Design and beam dynamic studies of a 30-MW superconducting linac for an accelerator-driven subcritical system

The Japan Atomic Energy Agency (JAEA) is working on the research and development of a 30-MW continuous wave (CW) proton linear accelerator (linac) for the JAEA accelerator-driven subcritical system (ADS) proposal. The linac will accelerate a 20 mA proton beam to 1.5 GeV, using mainly superconducting cavities. The main challenge for an ADS accelerator is the high reliability required to prevent thermal stress in the subcritical reactor; thus, we pursue a robust lattice to achieve stable operation. To this end, the beam optics design reduces the emittance growth and the beam halo through the superconducting part of the linac. First, we simulated an ideal machine without any errors to establish the operation conditions of the beam. Second, we applied element errors and input beam errors to estimate the tolerance of the linac design. Finally, we implemented a correction scheme to increase the lattice tolerance by reducing the beam centroid offset on the transverse plane. Massive multiparticle simulations and a cumulative statistic of $1\times {10}^8$ macroparticles have shown that the JAEA-ADS linac can operate with less than $1\mathrm{W}/\mathrm{m}$ beam losses in error scenarios.

Figure 1

General scheme for the ADS.

Figure 2

Layout of the JAEA-ADS linac.

Figure 3

Tune depressions along the main linac.

Figure 4

Hofmann chart shows the work points for each of the different SC periods of the main linac, the equipartitioning condition (black dotted line) and the instabilities areas of the second-, third-, and fourth-order modes.

Figure 5

MEBT layouts for the JAEA-ADS. The upper section depicts the previous design [24], while the lower section depicts the current configuration.

Figure 6

Transverse beam envelopes for different beam fractions along the MEBT.

Figure 7

Transverse beam distribution with and without scraping at the end of the MEBT.

Figure 8

Transient time factor $T$ for each SC cavity as a function of relativistic $\beta$. The black dotted lines indicate the energy transition between the types of cavities.

Figure 9

The different SC cavity period lattices for the JAEA-ADS: (a) for the HWR section, (b) and (c) correspond to the spokes sections SRR1 and SSR2, respectively, and (d) is for EllipR1 and (e) for EllipR2.

Figure 10

Synchronous phase (top) and the accelerating gradient (bottom) of the SC cavities. The vertical lines show the transition between different SC sections. The red dotted lines indicate the region of frequency jumps and the change of SC regions.

Figure 11

Phase advance per period (top) and per unit length (bottom). The $x$ axis is represented by the red solid line, the $y$ axis by the blue dashed line, and the $z$ axis by the green dotted line.

Figure 12

Longitudinal acceptance at the entrance of the main linac.

Figure 13

Beam energy of the linac (blue dashed line) and $1\mathrm{W}/\mathrm{m}$ beam loss rate (red dotted line).

Figure 14

Beam distribution at the entrance of the MEBT.

Figure 15

Hofmann chart for the main linac.

Figure 16

Rms beam size along the MEBT and main linac. The upper plot shows the transverse rms envelopes. The middle plot is the rms phase width. Finally, the bottom one presents the ratio between the acceptance, aperture radius for the transverse plane and $2|{\varphi }_s|$ for the longitudinal, and the beam rms size.

Figure 17

Normalized rms emittance evolution along the MEBT and main linac.

Figure 18

Beam evolution along the linac for different beam fractions. The aperture line is the physical aperture of the linac elements and the phase acceptance one is the $2|{\varphi }_s|$ of the cavities.

Figure 19

Halo behavior through the linac for different beam emittance fractions.

Figure 20

Beam distribution at the end of the linac.

Figure 21

Error amplitude scan of the emittance growth for the static EE2 case. Error bars represent the standard deviation error of the mean of 1000 runs.

Figure 22

Summary of the average emittance growth for the different static EE defined in Table 7.

Figure 23

$y_{\mathrm{rms},\mathrm{roe}}$ (top) and maximum $y$ size (bottom) along the linac for the different error amplitude (EA) cases.

Figure 24

Maximum transverse size (top) and beam power lost per meter (bottom) along the linac when all the static EE cases were applied simultaneously with the error value uniformly distributed within plus-minus their amplitude.

Figure 25

Orbit correction schemes implemented for the JAEA-ADS linac. The scheme (a) is implemented for the HWR and SSRs sections, and the (b), for the elliptical ones.

Figure 26

Transverse (top) and longitudinal (bottom) rms of residual orbit error for the cases without and with orbit correction when all the static EE are combined.

Figure 27

Summary of the average emittance growth for the different dynamic EE defined in Table 7.

Figure 28

Emittance growth of the input beam errors with respect to the ideal machine at the exit of the linac. The error bars are the standard error of the mean.

Figure 29

Maximum transverse and phase envelopes for the ideal machine and error cases. The envelopes plotted correspond to a cumulative statistic of ${10}^8$ macroparticles.