Fig. 6-1 Superconducting quarter-wave accelerating resonator In this type resonator (cavity), an electromagnetic standing wave is induced if one-fourth of the wavelength of the rf input coincides with the length of the center conductor. A strong electric field is generated around the drift tube that is supported by the center conductor. The rf current is dominant on the upper resonator wall. The rf loss is very small because of superconductivity. The fields in the gaps before and after the drift tube are in oposite directions and alternate at the resonant frequency (130 MHz for the resonator shown here). Pulsed beams, which are accelerated in the first gap, can be accelerated in the second gap as well because the field in the second gap changes to the direction of the beam during the transit in the drift tube. The resonator can generate an accelerating field of about 6 MV/m with an rf loss of only 4 watts.

[ Energy performance ] The acceleration energy by the booster is shown. Nuclear reactions can take place if the heavy-ion energy is above the Coulomb barrier energy shown by the dashed line. In the past, ions no heavier than medium heavy ions, like copper, could be used for these nuclear reactions. The booster raised the limit of mass numbers to about 200, which includes gold, for various experiments associated with these nuclear reactions.

Heavy ions can be accelerated to high energies by using a tandem type electrostatic accelerator. Heavy ions are accelerated first while in a negatively charged state (single excess electron). These ions are then transfered to a multiply charged positive state by an electron stripper and accelerated again with a high energy gain, a gain proportional to the charge number. To boost the energy further, a linear accelerator that accelerates pulsed ion beams by means of radio frequency (rf) electric fields is used. A high current flows on the surface of accelerating rf cavity wall when a high field is generated in the cavity. For a normally conducting cavity made from copper, etc., much rf power and cooling water are needed. In contrast, for a superconducting cavity made from niobium superconductor and cooled by liquid helium, very high continuous-wave accelerating fields can be generated with very little rf power consumption because the rf surface resistance is as small as one part in 100,000 compared to copper. The superconducting accelerating resonators (cavities) developed here are made from copper covered niobium. This concept comes from the principle of standing-wave resonance in a coaxial quarter-wave line. Forty of these devices are used in the booster linac for the tandem accelerator. They can generate an accelerating field as strong as 6 MV/m. The energy of highly charged heavy ions is a maximum of nearly 1 GeV (1,000,000,000 eV). As a result, the mass of heaviest ions with which nuclear reactions can take place with other heavy target elements was increased from medium heavy ions, like copper, to very heavy ions, like gold.

Reference T. Ishii, M. Shibata and S. Takeuchi, Construction of the Tandem Booster, Nucl. Instrum. Methods A328, 231, (1993).

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