Proceedings of the 19th Annual Meeting of Particle Accelerator Society of Japan
October 18 - 21, 2022, Online (Kyushu University)

PASJ2022 FROA07

ROBUST AND COMPACT DESIGN OF A 30-MW BEAM TRANSPORT LINEFOR AN ACCELERATOR-DRIVEN SUBCRITICAL SYSTEM
B. Yee-Rendon∗ , Y. Kondo, J. Tamura, K. Nakano, F. Maekawa and S. Meigo
Japan Atomic Energy Agency (JAEA), Tokai, Japan
Abstract
The Japan Atomic Energy Agency accelerator-driven subcritical system (JAEA-ADS) pursues the reduction of nuclear waste by transmuting minor actinides. JAEA-ADS
drives a 30-MW proton beam to a lead-bismuth eutectic(LBE) spallation target to produce neutrons for a subcriticalcore reactor. To this end, the JAEA-ADS beam transport tothe target (BTT) must provide a suitable beam profile andstable beam power to the beam window of the spallation
target to avoid high-thermal stress in the target and reactor components. The JAEA-ADS BTT was optimized bytracking a large number of macroparticles to mitigate thebeam loss in the region outside the reactor, performancewith high stability in the presence of errors, and fulfill thelength requirement. This work presents the design and thebeam dynamics studies of the first scheme of the BTT for
the JAEA-ADS design.

INTRODUCTION
The Japan Atomic Energy Agency (JAEA) is developing an accelerator-driven subcritical system (ADS) for thetransmutation of the minor actinides [1]. The JAEA-ADS
proposal employs a 30-MW proton beam to produce spallation neutrons for an 800-MWth thermal power subcritical
reactor, as shown in Fig. 1.

Figure 1: JAEA-ADS scheme.
Once the reference design for the JAEA-ADS linac wasestablished [2], the following step is the design of the beamtransport to the target (BTT) that carries out the beam fromthe end of the linac to the beam windows and onto the spallation target, with a specified beam profile, current density,and beam loss.
The BTT must satisfy the high-reliability operation of theADS designs; thus, a robust and simple design is pursued.
Table 1 presents the main specifications for the BTT based on∗ byee@post.j-parc.jp

the JAEA-ADS subcritical reactor design requirements [3,4].
In addition, the BTT must ensure the integrity of the beam
window by providing a low peak current density to avoid
excessive head load at the beam window [5]. This work
presents a short summary of the design criteria and the beam
dynamics performance of a BTT for the JAEA-ADS design.
The completed study has been published in Ref [6].
Table 1: Main Characteristics of the Beam Transport to theTarget for the JAEA-ADS Proposal
Parameter
Particle
ProtonBeam current (mA)
20
Beam energy (GeV)
1.5
Beam power (MW)30
Beam loss outside the reactor part (W/m)<1
Beam power stability± 1%
Beam energy stability± 1%
Peak current density (𝜇A/mm2 )< 0.3
Footprint stability at the beam window better than ± 10%
Final beam pipe aperture(mm)< 450Length (m)≈ 27

LATTICE DESIGNThe JAEA-ADS BTT must transport a 30-MW beam fromthe end of the linac to the beam window with a low peakcurrent density, high stability, and reliability. Besides therequirements of beam performance, the BTT must satisfy
other specifications, such as its length must be compatible
with the footprint of the design, 27 m [3, 4]. In addition,
to facilitate the exchanging work during the beam window
and fuel replacement, the dipole apertures were chosen to
be 100 m to reduce the dipole weight, but this demands awell-focused beam on the transverse plane to avoid beamloss. Therefore, we adopted a simple compact design using
normal conducting dipoles and quadrupoles. By using asimple design, we avoided complex arrangements using nonlinear optics that are prone to error, thus affecting its stability.
The BTT is divided into two sections: the beam optics
part, where the beam manipulations occur, and the reactor
zone, from the upper shield of the subcritical reactor until
the beam windows, as shown in Fig. 2. The beam optics section is subdivided into matching, bending, and final focusing
regions. Table 2 presents the parameters of the magnets
that composed the BTT. The matching section comprises
four quadrupoles that accept the beam from the linac andmanipulate it to achieve adequate beam parameters for thebending part. The bending sector is an achromatic lattice of- 179 -Proceedings of the 19th Annual Meeting of Particle Accelerator Society of Japan
October 18 - 21, 2022, Online (Kyushu University)

PASJ2022 FROA07

two 45∘ dipoles and two quadrupoles to avoid the increase of
the vertical size because of the dispersion. Finally, the focusing section handles the beam to achieve the required beamprofile at the beam window by employing five quadrupoles.
The BTT has a correction scheme composed of steering
magnets and beam position monitors to control the beam
centroid along the optics section. In addition, multi-wire or
luminescence monitors will be placed to survey the size andposition of the beam at the beam windows.

