Nuclear Fusion Research and Development

4-6

Realization of Fusion Intense Neutron Source－ Loop Antenna Development for RF Coupling in the RFQ Linac －Superconducting
RF LinacRadio-Frequency
Quadrupole Linac

1.00

H
140 mA 100 keV

5 MeV

9 MeV

M

Refl. Coefficient (–)

Injector

Test cell

Lithium Target

L40 MeV

RF Power system

High( > 20 dpa / y, 0.5 ℓ)Medium ( > 1 dpa / y, 6 ℓ)
Low
( < 1 dpa / y, 8 ℓ)

0.95

0.90

Operation mode

Dipole mode

0.85

0.80

170

175

180

185

190

Frequency (MHz)Fig.4-13 Configuration of IFMIF
A 40 MeV deuteron beam with at 250 mA is injected into a liquid lithium flow, anda 14 MeV neutron field similar to that of a D-T fusion reactor is produced by theD-Li stripping reaction. Materials are irradiated with these neutrons to evaluate theirrobustness for use in fusion reactors.

Fig.4-16 Reflection coefficient from RFQ
A frequency shift of less than 0.1% for operation
mode is attained, and it is sufficient far fromdipole mode.

2.527 mm
40 mm
45 mm
48 mm
73 mm

Coupling factor (–)

2.0

1.5(kA/m)
5.0

Beam center

Optimized
L=45 mmLoop antenna

1.0

2.50.50020406080

1000.0Loop angle (deg.)

Fig.4-14 Measured results of RF
coupling; RF coupling vs. loop
angle for the inserted depth

Fig.4-15 Magnetic field profile by 3-D
simulation code
RFQ magnetic field profile is satisfied
under the design conditions.

To realize a demonstration fusion reactor, it is indispensable
to evaluate the feasibility for use in fusion reactor materials
using 14 MeV neutron irradiation. For this purpose, theInternational Fusion Materials Irradiation Facility (IFMIF) is
being jointly planned by Japan and the European Union under
the agreement of Broader Approach Activities agreement.
In the IFMIF, a neutron field is produced by the deuteron(D)-lithium (Li) stripping reaction. The realization of adeuteron beam that can be accelerated and injected into liquid
lithium is a key issue (Fig.4-13). In this accelerator facility,
two 125mA beam lines are used. Continuous wave (CW)
operation is also required, and this is the world’s first attempt
at developing such a high-current beam and a CW operationlinac. One beam line consists of an ion injector, a radiofrequency quadrupole (RFQ) linac and a superconducting RF(SRF) linac; their designed output energies are 0.1, 5.0 and40 MeV, respectively. We developed a new RFQ, because
the RFQ had many technical issues to be overcome for therealization of high-current and CW operation.

Fig.4-17 Photograph of loop antenna
tip module employing an advanced
Japanese technology

In this RFQ linac, a driving RF power of 1.4 MW at 175 MHz
has to be injected to the RFQ cavity to accelerate a large
current of 125 mA in CW mode. For stable and continuous RF
power driving, it is indispensable to reduce the deformation of
the loop antenna and to avoid frequency shifts in the operation
mode. For this purpose, it is crucial to obtain an RF design byemploying a small loop antenna and a short insertion depth.
In this study, the precise RF properties (Fig.4-14) wereevaluated by measuring a real size RFQ mock-up module andanalyzing a 3-D simulation code (Figs.4-15, 4-16). The insertion
depth was successfully optimized. For the world’s first attempt,
an engineering design with cooling water channels in the loop
antenna was also developed. To make uniform cooling channels
following the loop’s curve, an advanced Japanese technology
using uniform minute grains of sand was applied. As a result, theloop antenna tip module with cooling channels was successfullymanufactured (Fig.4-17). With these results, the development of aloop antenna for CW operation is now in sight.

Reference
Maebara, S. et al., Engineering Design of the RF Input Coupler for the IFMIF Prototype RFQ Linac, Fusion Engineering and Design, vol.88, issues 9-10,
2013, p.2740-2743.
JAEA R&D Review 201355