Low emittance design of the electron gun and the focusing channel of the Compact Linear Collider drive beam

For the Compact Linear Collider project at CERN, the power for the main linacs is extracted from a drive beam generated from a high current electron source. The design of the electron source and its subsequent focusing channel has a great impact on the beam dynamic considerations of the drive beam. We report the design of a thermionic electron source and the subsequent focusing channels with the goal of production of a high quality beam with a very small emittance.

Figure 1

CLIC layout (not to scale). The main beams are accelerated at the center of the complex using the rf power extracted from the drive beam in the power extraction and transfer structures (PETS blue boxes) [2].

Figure 2

CLIC injector layout with a total length of 12.7 m consists of an electron source (ES), three subharmonic bunchers (SHB), a prebuncher (PB), a traveling wave buncher (TWB) and two accelerating structures (ACC-1,2). A solenoidal focusing channel covers the injector and the accelerating structures.

Figure 3

Numerical solution for the potential equation with linear decrease in the beam envelope to half of its initial value in comparison with a parabola.

Figure 4

The equipotential surfaces calculated from series expansion of $\mathrm{\Phi }(r,z)$ in terms of $\mathrm{\Phi }(0,z)$ and its derivatives.

Figure 5

Geometry of zero equipotential surface (the cathode plus the focusing electrode surfaces) in comparison with a parabola.

Figure 6

Analytical geometry of 140 kV equipotential surface (the anode surface) in comparison with a nose (hatched region) composed of a curve, straight line and a part of a parabola.

Figure 7

cst model for the final geometry of the CLIC thermionic gun with all its parameters. Left: the half of the whole structure is presented. Insets, a quarter of each one of the electrodes with all their parameters are shown. Right top: cathode plus focusing electrode; right middle: main part of the anode; right bottom: additional electrodes with the same voltage as the anode.

Figure 8

cst simulation results for variations of the beam energy along the CLIC thermionic gun.

Figure 9

cst simulation results for the variations of the rms beam radius (top panel) and the beam emittance (bottom panel) along the gun. For the rms beam radius also Eq. (12) is plotted (dashed line).

Figure 10

The complex of the focusing channel. Two matching coils after the electron gun and 148 coils in the solenoidal structure. See also Fig. 2.

Figure 11

parmela simulation results for the variations of the average bunch energy (top panel) and its current (bottom panel) in the focusing channel designed for the CLIC drive beam injector.

Figure 12

The current distribution of the magnetic coils (top panel) and the magnetic field profile (bottom panel) in the focusing channel from the beam waist designed for the CLIC drive beam injector.

Figure 13

Variations of the rms beam radius from the beam waist to the end of the injector using the envelope equation.

Figure 14

parmela simulation results for the variations of the beam rms radius (top panel) and the beam emittance (bottom panel) within the focusing channel designed for the CLIC drive beam injector.