Table 2: Magnet parameters that composed the beam transport to the target. For the quadrupoles, the gradient is presented. The positive sign in the quadrupole gradients indicates that the beam is focused on the 𝑥 direction.
ElementLength (mm)Gradient (MW)/Bfield (T)

Quad1
Quad2
Quad3
Quad4
Dipole1
Quad5
Quad6
Dipole2
Quad7
Quad8
Quad9
Quad10
Quad11808080803543300
3003543
300
300
300
300
300

11.69
-2.52
-1.08
-13.991.60-13.01
1.58
1.60-12.55
4.55
14.79
14.99
14.85

trolled inside the beam optics section to avoid beam losses
and smoothly expands inside the reactor.

Figure 2: Schematic layout of the BTT for the JAEA-ADS
based on subcritical reactor studies [4].

BEAM DYNAMICS STUDIES
We performed the beam dynamics studies using theTraceWin program [7]. The input distribution was obtained
from the JAEA-ADS linac beam tracking simulations [2],which comprised over 9 × 106 macroparticles. As a firstpart of this work, the lattice parameters were optimized toachieve a peak current density not higher than the accepted
Gaussian profile [5]. A smoothly beam scrapping was implemented to prevent significant perturbations in the beamcore, ensure beam control, and minimize the increase ofbeam loss to guarantee the requirement of beam power atthe beam window. Figure 3(a) and (b) show the transverse
root-mean-square (rms) beam size. The BTT is achromatic,
as is confirmed by the dispersion plot in Fig. 3(c). Figure 4
displays the maximum radial beam profile for the baseline
case, the solid red line. The radial envelope is well con-Figure 3: Horizontal (a) and vertical (b) rms beam size andthe dispersion function (c) along the BTT.
The radial beam profile from this study exhibits a slightly
lower peak current than the Gaussian model around the beam
center, as shown in Fig. 5. The low current density at thecenter is helpful in reducing the thermal stress and increasing- 180 -Proceedings of the 19th Annual Meeting of Particle Accelerator Society of Japan
October 18 - 21, 2022, Online (Kyushu University)

PASJ2022 FROA07

fluctuations in the beam delivered by the JAEA-ADS linac
because of baseline operation and fast recovery schemes. Inaddition, the errors were divided into static and dynamicerrors. The former remain constant for long periods; thus,
they can be measured and corrected. The latter are random
errors, therefore, stay uncompensated.
Table 3: Summary of the Static Element Errors for the BeamTransport to the Target
Element
Δ𝑥, Δ𝑦 [8] (mm)Δ𝜃𝑥, Δ𝜃𝑦 [9](mrad)Δ𝜃𝑧 (mrad)
Magnet field (%)

Figure 4: Comparison of the maximum radial envelope between the baseline (red solid line) and the double error caseI (blue dotted line) along the BTT.
the buckling pressure at the top of the beam windows withina radius of 25 mm. Thus, it is expected that this beam profile
will satisfy the feasible conditions of the beam windows. The
beam current density presents small fluctuations toward thebeam center that are attributed to statistical errors becauseof the smaller counting area.

Figure 5: Radial current density at the beam window obtained from the beam tracking simulations and the accepted
Gaussian profile [5]. The error bar is proportional to thesquare root of the proton number counted in each bin.
The accepted Gaussian model scrapes about 13% of thebeam. Consequently, the beam current should be increasedby the same amount to guarantee the 30-MW at the beam
window surface. For our model, 4.8% of the beam is scraped
before reaching the beam windows. Thus, our model has theadvantage of only requiring an increase of beam current of
one-third regarding the Gaussian model.
The second part comprised the evaluation of the robustness and estimation of the tolerances for the BTT design.
To this end, we applied beam error studies using a similar
proceeding to the JAEA-ADS linac error studies [2]. Theerrors applied in this study were element errors, e.g., misalignment of quadrupoles, and input beam errors, which are

Quadrupole

Dipole

0.45
1.2
6
0.5

0.5
2.4
2.4
0.05

Table 4: Summary of the Dynamic Element Errors for theBeam Transport to the Target
Element
Δ𝑥, Δ𝑦 (mm)Δ𝜃𝑥, Δ𝜃𝑦 (mrad)Δ𝜃𝑧 (mrad)
Magnet field (%)

