Proceedings of the 16th Annual Meeting of Particle Accelerator Society of Japan
July 31 - August 3, 2019, Kyoto, Japan

PASJ2019 THOH10CAVITY AND OPTICS DESIGN OF THE ACCELERATOR FOR THE
JAEA-ADS PROJECT ∗B. Yee-Rendon† , J. Tamura, Y. Kondo, K. Hasegawa, F. Maekawa, S. Meigo and H. Oguri
Japan Atomic Energy Agency (JAEA), Tokai, Japan
Abstract
The Accelerator Driven Subcritical System (ADS) becomes a prominent candidate for the transmutation of nuclear waste. To this end, the Japan Atomic Energy Agency(JAEA) is proposing the JAEA-ADS project which consists
in a CW superconducting proton linac coupling with a subcritical core reactor, the accelerator will operate with a beam
current of 20 mA and a final energy of 1.5 GeV. The firstpart of the work is focus in the design of five superconducting cavity models to accelerate the beam from 2.5 MeV to
1.5 GeV and the last one is dedicating to beam optic studies,with emphasis on the control of the emittance growth to
reduce the beam halos and mitigate the beam loss, which isone of the main challenges for the successful operation of
the ADS projects.

INTRODUCTION
The long-term and high level of radiotoxicity of the nuclear waste storage are some of the main challenges of nuclear energy. To deal with these problems, the scientificcommunity is proposing Accelerator Driven Subcritical System (ADS) as a promising solution to these issues. Currently,
several ADS demonstration projects are under commissioning around the world (MYRRHA [1] and Ci-ADS [2]). For
its part, the Japan Atomic Energy Agency (JAEA) is designed and ADS to solve the challenges of the nuclear waste
storage in Japan.
The JAEA-ADS project consists in high intensity accelerator coupling with a subcritical core reactor [3]. The requirements of high intensity accelerator with a beam power (someMW) together with the constrain to operate in Continuous
Wave (CW) mode (compatible with the steady state of thereactor operation) points out that a CW superconducting
proton linac is the favorable choice. Moreover, the requirements of the beam power (related to the thermal power of thereactor by the effective neutron multiplication (𝑘𝑒𝑓 𝑓 ) ) andthe efficient energy on the target (for the neutron production)
were important to select the beam current of 20 mA and thefinal energy of 1.5 GeV for the JAEA-ADS linac.
The biggest challenges are the lower numbers of beamtrips and its duration, MYRRHA has a limit of 10 trips longer
than 3 s per 3 months of operation [4], that the linac canaccepted to avoid thermal stress in the core reactor. These
conditions demand a high operation stability in the linacs
elements and a carefully design of the beam optics.
In particular, the JAEA-ADS linac presents the highest
beam power linac so far (30 MW). The most relevant char∗ Work supported by Subvention for ADS development.
† byee@post.j-parc.jp

acteristics of the linac design are: 1) High beam current(20 mA), strong space charge which could induce the emittance growth, 2) The double frequency jump (162 MHz to324 MHz to 648 MHz).
The first studies of the JAEA-ADS have been presented
in others particle accelerators conference [5–7]. This report provides a summary of the Superconducting Radio
Frequency Cavity (SRFC) models and a detailed discussionof the beam dynamics studies for the latest lattice model.

CAVITY MODELTo achieve a highly efficient acceleration, the JAEA-ADS
will use five types of families of SRFC (operating in a frequency of 162 MHz, 324 MHz and 648 MHz) to acceleratebeam from 2.5 MeV to 1.5 GeV. By using this scheme, thenumbers of cavity and its length can be reduced, the accelerating gradients (Eacc) and the cavity aperture can beincreased. At the first step, the types of cavities, the geometrical beta, the number of cell and energy range of theJAEA-ADS SRFC were selected and the results are summarized in Table 1.
Table 1: Parameters of the SRFC
CavityHalf Wave Resonator (HWR)
Single Spoke 1 (SSR1)
Single Spoke 2 (SSR2)
5-cell Elliptical 1 (EllipR1)5-cell Elliptical 2 (EllipR2)

Frequency 𝛽𝑔 Energy range[MHz][ MeV]
162324
324
648
648

0.08
0.16
0.43
0.68
0.89

2.5-1010-35
35-180180-500
500-1500

The geometry of the SRFC were designed by using theprograms SUPERFISH (SF) [8] for the two-dimensionalmodels (only for the elliptical cavities) and CST Microwave
Studio (CST) [9] for the three-dimensional ones. Figure 1presents the three main types of cavities used for the JAEAADS.
For each of cavity a full geometry parametrization was implemented, the details can be found in somewhere else [5, 6].
The optimization process was done by making variable scan,
the goals of the optimization process were the following:• Lower 𝐸𝑝𝑘/𝐸𝑎𝑐𝑐 and 𝐵𝑝𝑘/𝐸𝑎𝑐𝑐 (avoid electric breakdown, quench, etc.).
• Lower power dissipation (high value of 𝑅/𝑄 and 𝐺).
Table 2 presents a summary of the most important figuresof merit of the JAEA-ADS SRFC. The results presented
were computed for an operation temperature of 2 K.
The results of the SRFC models were efficient in terms oflower electromagnetic peak ratio and power dissipation, how-- 107 -Proceedings of the 16th Annual Meeting of Particle Accelerator Society of Japan
July 31 - August 3, 2019, Kyoto, Japan

