Ka-band linearizer structure studies for a compact light source

The CompactLight design for a next-generation x-ray free-electron laser utilizes a C-band injector. This requires that the harmonic system used to linearize the beam’s phase space must operate at X-band rf or higher. We investigate the optimum frequency for the harmonic system in the range of frequencies from 12 to 48 GHz. We describe the reasoning behind selecting 36 GHz (Ka-band) as our working harmonic frequency. The full linearizer system design including the power source, pulse compressor, and linearizing structure, along with options, is considered and presented. These designs are compared in terms of rf and beam dynamics performance. Two potential MW-level rf sources are discussed; a multibeam klystron and a gyro-klystron, while a klystron-based upconverter with an X-band driver is briefly discussed as an alternative path if even higher peak powers are needed. To further increase peak power, novel options for pulse compressors at Ka-band are discussed. Traveling and standing wave solutions for the structure are presented.

Figure 1

The fraction of the action amplification factor exceeding 1 under the effects of short-range wakefields in the Ka-band linearizer, as a function of the Ka-band length and of the iris aperture radius.

Figure 2

Example of start-to-end tracking in CompactLight, obtained with the code track1d.

Figure 3

Geometrical parameters of the single cell.

Figure 4

Parameter comparison of a $2\pi /3$ single cell at different frequencies for a 2-mm iris radius.

Figure 5

Parameter comparison of a 36-GHz single cell with a 2-mm iris radius for different phase advances.

Figure 6

Accumulated phase advance error given by manufacturing tolerances for a 30-cm structure and 2-mm iris aperture radius at different frequencies.

Figure 7

Accumulated phase advance error given by manufacturing tolerances for a 30-cm Ka-band structure with a 2-mm iris aperture radius as a function of the designed cell-to-cell phase advance.

Figure 8

Power requirement for a 30-cm, 36-GHz structure, to provide 12.75-MV integrated voltage, for different phase advances.

Figure 9

Power factor gained by a 20 ns pulse coming out of the pulse compressor, as a function of rf power source pulse width.

Figure 10

Geometric parameters of the nose cone for a reentrant cell option.

Figure 11

Example of the nose cone’s optimization process for a reentrant cell option. In this case, $\mathrm{NCH}=0.1\mathrm{mm}$ and ${\theta }_{\mathrm{NC}}=45°$.

Figure 12

Integrated voltage for a 30-cm long reentrant structure and 15 MW of input power.

Figure 13

Accumulated phase advance error and power needed to reach 12.75 MV of integrated voltage for a $2\pi /3$, 30-cm structure, as a function of the iris thickness for the disk-loaded simple cell.

Figure 14

Cross section of a single cell and cst® steady-state thermal simulations results at peak gradient.

Figure 15

Mode launcher geometry (top) and S-parameters for a 10-cm long structure (bottom).

Figure 16

TE20 to TM01 mode converter’s horizontal (a) and vertical (b) cross sections, field pattern distribution (c), and its matched S-parameters (bottom).

Figure 17

Geometrical parameters $rx$ and $hx$ (top). The magnitude of the mode converter’s minimum reflection, tuned to 36 GHz, as a function of $hx$ (bottom).

Figure 18

Side view of the simulated 30-cm TWS (top). The magnitude of the longitudinal (dash-dotted blue line) and transverse (dashed red line) short-range wakefields (bottom), for a single $300\mathrm{\mu }\mathrm{m}$, 75 pC electron bunch.

Figure 19

Magnitude of the longitudinal (dash-dotted blue line) and transverse (dashed red line) long-range wakefields on a 30-cm TWS, as seen by a second bunch.

Figure 20

Linearizing voltage as a function of dissipated power per unit length and pulse length for (a) Two SWS; (b) Four SWS.