SPES Target Group Data…… INFN-CISAS-CNR collaboration The Ablation Ion Source for refractory metal ion beams A preliminary design

SPES Target Group Data…… OVERVIEW 1- The Ablation Ion Source functioning 2- Laser Evaporation / Ablation (STEP A) 3- Electron Impact Ionization (STEP B) 4- Cathode – Anode preliminary design 5- Simulation activity with F3MPIC (CISAS) 6- Conclusions 7- Open problems and future developments

SPES Target Group Data…… The Ablation Ion Source functioning Anode (Ta) > > +300VCathode (Ta) Refractory metal vaporized by laser ablation Extraction STEP A: EVAPORATION OF THE REFRACTORY METALLaser Evaporation / Ablation STEP B: HIGH CHARGE STATE IONIZATIONElectron Impact Ionization Laser beam Plasma with high charge state ions Electron flux

SPES Target Group Data…… Laser Evaporation / Ablation (STEP A) “A high-voltage vacuum arc or a laser pulse creates a metal plasma, which expands and can be further ionized by an additional discharge” (B. Wolf, Handbook of Ion Sources, Chapter 3: Production of Ions from Nongaseous Materials)

SPES Target Group Data…… Cathode (Ta) Anode (Ta) > > +300V Electron flux 3- Electron Impact Ionization (STEP B) At present the idea is to obtain multiply stripped ions (from the laser-ablated atoms/ions) using a high- energy electron beam (Electron Impact Ionization Mechanism), keeping the anode at very high temperatures (close to 2200°C) in order to avoid the sticking of refractory atoms.

SPES Target Group Data…… REFERENCE 1: MK5 IS cathode (Ta) anode (Mo) 4- Cathode – Anode preliminary design

SPES Target Group Data…… Nuclear Instruments and Methods in Physics Research A236 (1985) 1-16 AN ELECTRON-BEAM-GENERATED-PLASMA ION SOURCE FOR ON-LINE ISOTOPE SEPARATION J.M. NITSCHKE Nuclear Science Division, Lawrence Berkeley Laboratory, University of California, Berkeley, California 94720, USA REFERENCE 2: EBGP IS 4- Cathode – Anode preliminary design

SPES Target Group Data…… REFERENCE 2: EBGP IS Nuclear Instruments and Methods in Physics Research A236 (1985) 1-16 AN ELECTRON-BEAM-GENERATED-PLASMA ION SOURCE FOR ON-LINE ISOTOPE SEPARATION J.M. NITSCHKE Nuclear Science Division, Lawrence Berkeley Laboratory, University of California, Berkeley, California 94720, USA 4- Cathode – Anode preliminary design

SPES Target Group Data…… REFERENCE 3: IRENA IS 4- Cathode – Anode preliminary design

SPES Target Group Data…… Anode (grid) Cathode Insulators OUR PRELIMINARY DESIGN Refractory metal sample 4- Cathode – Anode preliminary design

SPES Target Group Data…… Main geometrical parameters (ANODE) from MK5 anode grid from MK5 anode diameter increased respect to the MK5 extraction hole (  1.5), in order to allow the passage of the laser beam 4- Cathode – Anode preliminary design

SPES Target Group Data…… diameter defined in order to create a radial gap of 1.5 mm between the anode and the cathode; the value is similar respect to the axial anode-cathode gap of the MK5 Ion Source. thickness value representing a compromise between mechanical stability and electrical resistance (heating power generated by Joule effect). diameter that allows an effective thermal shielding of the anode avoiding important influences on the beam extraction. Main geometrical parameters (CATHODE) 4- Cathode – Anode preliminary design

SPES Target Group Data…… Preliminary electro-thermal analysis Temperature field correspondent to 750 A of heating current (I = 750 A) With heating currents between 500 A and 1000 A it is possible to heat the cathode to temperature levels high enough (approximately between 1800°C and 2200°C) to guarantee a sufficient electron emission. 4- Cathode – Anode preliminary design

SPES Target Group Data…… Cathode electron emission 1 cm Emitted electron current per unit length 4- Cathode – Anode preliminary design

SPES Target Group Data…… "Handbook of Ion Sources" (Bernhard H. Wolf) Ta: Z = 73 Ta 15+, ionization potential of about 400 eV. For the creation of ions of charge state q = n+ the optimal electron temperature / energy is about three or four times the n’th ionization potential (just as for the case of singly charged ions where the ionization cross section maximizes at an electron temperature of about three or four times the first inization potential). As a consequence the electron energy was fixed between 1200 and 1600 eV 4- Cathode – Anode preliminary design

SPES Target Group Data…… Space charge limited electron current per unit length r 0 (emitter)r (collector) The emitted electron current at 2200°C (Ta cathode) is approximately equal to 8 A/cm. This is the bottle neck for the electron current. 4- Cathode – Anode preliminary design

SPES Target Group Data…… SIMULATION STEP 1: Plasma generation (starting from BC1 e BC2) Beam extraction SIMULATION STEP 2: Laser-surface interaction and “calculated atom flux” Plasma generation (starting from BC1) Beam extraction BC1 electron flux BC2 atom flux (neutral) 5- Simulation activity with F3MPIC (CISAS)

SPES Target Group Data…… Taking into consideration this preliminary design, it is easy to observe that the electron current is controlled by the cathode temperature field (thermal electron emission). 2- The maximum electron current per unit length that can be transmitted from the cathode to the anode is approximately equal to 8 A/cm (the cathode, made of Ta, cannot sustain temperatures higher than 2200°C in a high vacuum environment). 6- Conclusions

SPES Target Group Data…… The time  needed for stripping to charge state “q” has to be calculated. 2- In our case, is the interaction time between ions and energetic electrons sufficient (bigger than  )? 3- Is F3PIC code capable to answer the aforementioned questions? 3- Dedicated experimental tests can be performed with the MK5 Ion Source (Ion Source currently in use at LNL), increasing the anode voltage up to 1 – 2 kV (a dedicated high voltage power supply has to be installed) and monitoring the beam current and the charge state of the ions (mainly Ar, Kr and Xe ions). 4- Other methods to increase the charge state of ions at high temperatures (to avoid sticking)? RF……? 7- Open problems and future developments

SPES Target Group Data…… THANKS FOR YOUR ATTENTION