Frequency jump using 704.4 MHz radio-frequency quadrupole and cross-bar $H$-type drift tube linear accelerators

A large-scale proton or ${\mathrm{H}}^{-}$ accelerator often requires a jump in radio frequency, typically in the beam velocity range of $\beta =0.4–0.6$. The MYRRHA linac has two frequency jumps: one at $\beta \cong 0.2$ and another one in the typical beam velocity range at $\beta =0.56$. Using the MYRRHA linac as an example, this study investigates two new solutions, whereby the frequency jump in the range of $\beta =0.4–0.6$ is eliminated. A single frequency jump at $\beta \cong 0.2$ can shorten the whole linac considerably and reduce the construction and operation costs. The proposed frequency jump sections will use 704.4 MHz radio-frequency quadrupole and cross-bar $H$-type drift tube linear accelerators. To ensure a safe and reliable cw operation at such very high frequency for these kinds of accelerating structures, careful design studies with respect to beam dynamics and rf structures including water-cooling channels have been performed. The results demonstrated the feasibility of both solutions.

Figure 1

Schematic layout of the MYRRHA facility.

Figure 2

Constructed CH cavities and the proposed frequency jump CH.

Figure 3

Temperature distribution of the 704.4 MHz CH with 2.5 kW of input rf power and water cooling.

Figure 4

Water-cooling concept for the proposed 704.4 MHz CH (left: front view; right: side view; darker blue: cooling channels for the stems and tubes; lighter blue: cooling channels for the tank wall).

Figure 5

Schematic layout of the frequency jump section based on four 704.4 MHz CHs.

Figure 6

Longitudinal particle distribution before (left) and after (right) frequency jump at the entrance to the first CH (the blue rectangles indicate the boundaries of the particle distributions).

Figure 7

Particle distribution at the entrance to the first CH after frequency jump (top) and particle distribution at the exit of the frequency jump section (bottom), where the blue rectangles indicate the boundaries of the particle distributions.

Figure 8

Main design parameters of the 704.4 MHz RFQ.

Figure 9

Front view of the 704.4 MHz RFQ with 28 water-cooling channels (blue dots).

Figure 10

Schematic layout of the frequency jump section based on a combination of one 704.4 MHz RFQ and one 704.4 MHz CH.

Figure 11

A comparison of the transverse and longitudinal beam envelopes between the two solutions.

Figure 12

Particle distribution at the entrance to the RFQ after frequency jump (top) and particle distribution at the exit of the frequency jump section (bottom), where the blue rectangles indicate the boundaries of the particle distributions.

Figure 13

A comparison of the transverse and longitudinal emittance evolutions for the two solutions.

Figure 14

A comparison of the phase width evolutions between the two solutions.

Figure 15

Beam transmission efficiency as a function of the run number for batch 3 of the CH-only solution (top) and batch 3 of the RFQ-based solution (bottom), respectively.

Figure 16

Output emittances as a function of the run number for batch 3 of the CH-only solution (top) and batch 3 of the RFQ-based solution (bottom), respectively.

Figure 17

Transverse and longitudinal beam envelopes of the worst run in S1B3 (green: nominal case without errors; red: worst case before correction; blue: worst case after correction).

Figure 18

Transverse and longitudinal beam envelopes of the worst run in S2B3 (green: nominal case without errors; red: worst case before correction; blue: worst case after correction).