Production of long bunch trains with $4.5\mu \mathrm{C}$ total charge using a photoinjector

A photoinjector, PHIN (PHotoINjector), has been realized at CERN by a joint effort of several institutes within the European Coordinated Accelerator Research in Europe program. The test facility has been installed and commissioned at CERN with the aim to demonstrate the beam parameters needed for the CLIC Test Facility 3 (CTF3). This beam is unique with respect to its long bunch train and high average charge per bunch requirements. The nominal beam for CTF3 consists of 1908 bunches each having a 2.33 nC charge and a bunch frequency of 1.5 GHz. Thus, a total charge of $\sim 4.4\mu \mathrm{C}$ has to be extracted and accelerated. The stability of the intensity and the beam parameters along this exceptionally high average current train is crucial for the correct functioning of the CLIC drive beam scheme. Consequently, extensive time-resolved measurements of the transverse and longitudinal beam parameters have been developed, optimized, and performed. The shot-to-shot intensity stability has been studied in detail for the electron and the laser beams, simultaneously. The PHIN photoinjector has been commissioned between 2008 and 2010 during intermittent operations. This paper reports on the obtained results in order to demonstrate the feasibility and the stability of the required beam parameters.

Figure 1

The layout of the PHIN photoinjector.

Figure 2

The schematic layout of the laser system for PHIN.

Figure 3

The PHIN rf gun.

Figure 4

(a) Photoinjector charge yield per bunch with the increasing laser pulse energy for different laser spot sizes and train lengths denoted by ${\sigma }_{L,x}$ and ${\sigma }_L$, respectively. (b) The linear behavior of the photoinjector in terms of the integrated charge and the laser energy over the pulse train.

Figure 5

Beam loading compensation. (a) rf power in the gun (1a) and reflected power (2a) when no beam is present. (b) rf power in the gun (1b) and reflected power (2b) when the beam is present.

Figure 6

Beam profile after the multislit mask, with Gaussian fit curves for individual beamlets.

Figure 7

Emittance measurement with respect to the laser spot size: (blue) ${\sigma }_{\mathrm{laser}}=4\mathrm{mm}$, $Q=1.28\mathrm{nC}$, $E=5.5\mathrm{MeV}$, $\mathrm{\text{train length}}=200\mathrm{ns}$; (green) ${\sigma }_{\mathrm{laser}}=2\mathrm{mm}$, $Q=1.1\mathrm{nC}$, $E=5.7\mathrm{MeV}$, $\mathrm{\text{train length}}=200\mathrm{ns}$.

Figure 8

The comparison of the statistical and the systematic errors on the multislit method for the emittance scan as a function of the focusing.

Figure 9

The illustration of the time-resolved OTR profiling of the electron beam. The scheme as shown in the figure would allow a measurement that is integrated over a region determined by the constant gate duration.

Figure 10

Beam size scan along the bunch train for a beam of 1.4 nC, 4.9 MeV. (a) Horizontal beam size scan as a function of the focusing. (b) Representation of the measurement under a focusing current of 202 A as a function of the gate position.

Figure 11

Emittance scan, with a beam of 1.6 nC, 4.8 MeV, along the bunch train with the steps of 400 ns.

Figure 12

Emittance measurement along the pulse train with the steps of 400 ns. The charge per bunch and the energy of the beam were 1.6 nC, 4.8 MeV, respectively, during the measurements.

Figure 13

The emittance fluctuations along the pulse train in comparison with the rf pulse shape. The measurement has been performed on a beam with the charge of 1 nC per bunch and the energy of 5 MeV.

Figure 14

An example from the energy measurement in the spectrometer by OTR monitoring. (a) The image captured by the CCD camera. (b) The horizontal profile of the image.

Figure 15

The time-resolved energy and energy spread measurement by the segmented dump. The contour plot of the energy change during the 1300 ns electron beam hitting the segmented dump.

Figure 16

Deviation of the mean energy in correlation with the rf amplitude pulse shape.

Figure 17

The correlation of the position stability between the laser and the electron beam for the horizontal axis.