Cavity and optics design of theaccelerator for the JAEA-ADS project
Bruce Yee-Rendon*, Jun Tamura , Yasuhiro Kondo, Kazuo
Hasegawa, Fujio Maekawa, Shin-ichiro Meigo and Hidetomo Oguri
Nuclear Transmutation & Accelerator Division
Japan Proton Accelerator Research Complex (J-PARC)
Japan Atomic Energy Agency (JAEA)
Acknowledgments: The members of the JAEA-ADS, J-PARC linac, KEK SRF, Riken SRF groupand F. Bouly (LPSC) for their help. This work was supported by ADS 補助金.
*byee@post.j-parc.jp

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B. Yee-Rendon1Contents• Goal• Introduction• Superconducting Radio Frequency Cavity (SRFC) models▪

Low beta cavities (Half-Wave Resonator (HWR) & Single Spoke (SS) )▪

High beta (Elliptical cavity (Ellip))▪

SRFC summary• Beam optics▪

Lattice design▪

Emittance growth study▪

Beam optics summary

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Goal
JAEA is proposing and Accelerator Driven Subcritical System (ADS). ADS
designed by JAEA will consist of a 30 MW superconducting CW proton
linac and a subcritical reactor core.
Requirements of the linac:• High beam power (~ MW )• High reliability (lower beam trip)

The goal is to design a CW Superconducting proton linac with a beam
current of 20 mA and a final energy of 1.5 GeV with a low beam loss.
To achieve that goal two main tasks were developed: Cavity design
and Beam optics studies.
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SRFC baseline
SRF cavity start point model
2.5 MeV

Cavity type

Freq [MHz]# of Cell

HWR

162

2SS

324

2

Elliptical

648

5500 MeV

35 MeV

10 MeV

1500 MeV

180 MeV

Voltage gain was computedassuming:• Sinusoidal E fields• Synchronous phase of -30⁰
• Vmax of 2.0,2.05,4.987,11.9
and 19.9 MV (values of PIP-II)

The optimization in the numbers ofcavities and 𝛽𝑔 were chosen to obtainmaximum voltage gain per cavity +smooth transition
JAEA-ADS superconducting linac layout

162 MHz
HWR
βg=0.08
2.5 MeV

648 MHz324 MHzSSR1
βg=0.16
10 MeV

SSR2
βg=0.43
35 MeV

EllipR1βg=0.68

180 MeV

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EllipR2βg=0.89
500 MeV1500 MeV4Low beta summary
Design goals• Lower Epk/Eacc & Bpk/Eacc (avoid electricbreakdown, quench, etc.)•Lower power dissipation (high value of R/Q and G)
Epk/Eacc Ring (ITX = ITY)
Bpk/Eacc Ring (ITX = ITY)
Epk/Eacc Ring (ITY = 24 mm)
Bpk/Eacc Ring (ITY = 24 mm)
Epk/Eacc Ring (ITX = 22 mm)
Bpk/Eacc Ring (ITX = 22 mm)

Goal
HWR & SSR Electric field surfaces

Inner center conductor
The values of Bpk/Eacc and Epk/Eacc as a function
geometry
the transverse dimensions of the inner conductor shape.

[1] A. Facco “Tutorial on LOW BETA CAVITY DESING”[2] G. Tae Park et al “ELECTROMAGNETIC DESING OF HALF WAVE RESONATOR WITH B=0.13, F=325MHZ FOR FUTURE HIGH POWER AND HIGH
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INTENSITY PROTON DRIVER KEK”

5High beta summary• Design goalso Epk/Eacc < 2.60 & Bpk/Eacc < 4.6 mT/MV/m

Goal

Lower power dissipation (high value of R/Q and G)

Bpk/Eacc [normalized]▪

Epk/Eacc [normalized]▪ Lower electromagnetic peaks ratios:Goal

5-cell elliptical cavity

Half cell parametrizationgeometry
PASJ19 THOH10The surface plot by changing the iris and thedome ratio to select the parameters tominimize the Epk/Eacc (top) and Bpk/Eacc(bottom).
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SRFC summary••

The first EM design of the JAEA-ADS SRFC were developed [1,2], this is an
important advance for several reasons:▪

SRFC is a key ingredient for the JAEA-ADS project (and necessary to design the beam optics)▪

The continuity & boost of the superconducting linac R&D in JAEA .

The models present an efficient performance in terms of the figures of merits
and their values are close with the ones obtained by similar projects (PIP-II andC-ADS).

[1] B. Yee-Rendon, et al, “Electromagnetic design of the low beta cavities for the JAEA ADS”, J. Phys.: Conf. Ser.
Accepted (2019).
[2] B. Yee-Rendon, et al, “Design of the elliptical superconducting cavities for the JAEA ADS”, J. Phys.: Conf. Ser.
Accepted (2019).
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Beam optics•The ADS requires an excellent control in the beam loss, the main source of thebeam halo, and emittance growth has a strong correlation with the beam halo.