PASJ2019 THOH10

Table 2: Figures of Merits of the JAEA-ADS Models
Parameters

HWR

SSR1

SSR2

EllipR1

EllipR2

Epk/Eacc
Bpk/Eacc [mT/MV/m]
R/Q [ Ω]
G [ Ω]

4.21
3.41
285
59

4.7
6.68
212
64

3.55
5.13
285
129

2.17
4.22
443
208

2.11
4.07
619
256

• Epk ≤ 30 MV/m (to ensure the stable operation in
the cavities by avoiding possible electric breakdown,quench, etc.).
• Continuity of the longitudinal acceptance (to reduce theemittance growth, specially in the region of frequency
jump [10]).
This study used new emittance inputs obtained from preliminary studies of the RFQ for the JAEA-ADS [11]. Consequently, a new phase law was implemented to satisfy theequipartition condition. A summary of these changes andthe comparison with the previous model [7] are presentedin Table 3.
Table 3: Comparison of the emittance between previous andnew studies. The unit of emittance is mm mrad.

Figure 1: JAEA-ADS main types of SRFC: Half Wave Resonator (HWR), Single Spoke (SSR) and 5-cell Elliptical(EllipR).
ever, the SSR1 cavity performance was lower than HWR1and SSR2.

BEAM OPTICS
The ADS has a high restriction in the numbers of beamtrips and its duration. Thus, one of the main priorities in thelinac design is an excellent control in the beam loss, whichis the principal responsible of the beam trips. The beam loss
has an strong correlation with the formation of beam haloand emittance growth in the linac. As a first step to achieve arobust lattice design, the beam optics will be focus to control
the emittance growth. To this end, the following conditions
were applied:• The phase advance (𝑘) in all the planes is kept lower
than 90 degree to avoid parametric resonances.
• The beam must satisfy the equipartitioning condition,
𝑇𝑥/𝑦 𝑘𝑥/𝑦 𝜖𝑛𝑜𝑟𝑚,𝑥/𝑦=𝑇𝑧
𝑘𝑧 𝜖𝑛𝑜𝑟𝑚,𝑧Parameter

New studies

Previous study

Phase advance law
𝜖𝑥/𝑦,𝑛𝑜𝑟𝑚,𝑟𝑚𝑠
𝜖𝑧,𝑛𝑜𝑟𝑚,𝑟𝑚𝑠

𝑘𝑧 = 0.54𝑘𝑥/𝑦
0.25
0.46𝑘𝑧 = 0.85𝑘𝑥/𝑦0.36
0.4

Another difference with the previous studies is the adoption of two new conditions: Epk ≤ 30 MV/m and continuityof the longitudinal acceptance. The first requirement comes
for the operational experience of SRFC around the word, thislimited could be recomputed and its side effect (reducing theEacc) could be improved by the optimization of the SRFC
electromagnetic peak ratios. The second new condition results in a change of the initial synchronous phase (𝜙𝑠 ) in all
the SRFC sections. This has a decrease effect in the voltage
gain for particle, which is translated as an increase in thenumbers of cavities and the linac length.
Using the same the lattice layouts as the previous studies(See Fig. 2), two new models were created to evaluate theadvantages of the continuity of the longitudinal acceptance.
Table 4 presents a summary of the most important featuresof the two models.

(1)

where 𝑇 is the rms kinetic energy on the plane, 𝑘 is phaseadvance and 𝜖𝑛𝑜𝑟𝑚 is the normalized emittance. This
is to prevent emittance exchange between the planes.
• Smooth envelope (an excellent beam matching between
different cavity sections).

- 108 -Table 4: Summary of JAEA-ADS Schemes
Parameter

Model A

Model B

Epk = 30 MV/mContinuity of the longitudinal
acceptance
Number of cavities
Number of magnets
Linac length [m]Yes
NoYes
Yes

278
138
423

374
190
532

Proceedings of the 16th Annual Meeting of Particle Accelerator Society of Japan
July 31 - August 3, 2019, Kyoto, Japan

PASJ2019 THOH10Figure 2: JAEA-ADS lattice layout. S= Solenoid, C = Cavity, DQ= Double Quadrupole.
The beam dynamics studies were developed by tracking
6-Dimensional Gaussian distributions (3𝜎 cut) with 100,000
macro-particles. Figure 3 shows rms beam size envelopes
along the linac for the model B, the horizontal (top red) andvertical (top blue) envelopes are bounded in a radius of 3 mm.
The ratio between cavity radius and rms beam size are from
8 to 19, and the lattice presents quasi-periodic envelope. On
the longitudinal plane, the rms phase is below 10 degree(Fig. 3 bottom).

