Electron beam deceleration measurements using the decelerator test beam line in the Compact Linear Collider test facility

We discuss beam deceleration through a series of 12 power extraction and transfer structures, at the CLIC test facility 3 at CERN, as a proof-of-principle of the CLIC deceleration scheme. Up to 36% of the kinetic energy of an electron drive beam is extracted and converted to 12 GHz rf power. We look at the average and maximum energy loss of the particles, and compare them with simulations performed with the placet tracking code. The measured final energy is also compared to predictions based on the measured beam current and rf power in the structures. In the analysis we make use of the charge distribution form factor, taking into account the bunch length and the bunch phase. Finally, we look at the evolution of the transverse emittance with deceleration and compare the measured emittance with simulations.

Figure 1

Energies of simulated particles in the TBL from the placet tracking code. Nominal beam parameters were used in the simulation, and the plot shows the first 20 ns out of the 140 ns bunch train. At the start one can easily see the high-energy transient time that occurs due to the field buildup in the PETS.

Figure 2

The current TBL lattice. Quadrupole magnets are shown as blue lenses, dipole magnets as orange rectangles, corrector devices as orange triangles, BPMs as green circles, OTR screens as purple pentagons, and PETS as brown corrugated structures.

Figure 3

One of the installed PETS tanks, which keeps the structure in a vacuum. In the front, one can see a waveguide arm, where the rf power is coupled out, measured, and dissipated in a load.

Figure 4

Measured beam current and RF power for one BPM and one PETS. The beam current has been multiplied by a factor of 2. The shape of the produced power pulse is somewhat different from the beam current pulse, which can be explained by a change in the form factor over the pulse. A reconstruction from the BPM signal and the multibunch form factor based on Eqs. (1), (16), (18), and the signal in Fig. 5 is shown with red triangles.

Figure 5

12 GHz phase measured in a PETS for an uncombined beam, which itself has a 3 GHz bunch structure and a longer bunch train than the combined beam.

Figure 6

Measured energy spectrum along the pulse at the end of the TBL for a beam current of 13.5 A. The contour lines show 10% increments of the maximum value of the signal, and the energy changes slightly along the pulse. The star shows the point with the most decelerated particles, while the crosses show the mean energies.

Figure 7

The measured energy spectrum at the time instant with the most decelerated particles in Fig. 6. In addition, we show a simulated steady-state energy profile from placet for a form factor of $F\{\lambda (z)\}=0.95$.

Figure 8

Comparison between measured mean deceleration and deceleration predicted from measured beam current and PETS rf power. The curves with markers represent the average values over the data series, while the corresponding colored bands show the standard deviations. The predictions are only shown where the multibunch form factor could be calculated. Outside of this, we show the remaining signal based on the PETS rf power for reference, as a black stapled line.

Figure 9

Measured final energy at different beam currents and levels of deceleration. The intercept derived from a linear fit estimates what the final energy would be without deceleration.

Figure 10

Measured energy spectrum along the pulse at the end of the TBL, for 21.4 A beam current. The contour lines show 10% increments of the maximum value of the signal, and there is a step in the measured energy because of a change in the form factor. The star shows the point with the most decelerated particles, while the crosses show the mean energies.

Figure 11

Measured and simulated horizontal quadrupole scan at the end of the TBL. The measurement is an average of four quadrupole scans, and a parabola was fitted to the data. The curve for the simulations has a green band representing the uncertainty in the Twiss parameters and emittance measured at the beginning of the beamline.