1. INFN-CISAS collaboration
The Ablation Ion Source for refractory metal ion beams: some preliminary indications
Daniele Scarpa & the SPES target group
20/04/2012

2. 1. Starting point Daniele Scarpa & the SPES target group 20/04/2012
07/03/2012
Ciao a tutti,
durante la videoconferenza di oggi sono stati definiti i seguenti step di simulazione per lo studio della nuova sorgente per fasci metallici n+:
- step 1 di simulazione con codice CISAS attuale
- step 2 di simulazione con codice da implementare per l’interazione laser-materia
Per quato riguarda lo step 1 io e Daniele Scarpa saremo in grado di fornire delle indicazioni sufficientemente dettagliate per la geometria e le condizioni al contorno (BC1 e BC2 della slide) necessarie per partire con la simulazione vera e prepria. Nel frattempo speriamo di ricevere delle indicazioni dal gruppo di ricerca contattato da Daniele Pavarin per quanto riguarda le sezioni d’urto elettroni-Ta, W, Mo e, se possibile, altri metalli.
Verso i primi giorni di Aprile io e Daniele Scarpa saremo in grado di fornire le info sopra indicate: coglieremo l’occasione per programmare l’attività di simulazione tenendo conto ovviamente della disponibilità offerta da Davide e dal gruppo CISAS.
Per qualsiasi dubbio/questione possiamo sentirci via mail o via skype come oggi.
Grazie a tutti,
buon proseguimento,
Mattia
Daniele Scarpa & the SPES target group
20/04/2012

3. 1. Starting point BC1 electron flux BC2 atom flux (neutral)
SIMULATION STEP 1:
Plasma generation (starting from BC1 e BC2)
Beam extraction
SIMULATION STEP 2:
Laser-surface interaction > atom flux
Plasma generation (starting from BC1)
Daniele Scarpa & the SPES target group
20/04/2012

4. The laser – material interaction and plume expansion
2. Focus on Boundary Condition n.2: the atom flux
The laser – material interaction and plume expansion
Refractory metal vaporized by laser ablation
Laser beam
Material plume
Extraction
Expansion volume
Daniele Scarpa & the SPES target group
20/04/2012

5. 3. Focus on Boundary Condition n.2: the atom flux
REFERENCE 1: Literature studies on energy deposition
Spectrochimica Acta Part B 58 (2003) 1867–1893
Laser ablation for analytical sampling: what can we learn from modeling?
Annemie Bogaerts , Zhaoyang Chen , Renaat Gijbels , Akos Vertes
University of Antwerp, Department of Chemistry, Universiteitsplein 1, Wilrijk-Antwerp, B-2610, Belgium and
George Washington University, Deparment of Chemistry, Washington, DC 20052, USA
LASER POWER THRESHOLD:
< 1010 W/CM2
IN THE ABLATION POINT
Daniele Scarpa & the SPES target group
20/04/2012

6. 4. Focus on Boundary Condition n.2: the atom flux
REFERENCE 2: Literature studies on expansion plume
J. Appl. Phys., Vol. 83, No. 10, 15 May 1998
Monte Carlo simulation of the laser-induced plasma plume expansion under vacuum: Comparison with experiments
F. Garrelie, J. Aubreton, and A. Catherinot
E.S.A C.N.R.S. ‘‘Matériaux Céramiques et Traitements de surface’’ Faculte ´ des Sciences,
123 Avenue A. Thomas LIMOGES, France
PLUME EXPANSION:
Distribution function ≈ COS2N
Ion velocity ≈ 4000 m/s
Daniele Scarpa & the SPES target group
20/04/2012