Figure 3: RMS beam size envelopes along the linac in thehorizontal (top red) and vertical (top blue), and longitudinalplane (bottom).
The Hofmann charts (Fig. 4) shows that both models A(top) and B (bottom) operates near the equipartition region(dotted black line, where the growth rates of the emittanceexchange is zero) and the two schemes have space charge𝑘dominant beams ( 𝑘 𝑥/𝑦 < 0.7).
𝑥/𝑦0

As a result of the continuity of the longitudinal acceptance,
the longitudinal acceptance of the model B is almost 3 times
larger than the one of A (See Fig. 5).
Model B presents a better control of the rms normalized
emittance ratio ( 𝜖𝜖0 ) through linac than A (Fig. 6 top andmiddle). It can be seen a large longitudinal emittance growth

Figure 4: Stability chart (Hofmann chart) provides information of the growth rate regions for the models A (upper) andB (lower).

- 109 -

Proceedings of the 16th Annual Meeting of Particle Accelerator Society of Japan
July 31 - August 3, 2019, Kyoto, Japan

PASJ2019 THOH10

Figure 5: Longitudinal acceptance and the corresponding
multiples of the initial longitudinal normalized rms emittance (yellow line) for the model A (top) and B (bottom).
oscillations (Fig. 6 bottom) at the beginning of the linac inspecial during the transition between HWR and SSR1.

CONCLUSIONS
The first electromagnetic designs of the JAEA-ADS SRFC
were completed [1,2], these are an important advance forseveral reasons: 1) The SRFC is one of the key ingredientsfor the JAEA-ADS project and represent a significant part ofthe linac cost, 2) The SRFC results are critical for the beam
optics design of the linac, 3) This work is the continuity andboost of the superconducting linac research and development
in JAEA.
The SRFC models have an efficient performance in termsof the figures of merits and their values are close withthe ones obtained by similar projects (PIP-II [12] and CADS [13]). The Epk/Eacc was under 2.2 for the high beta(elliptical) cavities and 4.8 for the rest of the cavities, similarly, Bpk/Eacc was under 7 in all the SRFC.
The JAEA-ADS linac is the highest beam power (30 MW)
linac proposal. The control of the beam loss, halo and emittance growth are big challenges for the success of the project,specially in the ADS projects. This upgrade version adopted
new emittance inputs in consequence a new phase law isused to achieve an equipartitioned beam. In addition, theFigure 6: RMS normalized emittance ratio (𝜖/𝜖0 ) along thelength of the linac for the model A (dashed red line) and B(dotted blue line). Horizontal (top) and vertical (middle),
and longitudinal planes (bottom).

Epk ≤ 30 MV/m, to operate the SRFC with high stability,decreases the Eacc gradients and the continuity of the longitudinal acceptance condition helps to control the emittance
growth during the frequency jump.
Two models were developed: A (Epk limit, this scheme
is similar as the previous model [7]), B (Epk limit and Continuity of the longitudinal acceptance). The beam dynamics
studies showed that both models are space-charge dominantbeams and theirs working points are near the equipartition re-- 110 -Proceedings of the 16th Annual Meeting of Particle Accelerator Society of Japan
July 31 - August 3, 2019, Kyoto, Japan

PASJ2019 THOH10gion. Additionally, the two schemes have longer linac length(8% for A and 36% for B) and emittance growth (almost thedouble on the transverse plane and half on longitudinal onefor A and half on all the planes for B) in comparison withprevious model.
These increases in the emittance growth with respect to theprevious study are consequence, principally, of the change of𝜖the 𝜖𝑥/𝑦,𝑛𝑜𝑟𝑚,𝑟𝑚𝑠,0from 0.85 to 0.54, the large difference in the𝑧,𝑛𝑜𝑟𝑚,𝑟𝑚𝑠,0emittance enhanced the energy transfer between transverseand longitudinal planes.
Model A and the previous scheme have similar longitudinal acceptance about 10 ×𝜖𝑧,𝑛𝑜𝑟𝑚,𝑟𝑚𝑠 , on the contrary, B has
almost 3 times that value. B has a better emittance growth
control than A, specially on the transverse plane. This is dueto the emittance interchange between the longitudinal plane
and the transverse ones (See Fig. 6). The plots pointed out
the advantage of the continuity of the longitudinal acceptance.
Finally, the study is a step forward in the robust designof the JAEA-ADS linac which is fundamental to the JAEAADS project and will help in the development of the futureof the high intensity linacs around the world.

ACKNOWLEDGEMENTS
The authors would like to thank to F. Bouly (LPSC) for
the fruitful discussions and the members of the JAEA-ADS,
J-PARC linac, KEK SRF and Riken SRF group for theircomments and suggestions.

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