IH-structure

Linear accelerator

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

IH-structure

The IH-structure accelerates the ions from 0.3 MeV/u to an energy between 1.1 and 1.2 MeV/u. The variation of the final energy of the IH-structure can be achieved by adjusting the gap voltage distribution via capacitive plungers. The lower final energy of the IH-structure is important for deceleration of the ions down to 0.8 MeV/u. Otherwise the deceleration from 1.2 MeV/u down to 0.8 MeV/u through the 7-gap resonators would perform a non-acceptable energy spread of the ions at the target. The IH-structure [1] for REX-ISOLDE is an efficient drift-tube structure which was derived from similar structures like the GSI HLI-IH-structure or 'Tank 1' from the IH-structure of the lead LINAC at CERN [2]. The used beam dynamics concept of the 'Combined Zero Degree Structure' [3] was developed and applied for both IH-DTL's mentioned above. After a first acceleration section with a 0°-synchronous particle the ions are focused in transverse direction by an inner-tank triplet lens. Just behind the triplet the ions are rebunched by a -30°-synchronous particle section (3 gaps), followed by a second acceleration section. The whole structure has 20 gaps and a total length of 1.5 m. The resonator tank is cut into an upper and a lower shell. The drift tubes and the magnetic flux inductors are mounted on the center frame.

Figure 1. Photo of the REX IH-structure with upper half shell dismounted. The large drift tube in the centre houses a magnetic triplet lens.

For beam dynamics calculations and drift tube design of the IH-structure normalized injection emittances of 0.6 π mm mrad and ε_long = 2.31 keV/u*ns have been assumed. The accepted energy/phase spread are ± 10° / ± 3.4% respectively, achieved by a rebunching section. The final emittances of the present design are 0.6 π mm mrad in x, 0.63 π mm mrad in y-direction and ε_long = 2.65 keV/u*ns. The final energy/phase spread are ± 7° / ± 1.2%.

The development of the IH structure was done by the university of Munich.

Figure 2. 3D figure of the IH-structure.
The drift tubes are fixed on the girders
welded to the middle frame.

Data

Energies Ein = 300 keV/u Eout = 1.1-1.2 MeV/u

Tank length L = 1.5 m 

Number of gaps = 20 

Electrode voltage Veff = 4.05 MV (A/q = 4.5)

Quality factor Qo = 16400 

rf-power consumption 65 kW (A/q = 4.5) 

Shunt impedance R = 330 MΩ/m

Re-buncher section

To match the beam to the transverse and longitudinal acceptances of the IH-structure a section consisting of two magnetic quadrupole triplet lenses and a rebuncher is placed in front IH-cavity. This section is similar to those of the GSI HLI-accelerator and of the CERN lead-LINAC. The first triplet lens focuses the beam through the rebuncher and produces a waist for diagnosis. The rebuncher is a so called splitting structure with three gaps and integrated voltage 70 kV at A/q = 4.5 built by A. Schempp, Uni Frankfurt.

Data

Quadrupole triplets lenses, 

k = 60 T/m, a = 30 mmLength L = 0.2 m, three gaps

Frequency: 101.28 MHz 

Shunt impedance R = 26 MΩ/m

Quality factor Qo = 3500

Maximum rf power Ppeak = 2 kW

Δφ = ± 14° at RFQ exit to Δφ = ± 10° at IH entrance

Figure 3. View inside the tank of the 3-gap Split-Ring Rebuncher

References

E. Nolte, et al., Nucl. Inst. and Meth. 158 (1979) 311
D. Warner et al., "CERN Heavy-Ion Facility Design Report", CERN93-01 (1993)
U. Ratzinger, Proc. IEEE-PAC91, San Francisco, 567-571 (1991)