An ECR-based charge breeder for the SPES project

A. Galata1, G. Patti1, J. Angot2, T. Lamy2,

1 INFN, Laboratori Nazionali di Legnaro, Legnaro (Padova), Italy.2CNRS-LPSC, Grenoble, France.

INTRODUCTION

SPES (Selective Production of Exotic Species) [1] is anINFN project supported by the two national laboratoriesLNL and LNS, and is currently under way at LNL. Themain goal of the project is the development of an ISOL(Isotope Separation On Line) facility for neutron-rich exoticbeams production to perform advanced research in nuclearstructure, reaction dynamics and interdisciplinary fields.

To boost the charge state of the radioactive ions produced,and allow post-acceleration at high energies by the ALPI [2]superconducting linac in operation at LNL, an ECR-basedcharge breeder [3] was chosen (SPES-CB): in particular, anupdated version of the Phoenix booster [4], into operationat Laboratoire de Physique Subatomique & Cosmologie(LPSC) since about twenty years. Such devices are basicallyECR sources where a radioactive 1+ beam is injected,captured by the plasma, further ionized and then extractedas a q+ beam.

In June 2014 a Research Collaboration Agreement wassigned between INFN and CNRS, for the delivery at LNL ofan upgraded version of Phoenix ECR booster and ancillariesby LPSC, satisfying the SPES requirements. The acceptancetests were succesfully peformed between March and April2015 on the LPSC test bench (details can be found in [5]),and the charge breeder has been delivered at LNL at theend of November 2015. The SPES-CB can be divided inthree main subsystems: (i) the source body and the plasmachamber, (ii) the gas injection system and the ∆V -rack,and (iii) the extraction system. A brief description of eachsubsystem will be given in the following.

THE SOURCE BODY AND THE PLASMA CHAMBER

A schematic view of the SPES-CB is shown in figure 1:it is a 2nd generation ECR source [6],usually operating at14.521 GHz, with a B-minimum configuration created bythree coils and a permanent magnet hexapole. The maximaare, respectively, Bin j∼1.2 T, Bext∼0.8 T and Brad∼ 0.8 Ton the plasma chamber walls, obtaining mirror ratios of 2.3and 1.5 in the axial and radial directions. With respect to thevery first version of this device, the magnetic profile has beenmodified, so as to optimize the magnetic field gradient at theresonance, and so electron heating. Two physical boundariesdelimit axially the plasma chamber: the plasma electrodefrom one side (φ=8mm) and the so called HF-Blocker fromthe other, an empty aluminum cylinder with a φ=28mm

central hole allowing the 1+ beam injection.

Fig. 1. Schematic view of the SPES-CB: all the main componentsare indicated by coloured rectangles; the ceramic at extraction isnot in its final design.

The SPES-CB will have the possibility to work withtwo distinct frequencies (only one microwaves input isshowed in figure 1, the other being rotated of 90◦), so asto explore both the frequency tuning and the two frequencyheating: initially, both frequencies will be around 14.5GHz or less, but tests at 18 GHz are expected for thefuture. For each of the two circuits a microwave signalwill be produced by a wide-band signal generator andamplified by a Travelling Wave Tube Amplifier (TWTA):microwaves will pass then through a circulator, to protectthe amplifier from the reflected power. By using twodirectional couplers, it will be possible to measure boththe forward and the reflected power, P f and Pr, very closeto the charge breeder, so as to have a measurement of theeffective power fed to the plasma. Finally, a DC breaker anda vacuum window separate, respectively, the high voltagepart from the grounded one and the vacuum part from theatmospheric one. Microwaves will enter then into a doublewall plasma chamber cooled down by deionized water. Tolimit the contaminants introduced by the charge breeder,special attention was paid to the treatment of the surfacesexposed to vacuum, to ensure an high level of cleanliness(see details in [7]).