7. ASSUMPTION: presented FIRB2012 ATOM PRODUCTION: 1014 ATOMS/PULSE
5. Focus on Boundary Condition n.2: the atom flux
ASSUMPTION: presented FIRB2012
A rough evaluation of the required laser performances can be done as follows. Let us consider a refractory material such as Tantalum, widely used in evaporation ovens. Due to its high boiling temperature, Tantalum is quite toughly evaporated in traditionally used ovens. By assuming as a general goal in terms of high-charge-state ion (Ta(20+)) current generation at the output of the whole apparatus a value in the order of 1 µA, the equivalent singly-ionized tantalum ion current is 50 nA. This current is produced by an laser-induced atom removal rate of about 3 x 10^(11) atoms/s. By considering an ionization efficiency in the order of 10^(-6) and a geometrical coupling efficiency to the ionization chamber of 0.25 (see module A) the required laser ablation rate has to be 1.2 x 10^(18). Given the Tantalum atomic weight (3 x 10^(25) kg), the laser ablation rate should be about 3.6 x 10^(-7). By using the value of the Tantalum density (16.7 x 10^(3) kg/m^(3)), the volume ablation rate can be estimated as 2.2 x 10^(-2) mm^(3). By assuming a laser repetition rate of 10 kHz, the material volume to be ablated by a single pulse has to be 2.2 x 10^(-3) µm^(3).
By assuming a laser spot on the sample with a beam waist of 150 µm (at a 1/e^(2) intensity drop) with a corresponding area S=7 x 10^(4) µm^(2) the ablated material thickness related to the previously calculated value is pretty modest and approximately equal to 30 nm. It is reasonable and conservative to assume that this material removal thickness could be achieved with a laser pulse with a peak intensity in the order of 1 GW/cm^(2), close to the plasma generation threshold. Indeed plasma generation strongly reduces the laser absorption efficiency of the material itself due to a plasma shielding effect. A peak intensity of 1 GW/cm2 could be obtained on a laser spot S if the peak power is about 700 kW. By assuming to use a pulsed laser source with 3.5-ns-long pulse duration, the corresponding pulse energy is about 2 mJ, and consequently at a repetition rate of 10 kHz the average output power is 20 W.
LASER: 10 KHz REP RATE
ATOM PRODUCTION: 1014 ATOMS/PULSE
Daniele Scarpa & the SPES target group
20/04/2012

8. EXPERIMENTAL SETUP: TOF design
6. Focus on Boundary Condition n.2: the atom flux
EXPERIMENTAL SETUP: TOF design
Laser
Target
(ion generation)
Extractor
Faraday cup
Daniele Scarpa & the SPES target group
20/04/2012

9. EXPERIMENTAL SETUP: TOF simulation
7. Focus on Boundary Condition n.2: the atom flux
EXPERIMENTAL SETUP: TOF simulation
Ion Generation
TOF at Faraday cup
CHARGE DISTRIBUTION:
Gaussian centered on 10+
Daniele Scarpa & the SPES target group
20/04/2012

10. 8. Focus on Boundary Condition n.2: the atom flux
EXPERIMENTAL SETUP: TOF simulation
CHARGE DISTRIBUTION => TOF distribution
Evaluation on ablated material amount => atom ablated
Daniele Scarpa & the SPES target group
20/04/2012

11. 9. Conclusions
1- 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).
3- Electrons and ions plasma performs shield effect preventig laser beam to reach target material.
4- Ions escape with 4000 m/s velocity from the target, time to provide ionization by electron interaction is reduced ( ≈ 60 µs )
Daniele Scarpa & the SPES target group
20/04/2012

12. 10. Open problems and future developments
1- The time of interaction between the electrons and the atoms/ions is probably not sufficient to allow high charge state ionization.
2- Dedicated experimental tests will be performed with the MK5 Ion Source (an 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).
3- Dedicated experimental tests will be performed with experimental chamber in order to define atom production for Ta with available lasers.
Daniele Scarpa & the SPES target group
20/04/2012

13. 11. Preliminary installation
Daniele Scarpa & the SPES target group
20/04/2012

14. 11. Preliminary installation
Prototype chamber:
Rough and high vacuum pump
Optical windows
TOF (to be build) HV
Vacuum-meters
1064 optics
Diode laser for absorption
Laser:
308 nm Excimer laser
Nd:YAG laser (to be delivered)
Electronic:
Oscilloscope
Dedicated measurement system
Daniele Scarpa & the SPES target group
20/04/2012

15. Ablation: ionization measurements
11. First studies
Ablation: ionization measurements FC
Target
Daniele Scarpa & the SPES target group
20/04/2012

16. ABSORBTION: plume definition
11. First studies
ABSORBTION: plume definition
DIODE
Laser
Photodiode
Target
Daniele Scarpa & the SPES target group
20/04/2012