rf design of a pulse compressor with correction cavity chain for klystron-based compact linear collider

We present an X-band high-power pulse compression system for a klystron-based compact linear collider. In this system design, one rf power unit comprises two klystrons, a correction cavity chain, and two SLAC Energy Doubler (SLED)-type X-band pulse compressors (SLEDX). An rf pulse passes the correction cavity chain, by which the pulse shape is modified. The rf pulse is then equally split into two ways, each deploying a SLEDX to compress the rf power. Each SLEDX produces a short pulse with a length of 244 ns and a peak power of 217 MW to power four accelerating structures. With the help of phase-to-amplitude modulation, the pulse has a dedicated shape to compensate for the beam loading effect in accelerating structures. The layout of this system and the rf design and parameters of the new pulse compressor are described in this work.

Figure 1

(a) Center part of the transmission spectrum and (b) waveforms of SLED-II. Operation frequency is 11.994 GHz. Output pulse length is 250 ns.

Figure 2

(a) Scheme and (b) network of pulse compressor with a storage cavity and $n$ correction cavities. The arrows in green represent the rf power flow, which goes through the cavities one by one. The small ones are correction cavities, being of relatively low unloaded quality factor (Q-factor) and small size. The large one is the storage cavity with a large unloaded Q-factor, thereby offering the high efficiency of the pulse compression system. $M^n$ is the S-matrix of the $n$th cavity, and $M^{[n]}$ is the S-matrix of the cavity chain with $n$ cavities. The input waves of the $M^{[n]}$ are $a_1$ and $a_2$, and the output waves are $b_1$ and $b_2$. The input waves of the $M^n$ are $a_{1m}$ and $a_2$, and the output waves are $b_{1m}$ and $b_2$.

Figure 3

Typical transmission spectrum of the pulse compressor with correction cavities. The number of the correction cavities in this case is 8 and all of them have the same unloaded Q-factor of $4.5\times {10}^4$ and coupling coefficient of 1.5. Furthermore, the unloaded Q-factor of the storage cavity is $1.77\times {10}^5$ with a coupling coefficient of 5.98. The central frequency of the spectrum is 11.994 GHz and the frequency difference between two nearby peaks is 4 MHz, which corresponds to the pulse length of 250 ns.

Figure 4

(a) Input waveform, waveform after passing through the correction cavity chain, and the output waveform. (b) Input phase, phase after passing the correction cavity chain, and the output phase. (c) Output phase within the period with high peak power.

Figure 5

Top shapes of the output pulses with different correction cavity numbers ($N_c$). The output pulse length is 250 ns and the compression ratio is 6. The $Q_s$ and $Q_c$ are both $1.77\times {10}^5$.

Figure 6

Top shapes of the output pulses with different unloaded Q-factors of correction cavities. The output pulse length is 250 ns and the compression ratio is 6. The $Q_s$ is $1.77\times {10}^5$ and the number of correction cavities is 32.

Figure 7

Scheme of phase-to-amplitude modulation.

Figure 8

(a) Waveforms of the input and output pulses after phase-to-amplitude modulation and (b) the modulated phases of the two klystrons. In this case, the compression ratio is 9 and the output pulse length is 250 ns. The unloaded quality factors of the storage and correction cavities are $1.77\times {10}^5$ and $4.5\times {10}^4$, respectively. The number of the correction cavities is 8.

Figure 9

Field patterns from different views of spherical resonant cavities with ${\mathrm{TE}}_{012}$ and two ${\mathrm{TE}}_{112}$ modes.

Figure 10

(a) Field patterns and (b) reflection coefficient of a correction cavity unit.

Figure 11

Geometry and E-field distribution at 1 W input power injected from port 1.

Figure 12

S-parameters of the dual-mode polarizer.

Figure 13

Assembly of correction cavities and polarizers.

Figure 14

(a) S-parameters of the correction cavity chain and (b) the output pulse with simulated S-parameters of correction cavity chain from HFSS and storage cavity with unloaded Q-factor of $1.77\times {10}^5$ and coupling coefficient of 5.98.

Figure 15

Conceptual design of an rf unit for a klystron-based CLIC main linac.

Figure 16

Power gain and efficiency of the pulse compressor as functions of compression ratio.