Ⅱ-8. Accelerator

RIKEN Accel. Prog. Rep. 47 (2014)

Design and construction of drift tube linac cavities for RIKEN RI
Beam Factory†
K. Suda,∗1 N. Sakamoto,∗1 K. Yamada,∗1 S. Arai,∗1 Y. Chiba,∗1 M. Kase,∗1 H. Okuno,∗1 Y. Watanabe,∗1 andO. Kamigaito∗1
A recent intensity upgrade for uranium and xenon
beams at the RIKEN RI Beam Factory required theconstruction of a new injector linac, RILAC2. Theacceleration system consists of three drift tube linaccavities (DTL1, DTL2, and DTL3) that operate at
f0 = 36.5 MHz in CW mode. The cavity structure isbased on a quarter-wavelength resonator, since its sizeis the smallest in this frequency range among the available cavity structures. The DTL3 was built by modifying the decelerating cavity of the Charge State Multiplier (CSM)1–3) . Because specifications for the DTL3
were similar to those for the CSM, the design was performed carefully, comparing our simulation with theactual cavity to check the validity of the design procedure. Finally, the DTL3 was built by removing amovable shorting plate and relocating the drift tubes.
The other two cavities were newly constructed.
The most significant characteristic of the design isthe adoption of the direct coupling method for amplifiers connected to the cavity. The amplifier using atetrode 4CW50,000E (Eimac) is directly connected tothe cavity with a capacitive coupler. Load resistancefor the tetrode, or an input impedance, was assumed
to be Z0 = 700 Ω in the design. Direct coupling reduces the number of parts, such as the stub and output capacitor, thereby reducing size and construction
cost. However, as the resonant frequency of the cavity changes significantly because of the capacitance ofthe tetrode, the cavity design cannot be independentof the amplifier design.
The design procedure we used comprises the following two steps. We first design the cavity itself without the coupler. We then design the combined cavity
and amplifier system, determining the coupling capacitance and size of the cavity. It is helpful to evaluate load impedance of the tetrode using the lumped
circuit model, but modeling the coupler as a lumped
element neglects some effects; namely, the coupler occupies a certain volume inside the cavity, so capacitance between the outer conductor and the coupler is
non-negligible, making it difficult to estimate the resonant frequency shift due to the coupler. Because ofthis frequency shift, geometric parameters such as cavity height must be carefully determined. we design theThe cavity without a coupler was designed first using
CST Microwave Studio 2009 (MWS)4) . We optimized
the shape of parts constituting the resonator, such as†∗1

Condensed from the article in Nucl. Instrum. and Methodsin Phys. Res. A 722, 55–64 (2013)
RIKEN Nishina Centergaps between the drift tubes, stem geometry, and theinner diameters of the coaxial section, using the eigenmode solver of the MWS to obtain a high parallel shunt
impedance considering height and radius limitations.
We also calculated RF power loss distributions to determine the flow rate of cooling water for each part.
When determining the frequency of the resonator, it is
crucial to consider the effect of the coupler; attaching
the coupler to the cavity can result in a frequency shift
as large as −300 kHz. The target frequency f0 of thecavity determined by considering the coupler effect canbe realized by adjusting the cavity height. The measured frequency shift against a cavity height of DTL3
was approximately 18 kHz/mm. We must determine
the cavity height within an accuracy of ±4 mm to realize a frequency within ±73 kHz around f0 .
RF simulations of the cavity including the coupler
were performed next. The calculated frequency shift
due to the coupler was −290 kHz. Further frequency
shifts due to the tetrode were estimated with the aidof the frequency domain solver. The load impedanceof the tetrode Z  (f ) was roughly estimated by adding
a lumped capacitance of tetrode Cp in parallel as
1/Z  (f ) = 1/Z(f ) + j2πf Cp . Z  (f ) takes a real valueof 750 Ω, which was close to Z0 . The frequency shift
due to the tetrode was estimated to be −19 kHz.
The cavity height of the DTL3 was finally determined by taking these frequency shifts into account.
The calculated shift with the tetrode and coupler
was −309 kHz, and the measured frequency shift was−288 kHz. The estimation agreed well with the measurement. We also estimated the coupling strength using the frequency domain solver. Input impedance wascalculated with various diameters of the coupling disk.
Combining the result with the frequency dependence of
the impedance Z  (f ), we concluded that a plate disk
with a diameter of approximately 130 mm would besuited to obtain the desirable load resistance of 700 Ω
at f0 . Based on these estimations, the diameter of thecoupling disk was adjusted by making iterative measurements with the real structure of the cavity, so thatthe desirable load resistance was successfully obtained.
References
1) O. Kamigaito et al., Proceedings of LINAC’98, Chicago,TU4085, 603 (1998).
2) O. Kamigaito et al., RIKEN Accel. Prog. Rep. 34, 322(2001).
3) O. Kamigaito et al., Rev. Sci. Instrum. 76, 013306(2005).
4) http://www.cst.com.

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