, obtained with Placet, has been used as input. The width $\Delta x$ refers to the full width at half maximum.

Figure 11

Response of individual segments. The response is calculated by scanning the beam over each segment separately, then taking the maximum of the integrated profile. This plot is a refined analysis of data presented in Fig. 24 in Ref. 18.

Figure 12

Comparison of the segmented dump with the OTR screen. After applying corrections to the measurement, the segmented dump measurement gives an up to 4% overestimation compared to the OTR screen. For larger beams the discrepancy is negligible.

Figure 13

Beam energy spectrum before entering TBL (a) and after the deceleration in 13 PETS (b). The first bunch arrives at time $t=0$. Note that the spectrum to the left is acquired over many pulse trains while the spectrum to the right is a single-shot measurement.

Figure 14

The measured beam profile after deceleration in 13 PETS compared to the profile expected from Placet simulations.

Figure 15

The measured energy along the pulse train after deceleration in 4 PETS. The prediction from the extracted rf power and from the beam current measurements are shown for comparison. Incoming beam: $I_{\mathrm{beam}}=18\mathrm{A}$, $E=120\mathrm{MeV}$.