THE GAS INJECTION SYSTEM AND THE ∆V - RACK

The ∆V - rack regulates the energy of the ions injectedinside the charge breeder, in order to optimize the captureby the plasma. In the SPES model the high voltage ofthe 1+ source (whose power supply will be installed inanother rack) will work as reference for a power supplymounted inside the ∆V -rack (∆V -supply), whose output

Fig. 2. Design of the gas panel and the leak valves: three differentgas connections are foreseen, with automatic switching from oneto the other.

will fix the voltage of the charge breeder, determining thepotential difference between the 1+ source and the chargebreeder. It basically consists in a big outer rack, withinside an insulating transformer that brings the main to aninner rack insulated from ground. The inner rack housesthe ∆V -supply, a complete gas feeding system with vacuumgauges and a purging pump, and all the necessary electronicsto remotely drive the above mentioned equipments. Thegas panel, shown in figure 2, was designed under LNLspecifications: it consists in two independent branches,allowing the possibility to inject two different kinds of gasesat the same time. If only one gas should be used, the secondconnection to the plasma chamber could serve to mount avacuum gauge, so as to measure the pressure as close aspossible to the plasma chamber. One branch will inject noblegases (He and Ar), the other Oxygen or Nitrogen: both willuse precise VAT series 59 leak valves, able to adjust a flowrate down to 10−10 mbar*l/s; such valves have an integratedmotor+encoder system remotely controlled through a serialport. The pressure of each branch will be monitored througha Pirani gauge and a purging port will allow cleaning ofthe entire gas panel. The ∆V - rack will mount a CPU toremotely execute any operation with the gas panel (includingpurging and switching from one gas to the other), and withthe ∆V -supply, through a fiber optical link.

THE EXTRACTION SYSTEM

The beam extracted from the SPES-CB will show thetypical charge state distribution of ECR sources, withdifferent peaks corresponding to different A/q ratios: theexpected values will span roughly from 4 to 7 for the speciesproduced at SPES. The beam will be injected into a roomtemperature RFQ, for a first acceleration to 727*A keV: thiskind of accelerator accepts and accelerates ions with a giveninjection energy, in this case 5.7*A keV. To keep constant the

injection energy of ions of different A/q’s, as those comingout from the SPES-CB, will imply the use of an extractionvoltage VCB variable between 20 and 40 kV. For this reason,it was decided to substitute the single gap extraction system,presently mounted on the Phoenix booster, with a moreflexible three electrode extraction system, as the one shownin figure 3. Two bellows, with a maximum excursion of±10.5 mm in longitudinal direction and 3 mm in radialdirection, will allow the alignment of the two electrodesindependently from the charge breeder and the first opticalelement downstream, a magnetic solenoid. The design of theextraction system foresees the possibility to install opticalsphere for the alignement thorugh a laser-traker; as a furtherpossibility, a special tool has been machined (see figure 3)in order to mechanically align the plasma and the extractionelectrodes.

Fig. 3. The three electrodes extraction system adopted for theSPES-CB.

The potentials will be set so as to always work with aconstant potential difference VCB-Vpuller= 20 kV in the firstextraction gap. The extraction system is based on a LPSCdesign, but its configuration in terms of reciprocal distancesand potentials between the electrodes was optimized atLNL by using the numerical code Kobra-3D [8]: this wasdone keeping in mind the requirement for SPES, that is anormalized root-mean-square (rms) emittance εrms

norm < 0.1πmm mrad for the extracted beam.

[1] G. De Angelis et al., Jour. of Phys. Conf. Series, DOI10.1088/1742-6596/590/1/012010 (2015).

[2] G. Bisoffi et al., NIM B, DOI: 10.1016/j.nimb.2016.01.024(2016).

[3] P. Delahaye et al., Eur. Phys. J. A, (2010), 46, pp. 421-433.[4] J. Angot at al., Proc. of the 20th ECRIS Workshop

(ECRIS-2012), Sydney, Australia, 2012.[5] A. Galata et al., this annual report.[6] R. Geller, Electron Cyclotron Resonance Ion Source and ECR

plasmas, IOP, Bristol, UK, 1996.[7] C. Roncolato et al., this annual report.[8] A. Galata et al., DOI 10.1016/j.nimb.2015.12.031 (2015).