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The laser - INFN-LNL

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The laser - INFN-LNL
INFN-CISAS collaboration
The Ablation Ion Source for
refractory metal ion beams:
some preliminary indications
Daniele Scarpa & the SPES target group
20/04/2012
1. Starting point
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
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)
Beam extraction
Daniele Scarpa & the SPES target group
20/04/2012
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
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
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. 6015 C.N.R.S. ‘‘Matériaux Céramiques et Traitements de surface’’ Faculte ´ des Sciences,
123 Avenue A. Thomas 87060 LIMOGES, France
PLUME EXPANSION:
Distribution function ≈ COS2N
Ion velocity ≈ 4000 m/s
Daniele Scarpa & the SPES target group
20/04/2012
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).
LASER:
KHzwaist
REPofRATE
By assuming a laser spot on the sample
with a10beam
150 µm (at a 1/e^(2) intensity drop) with a
14
corresponding area S=7 x 10^(4)
µm^(2)
the ablated material
related to the previously calculated value is
ATOM
PRODUCTION:
10 thickness
ATOMS/PULSE
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.
Daniele Scarpa & the SPES target group
20/04/2012
6. Focus on Boundary Condition n.2: the atom flux
EXPERIMENTAL SETUP: TOF design
Laser
Target
(ion
generation)
Extractor
Daniele Scarpa & the SPES target group
20/04/2012
Faraday cup
7. Focus on Boundary Condition n.2: the atom flux
EXPERIMENTAL SETUP: TOF simulation
Ion
Generation
CHARGE DISTRIBUTION:
Gaussian centered on 10+
Daniele Scarpa & the SPES target group
20/04/2012
TOF at
Faraday cup
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
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
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
11. Preliminary installation
Daniele Scarpa & the SPES target group
20/04/2012
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
11. First studies
Ablation: ionization measurements
Target
FC
Daniele Scarpa & the SPES target group
20/04/2012
11. First studies
ABSORBTION: plume definition
DIODE
Laser
Photodiode
Target
Daniele Scarpa & the SPES target group
20/04/2012
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