9-gap resonator

Linear accelerator

Linear accelerator | RFQ | IH | 7-gap | 9-gap

The IH 9-gap resonator

In order to make a wider range of isotopes from ISOLDE available for nuclear physics experiments at REX, an energy upgrade of the accelerator from 2.2 MeV/u to 3.0 MeV/u was proposed [1]. By installing an additional IH 9-gap cavity after the 7-gap resonators the energy could be boosted, see fig. 1. The further energy upgrades to 5.5 MeV/u and later 10 MeV/u are treated in HIE-ISOLDE.

Figure 1. The first energy upgrade of the REX-ISOLDE Linac to 3 MeV/u

For the MAFF [2] project a design has been worked out for two identical short 7-gap IH structures, providing the desired energy variation for the MAFF-Linac. The main advantage of this accelerator type compared to the split-ring resonators of REX-ISOLDE lies in the higher shunt impedance, allowing a variation of the final energy over a comparatively wide range (3.7 – 5.9 MeV/u) with only two short cavities.

In the first design for the REX 3 MeV/u upgrade, it was foreseen to change one of the MAFF resonator from a 7-gap to a 9-gap resonator – keeping a constant cell length, corresponding to 2.5 MeV/u synchronous particle energy. Nine gaps were necessary to match the lower injection energy of 2.2 MeV/u instead of MAFF's 3.7 MeV/u. However, measurements at the Tandem accelerator of the Maier-Leibnitz Laboratory in Garching showed that this gap geometry leads to a rather low transit time factor [3]. Thus the drift tube geometry was changed to a βλ/2 profile for fixed input and output energies. Table 1 shows the geometry and rf-parameters of the resonator.

Frequency [MHz]	202.56
outer tank length [mm]	676
inner tank length [mm]	520
half shell radius [mm]	145
cell length [mm[	38.5-58.5
gap length [mm]	19 - 27
drift tube length [mm]	32
drift tube diameter in./out. [mm]	16 / 22
maximum rf-power [kW]	100
duty cycle [%]	10
Kilpatrick	1.5
shunt impedance (pert.) [MW/m]	218
Q₀	10100

Table 1: Resonator parameters of the 9-gap IH-cavity

Figure 2. The 9-gap IH resonator after installation in the REX beam line, before the lead shielding was installed.

The input for the LORASR particle dynamics simulations was given by the original LINAC design calculations for the 7-gap resonators, which were verified in detail during the commissioning phase of REX-ISOLDE [4]. The design injection energy produced by the 7-gap resonators is 2.25 MeV/u at a phase spread of ± 15° (after 1.3 m drift) and at an energy spread of ± 0.45 %. Transversely, the beam is injected with an emittance of e_n,x,y = 0.6 p mm mrad in both planes convergent. The calculations were done to fix the drift tube geometry for the resonator, but also to check the possibility of energy variation. Table 2 shows the results of the calculations.

input energy [MeV/u]	2.2
output energy [MeV/u]	2.55 - 3.0
energy spread [%]	1.0 – 1.6
phase spread [°]	25
transmission [%]	100
TTF on axis in gap No. 5 (2.55 – 3.0 MeV/u)	0.855 – 0.866
maximum A/q (90kW)	3.5
radial acceptance a_x,y,norm [p mm mrad]	1.4

Table 2: Design parameters of the 9-gap IH-cavity.

The simulations could be verified during the first beam tests at REX-ISOLDE. The good flexibility in output energy of the accelerator allows for a wider range of mass-to-charge ratios to be available at energies around 3.0 MeV/u, than limited by the currently maximum available rf-power. With an rf-power level limited to 90 kW, the maximum A/q at 3.0 MeV/u is at the moment A/q = 3.5. Thus, during the first runs with radioactive ions, compromises could be found, like e.g. by accelerating ⁷⁶Zn²⁰⁺ ions (A/q = 3.8) at 90kW to ~2.9 MeV/u.

Figure 3 shows energy spectra measured with a N⁴⁺ residual gas beam from the REXEBIS. The decrease of the beam current at higher energies occurs because the beam transport was optimized for a parallel 2.25 MeV/u beam through the spectrometer instead of a convergent injection into the 9-gap. With an optimized injection and a beam transport scaled to the different energies, the transmission through the 9-gap was close to 100%.

Figure 3. The final beam energy for varying power levels in the 9-gap IH resonator.

References

1. T. Sieber et al., Test and First Experiments with the New REX-ISOLDE 200MHz IH Structure, Proc. of the LINAC 2004, Lübeck, August 2004

2. H.Bongers et al., The IH-7-Gap Resonators of the Munich Accelerator for Fission Fragments (MAFF) Linac,
proceedings of the PAC2001, Chicago, June 2001, p.3945

3. O. Kester et al., An Energy Upgrade of the REX- ISOLDE Linac, PAC’2003, Portland, Oregon, USA, May 2003, p.2869

4. S. Emhofer et al., Commissioning results of the REX-ISOLDE LINAC, PAC’2003, Portland, Oregon, USA, May 2003, p.2872