Quadrupole

Dipole

0.01
0.01
0.06
0.05

0.01
0.02
0.02
0.005

Tables 3 and 4 summarize the element errors. Their amplitudes were defined based on other error studies [2, 10]
and corrected to achieve the demanded stability presented
in Table 1. Table 5 presents the input beam errors. For thestatic case, we used the results of the fast recovery compensation studies for the JAEA-ADS linac, which are rematched
schemes to enable a fast-stable linac operation recovery withan acceptable beam quality to achieve high availability incase of a faulty element [11]. The cases were classified
according to the type of failure in single superconducting
radiofrequency cavities (SSRFC), multiple superconducting
radiofrequency cavities (MSRFC), and magnets (Mag) failures. The amplitudes presented in the table are increased by
20% to increase the severity of the cases.
Table 5: Summary of the Input Beam Errors for the BTTParameters
Δ𝑥, Δ𝑦 (mrad)Δ𝜙 ((deg))
Δ𝑥 ′ , Δ𝑦′ (mrad)
Δ Beam energy (keV)(Δ𝜖/𝜖0 )𝑡 [12] (%)(Δ𝜖/𝜖0 )𝑙 (%)
𝑀𝑡 (%) [13]𝑀𝑙 (%)

SSRFC MSRFC Magnet Dynamic
1
1
10.4
428
5
111
1
10.4
28
192
1528

1
1
10.4
27
21
14
140.02
0.02
0.02
0.1
111
1

We applied double errors consisting of the simultaneous
application of the element and the input beam errors. The errors were separated into static and dynamic cases, as shown- 181 -Proceedings of the 19th Annual Meeting of Particle Accelerator Society of Japan
October 18 - 21, 2022, Online (Kyushu University)

PASJ2022 FROA07

in Table 6. Each double error case comprised 1,000 independent runs with a beam distribution of 9 × 106 macroparticles.
In each run, the amplitude error is assigned independently
and uniformly plus-minus the maximum value presented inTables 3, 4, and 5.
Table 6: Double Error Description
Case TypeIIIIIIIVDefinition

StaticElement errors + SSRFC input beam errors
StaticElement errors + MSRFC input beam errors
StaticElement errors + Mag input beam errors
Dynamic Element errors + input beam errors

Figure 6 shows the difference in beam power for each runof the double error case I regarding the baseline. In addition,
Fig. 4 compares the average maximum radial envelope ofthe double error case I against the baseline. The study reveals that the beam performance for both cases was similar,indicating an adequate beam stability.

Figure 6: Beam power fluctuations to the baseline for doubleerror case I.
Figure 7 summarizes the beam performance of the double
errors with a confidence level of 99%. All the beam losses
were confined inside the reactor part. Figure 7(a) shows thebeam energy is lower than ± 7.5 MeV, equivalent to 0.5%of the baseline beam energy. The beam power fluctuations
are inside ± 1%, as shown in Fig. 7(b). Figures 7(c) and (d)display the rms size growth is below ± 6%, and the transverse offset is within ± 9 mm, respectively. The dynamics
offset will be removed using active beam correction fromthe beam position observed by the instrument placed at thebeam windows.

Figure 7: Summary of the beam error degradation for different error cases defined in Table. The plot (a) is the differencein kinetic energy, (b) beam power, (c) radial rms size, and(d) transverse offset. The error bars denote 2.57 standard
deviation, for a 99% confidence interval.

CONCLUSIONSThis work discusses the design and presents the beam
dynamics studies of the beam transport to the target for theJAEA-ADS proposal. The JAEA-ADS BTT must properlytransport a 30-MW beam from the end of the linac to thebeam windows and onto the spallation target while fulfilling
stringent conditions of beam stability and engineering specifications. The design provides an efficient beam profile at thebeam windows with low peak current density to satisfy thefeasibility of the beam windows and small beam scraping,which reduces the extra beam power required. This study
applied double errors that comprise input beam and element
errors to investigate the viability of the fast-recovery compensation schemes, which are crucial strategies to operatewith high reliability, and set up the element tolerances. The- 182 -Proceedings of the 19th Annual Meeting of Particle Accelerator Society of Japan
October 18 - 21, 2022, Online (Kyushu University)

PASJ2022 FROA07

analysis confirms that the BTT design meets the stringent
beam window stability of the JAEA-ADS.

ACKNOWLEDGMENTS
The authors would like to thank to Hiroki Iwamoto,Takanori Sugawara, Kenji Nishihara, and the members ofthe JAEA-ADS group for their comments and suggestions.
This work was supported by the Subsidy for Research andDevelopment on Nuclear Transmutation Technology.

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9720974[6] B. Yee-Rendon et al., “Beam physics design of a 30-MW
beam transport to the target for an accelerator-driven subcritical system”, JINST, vol. 17, p. P10005, Oct. 2022.
doi.org/10.1088/1748-0221/17/10/P10005[7] TraceWin Manual,
http://irfu.cea.fr/dacm/logiciels

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