Charge breeding -charge breeding of intense radioactive beams

The new concept of the MONO1000 magnetic structure has been used to design the new MINIMONO ECR ion source. It has permitted to reduce the size of the source and thus to make it suitable to the ISOLDE weight and volume constraints while conserving a good ionisation efficiency. In case of stable noble gases, the ionisation reach up to 95% for masses heavier than Ne. For 4He, the ionisation efficiency is equal to 22%. Associated to a carbon target for radioactive ion production on line, the production efficiencies measured at GANIL for 35Ar+, for example, including the diffusion efficiency out of the target, the effusion efficiency in the target and in the source chamber and the ionisation efficiency was close to 15%. Moreover, production of several molecular ions was observed. The largest emittance was observed for a H+ beam. At the exit of the source it was equal to 60 pi.mm.mrad for an extraction voltage of 14kV and for a current of H+ equal to 120mA.To adapt the ECR ion source to the ISOLDE mechanical environment, several modifications have been made before testing it on line. Both experiments performed at GANIL and at CERN reveal a strong sensitivity of the source to the gas present in the chamber, coming from the gas feeding or from the production target, which can remove the working point from the best working conditions.

The ECRIS charge state breeder has been extensively studied at LPSC (ex-ISN) Grenoble, 22 different stable elements have been injected as a 1+ beam, multi-ionised and extracted as n+ ones. It has been proved that in case of gaseous ion beams the only requirement is to give enough energy to overcome the plasma potential of the ECR, the efficiency is then optimum and almost constant with increasing energies, the efficiency can easily reach 10% on a specific charge state. For metallic and alkali ions, the final energy of the 1+ beam must be carefully adjusted within an eV range. The efficiency for such beams is up to 6% on one charge state.

The 1+ -> n+ method is a fast and efficient way to multi-ionise a low energy 1+ ion beam. The process consists in a direct injection of the 1+ beam into an ECR plasma. The multi-ionisation is obtained when the capture of the primary beam is insured by the accurate tuning of specific parameters of the system. An ECR ion source (MINIMAFIOS) has been used in the so called 1+ (R) n+ method developed for the PIAFE project at ISN Grenoble. This ECR ion source gave interesting results (i.e.: an efficiency of 2.5 % was obtained for the Rb1+ -> Rb15+ multi-ionisation).Based on the efficiency obtained with the MINIMAFIOS ECR ion source, a new ECR charge breeder has been studied, designed and built at LPSC Grenoble. This ECR charges Breeder, named PHOENIX, permits to reproduce the axial magnetic configuration and the RF input geometry of the MINIMAFIOS source. The main improvements concern the radial magnetic field, which has been almost doubled to permit the use of an RF input from 10 to 18GHZ, the high voltage insulation has been improved to reach 60kV, and the mechanics has been improved to simplify the maintenance. The efficiency obtained with PHOENIX can reach 6% on a single charge state.

The charge breeder process make a natural representation of the test particle approach, which is an importanttopic of plasma physics: in detail, the exotic particles injected are the 'test particles', which propagates insidethe ECRIS plasma, which act as background: our particle motion description (theoretical and with numerical programs) includes full 3D details of collisions (affecting beam stopping and diffusion) and ECRIS field (solenoids, hexapoles, and ambipolar field models).At the same time, the progressive ionisation of particles (or recombination) is considered. The statistical properties of collisions are represented with an ensemble of particles. The numerical simulation confirm a three phase model of charge breeding: a) injection and slowing down in a typical length (also analytically estimated);b) diffusion mainly towards a central plasma core, due to ambipolar potential (this phase is longer, so mostionization happens here); c) random collision causes extraction and beam formation. Application includes: 1) optimisation of the charge breeder conditions; 2) validation of estimate of the background condition; 3) extension and comparison to other physics regimes. On the other side, the ionisation dynamic equilibrium that attains in phase b) can be studied with analytical model giving precise bounds on charge breeder efficiency, comparable with EBIS case. Therefore charge breeder studies leads to a better understanding of ECRIS physics. LNL-INFN is also involved in many research and projects on acceleration of exotic nuclear beams.

That result is derived from several measurement campaigns with the REX-ISOLDE charge state breeder. The results of those measurements provide data about the different charge breeder components such as beam quality, efficiencies, cycle times and intensities. The REXTRAP accumulates the continuous ISOL-beam, bunches (extraction time -10ms) and phase space cools within 10-20ms from 30p.mm.mrad to a transverse emittance of 10 p.mm.mrad for 80% at 30keV. All elements except for He can be handled by the trap with efficiencies up to 50%. Space charge effects start occurring for more than 105 ions/pulse, with an efficiency decrease and emittance increase as a result. The REXEBIS has a theoretical capacity of -5x1010 charges/pulse, and so far 2.3x1010 charges of residual gas beam have been extracted. The extracted charge-bred ions have a charge state distribution, with approximately 25% of the ions in the main charge. The best attained efficiency result for the REXEBIS is -10% for potassium breeding without trap efficiency. The average efficiency over the whole range Li to Cs is between 5 and 10%, and for higher intensities the efficiency can drop to as low as 2%. The acceptance is approximately 10p.mm.mrad (95% at 60 kV), which in many cases is smaller than the emittance of a primary radioactive ion source. The geometrical emittance of the REXEBIS has been measured as -10p.mm.mrad (95% at 20kV), but the value is strongly dependent on the ion neutralisation of the electron beam. The REX-ISOLDE charge breeder results can be easily scaled to higher intensities, resulting in advanced high current EBIS devices like the BNL RHIC-EBIS.

This result is one mayor goal of the RTD-project. The determination of the characteristic parameter of the two charge breeding systems, based on EBIS or ECRIS gives detailed data of the strong and weak points of each breeding system. Strong points of the ECRIS are simple operation, continuous and pulsed operation, very high intensities and good transverse beam acceptance. The weak points are the beam purity, especially for low intensities, the larger beam emittance and the longer breeding times. The strong points of the EBIS are the very high beam purity, short breeding times, high charge states and good beam emittance. The weak points are the beam matching for injection, which requires buffer gas emittance cooling and the restricted ion capacity. Within the project both systems have shown to be quit complementary. For very high intensities the ECRIS is best suited. The longer lifetimes of isotopes with high beam intensities corresponds to the longer breeding time concerning the ECRIS breeder. For short living isotopes the maximum intensities proposed for next generation RIB facilities are very well suited for an EBIS type charge state breeder. An buffer gas filled ion cooler would be beneficial for both breeding systems and is required for good isobar separation anyhow.

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