H. Ingwersen

A time-of-flight spectrometer using short heavy ion pulses

Abstract A time-of-flight spectrometer consisting of a rebuncher (START signal) and selected surface barrier detectors (STOP signal) has been tested at the Heidelberg post-accelerator. Using the reaction 32 S + 27 Al in an incident energy range of E Lab = 140−320 MeV a time resolution of Δt ∼ 70 ps was routinely achieved. This resulted in a mass resolution for medium-heavy reaction products ( m ≲ 60 u) of Δm ≲ 0.5 u.

The Heidelberg 3MV-CW Heavy Ion Postaccelerator Section Using Independently Phased Spiral Resonators

The Heidelberg 3MV heavy ion postaccelerator section based on independently phased normal conducting spiral resonators working at 108 MHz is described. The design veloctity of the spiral resonators is ß = 0.10. The peak voltage drop is 0.37 MV at 20 kW CW input power (0.74 MV at 80 kW, duty factor 0.25) corresponding to a peak accelerating field of 1.7MV/m (3,4MV/m) averaged over the external length of the resonator. Using a synchronous phase of ¿S = -20° gives a maximum effective voltage of 0.35 MV (0.7 MV). The resonators are stacked in modules of four with one external quadrupole doublet. The total effective voltage of this first test section of the Heidelberg postaccelerator under construction is 3 MV-CW (6 MV d. f. 0,25) providing ion energies up to 6 MeV/ nucleon at mass A=40. The postaccelerator will be integrated into the existing experimental area. Beam matching to the longitudinal acceptance of the postaccelerator is done by a separate spiral resonator as rebuncher. The high quality of the tandem beam with respect to longitudinal and transverse phase space is maintained in the designed postaccelerator.

The Heidelberg Heavy Ion Postaccelerator

The Heidelberg Heavy Ion Postaccelerator, a linear accelerator booster for the upgraded MP-Tandem is operational in its extended version since end of 1979. Yielding 9.7 MV in CW and 18.5 MV in pulsed mode (duty factor 0.25) it has considerably increased the mass and energy range of heavy ions for Nuclear Physics experiments at Heidelberg. The high flexibility of the booster is guaranteed by using 32 independently phased spiral resonators allowing operation of the machine in the mass range of A=10 to A=100 with almost constant accelerating voltage; although even ions as heavy as 197Au have been successfully postaccelerated. Typical examples of postaccele-rated beams are: 164 MeV 12C, 332 MeV 32S, 476 MeV 79Br, 511 MeV 127J and 640 MeV 197Au. Longitudinal and transversal beam quality are tandem like with ¿r < 2¿ mm mrad and ¿l < 3.2¿ MeV .deg., the debunched energy resolution being well below 10-3. The overall availability of the postaccelerator together with the MP-Tandem was 83% of the scheduled user beam time in 1980.

The Computer-Control-System of the Heidelberg Postaccelerator

The 3MV section of the Heidelberg postaccelerator is operated under full computer control. A PDP 11/34 equipped with a JY 411-CAMAC interface services the operators console via a parallel branch as well as the accelerator components via an extended serial highway. All parameters are controlled from the console by use of touchpanels and digital knobs. The basic software consists of a core resident data base and three control tasks which are functionally completely equivalent to a conventional console. Numerous other tasks are running under the operating system RSX11M for more complex operations and special control processes.

Application of a Spiral Resonator for Bunching Picosecond Heavy-Ion Pulses

A normal conducting spiral resonator (design ßo = vo/c = 0.06) with a maximum effective voltage of 0.33 MV at 20 kW CW input power has been used to bunch 156 MeV 58Ni and 96 MeV 32S ions. For an injected DC beam of 32S13+ of 96 MeV a pulse width of ¿tFWHM = 70ps was measured behind a drift length of 3.4 m. Phase locked to the 6.78/13.56 MHz nanosecond beam pulsing system of the MP tandem, pulse widths in the range of 200ps were obtained. In this case the pulse spacing is 147.5 ns with a background to pulse ratio of about 10-5. This spiral resonator is a standard section of the Heidelberg postaccelerator. It will also be used to match the pulsed heavy-ion beam to the postaccelerator over the whole mass range of interest A = 12-80. Furthermore it is installed in a normal beam line for heavy-ion time-of-flight experiments allowing a mass resolution of A < 1 for relatively heavy reaction products.

Operation Experiences with the Test Section of a Superconducting Heavy Ion Post Accelerator

The 0,5 MV test section of a superconducting heavy ion post accelerator built at Karlsruhe was operated for 1000 h at the beam line of the MP tandem at Heidelberg. Prototype tests carried out under realistic conditions now ensure the basis for the detailed design of a full scale accelerator. The performance tests reported here include 500 h of high field operation and 40 h of beam acceleration time. Poor vacuum conditions were found to increase electron emission in the superconducting resonators. The surface properties however could be restored by a single oxipolishing step. Beam loss during long term operation was simulated by direct ion irradiation at operating temperature and at 300 K. A total dose of up to 2 × 1014 Ni-ions per cm2 (average value over 12 cm2) was found to have no influence on the superconducting surface properties. Cooling system and rf control system were found to operate satisfactory. The required field stability was reliably obtained.

Heavy Ion Acceleration at the Heidelberg Tandem Postaccelerator Combination

At the MPI für Kernphysik the range of heavy ions available with energy higher than 5 MeV/N has been extended to masses of about A=100 by combining the upgraded 13 MV MP-Tandem with a normal conducting RF-linac consisting of 32 independently phased spiral resonators. A first part of the linac of 10 resonators had already been operated between 1977 and 1979 with maximum effective voltages of 3.3 MV (CW) and 5.5 MV (pulse mode). After completion the extended booster now yields 9.7 MV in CW mode and 18.5 MV in pulse mode (duty factor 0.25). The high flexibility of the booster guaranteed by the independent phasing principle allows operation of the machine in the mass range of A=10 to A=100 with nearly constant acceleration voltage and with almost negligible set up times due to the flexible computer control. The maximum energies obtained so far are for some ions: 164 MeV 12C, 332 MeV 32S, 337 MeV 35Cl, 476 MeV 79Br and 511 MeV 127J. The tandemlike beam quality of the Tandem Booster combination (radial emittance < 2 II mm mrad, energy resolution < 10-3 (with debuncher)) and the available beam pulse width of less than 100 ps are an important contribution to the heavy ion nuclear and atomic physics experiments at MPI since March 1980. The overall availability of the postaccelerator together with the MP Tandem was above 80% of the scheduled user beam time.