•

To control the emittance growth, the next conditions are applied:▪

The phase advance (𝑘𝑥/𝑦/𝑧 ) < 90⁰ to avoid parametric resonances
𝑇𝑥/𝑦𝑘𝑥/𝑦 𝜀𝑛𝑥/𝑦▪

The beam should satisfy the equipartition condition (== 1) to avoid𝑇𝑧
𝑘𝑧 𝜀𝑛𝑧emittance exchange between the transverse and longitudinal planes▪

Smooth envelope (an excellent beam matching between different cavity sections)▪ Epeak <= 30 MV/m (to ensure the stable operation in the cavities)▪ Continuity of the longitudinal acceptance (to reduce the emittance growth,specially in the region of frequency jump)• After the first beam optics studies[1] (called in this work as previous study), thelast two conditions were added.
[1] B. Yee-Rendon, et al, , “Beam optics design of the superconducting region of the JAEA ADS”, J. Phys.: Conf.
Ser. Accepted (2019).
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JAEA-ADS scheme upgrade• The lattice scheme for the different SRFC section of the JAEA-ADS

JAEA-ADS lattice layout. S= Solenoid, C = Cavity, DQ= Double quadrupole• The initial emittance were changed, consequently emittance ratio changed
too. Thus, a new phase law was implemented to keep with the equipartition
condition.
Parameter
New studies
Previous study

1 IPAC19 model

𝜀𝑥/𝑦𝑛𝑜𝑟𝑚𝑟𝑚𝑠 [𝜋 𝑚𝑚 𝑚𝑟𝑎𝑑]

0.25

0.36𝜀𝑧𝑛𝑜𝑟𝑚𝑟𝑚𝑠 [𝜋 𝑚𝑚 𝑚𝑟𝑎𝑑]

0.46

0.40

Phase law𝑘∥ = 0.54𝑘⊥𝑘∥ = 0.85𝑘⊥

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JAEA-ADS cavity summary & beam
tracking setup• The effect of the new two conditions (Epeak & Continuity of the longitudinalacceptance) were studied in detail by developed two schemes. The
description and some of the results are summarized in the next table.
Parameter/condition

Model A1

Model B

𝐸𝑝𝑒𝑎𝑘 = 30 [𝑀𝑉/𝑚]

Yes

Yes

Continuity of the longitudinal acceptance

NoYes# of Cavities

278374# of magnets

138190Linac length423.102532.155• The beam dynamics studies were developed by tracking 6-Dimensional
Gaussian distributions (3𝜎 cut) with 100,000.

1 This model is similar as the IPAC19 model

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JAEA-ADS Model B rms envelope
HWR

SSR1

SSR2

EllipR2

EllipR１

Freq= 324 MHz

Freq =648 MHz

On the top the rms beam size envelopes along the length of the linac, on the horizontal plane (red)and vertical (blue) one. The ratio between cavity radius and rms beam size are from 8 to 19, the lattice
has quasi-periodic envelope. On the bottom, the rms phase is below 10 degree.
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JAEA-ADS Hofmann chart &longitudinal
acceptance
Equipartition region
Equipartition region
Model A

11 x 𝜺𝒛,𝒓𝒎𝒔,𝒏𝒐𝒓𝒎

Model B

31 x 𝜺𝒛,𝒓𝒎𝒔,𝒏𝒐𝒓𝒎

The Hofmann charts (top) that both model A (left) and B (right) operate in the equipartition region(growth of emittance exchange is zero) and both models are space charge dominant beams(𝑘𝑥/𝑦 /𝑘𝑥/𝑦0 < 0.7). The longitudinal acceptance plots (bottom) show that Model B has almost 3 times
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Norm rms emittances evolution
12 %
8.2 %
10 %

8%

HWR to SSR1

8.1 %
7%

𝜀Model B presents a better control of the RMS normalized emittance ratio ( 𝜀 ) along the length of the0linac than A (top plots). It can be seen a large longitudinal emittance growth (bottom plot) at thebeginning of the linac in special during the transition between HWR and SSR1.
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Beam optics summary I• The JAEA-ADS linac is the highest beam power linac proposal so far. The control of thebeam loss, halo and emittance growth are big challenges for the success of the project.
• The most relevant characteristics of the linac design are:▪

High beam current (20 mA) → strong space charge → emittance growth▪

The double frequency jump (162MHz-324MHz-648MHz) → emittance growth• From the previous model (IPAC19) new inputs and conditions were implemented toimprove the design:▪ New initial (ratio) emittances → New phase law → changes in phase advance (𝑘) &the synchronous phase of the cavities (𝜑𝑠 )▪ Epeak = 30 MV/m → decrease the Eacc
▪ Continuity longitudinal acceptance → affects 𝜑𝑠• Two models were developed:▪

Model A : Epeak = 30 MV/m (similar as the previous model)▪

Model B : Epeak = 30 MV/m & Continuity of the longitudinal acceptance
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Beam optics summary II•

The summary of the results are:ModelsCavities

Len [m]Growth (X,Y,Z) [%]Acceptance long [𝜀𝑧,𝑛𝑜𝑟𝑚,𝑟𝑚𝑠 ]A

278

423.10212/8.2/7

11B

374

532.155

10/8/8.1

31• Both models are space-charge beam which operates near the equipartition region.
• Both models have longer linac length and emittance growth comparing with old model.
These are consequence of the Epeak & Continuity of the longitudinal acceptanceconditions and the new emittance inputs (new phase law).
• Model A and the previous model have similar longitudinal acceptance about 10 x𝜀𝑧,𝑟𝑚𝑠,𝑛𝑜𝑟𝑚 , on the contrary, Model B has almost 3 times that value.

• Model B has a better emittance growth control than A, specially in the transverse plane.
This is due to the emittance interchange between the longitudinal plane and thetransverse one (see the rms emittance plot). This show the advantage of the continuityof the longitudinal acceptance.
• The study is a step forward in the JAEA-ADS linac design which will help to thedevelopment of the JAEA-ADS project and the future of the high intensity linacs aroundthe world.
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