Laser-Driven Ion Acceleration In A Hybrid RPA

Laser-Driven Ion Acceleration In A Hybrid RPA-TNSA Regime
D. Gwynne,1 S. Kar,1 H. Ahmed,1 M. Cerchez,2 D. Doria,1 J. Fernandez,3, 4 R. Gray,5 J. Green,4 F. Hanton,1 D. Maclellan,5 P. McKenna,5 Z. Najmuddin,6 D. Neely,4 J.A. Ruiz,3 M. Streeter,6 M. Swantusch,2
O. Willi,2 M. Zepf 1 and M. Borghesi.1
1) Centre for Plasma Physics, School of Mathematics and Physics, Queen's University Belfast. 2) Institut für Laser-und Plasmaphysik, Heinrich-Heine-Universität, Düsseldorf, Germany. 3) Instituto de Fusion Nuclear, Universidad Politecnica de Madrid, Spain. 4)
Central Laser Facility, Rutherford Appleton Laboratory, Didcot, Oxfordshire. 5) Department of Physics, SUPA, University of Strathclyde, Glasgow. 6) Imperial College, London.
The production of proton and ion beams during ultraintense laser interactions with thin foils is a subject which has recently received a great deal of attention from both theoretical and experimental points of view [1]. Laser-driven ion acceleration has the
potential to be a cost effective, compact alternative to conventional acceleration sources for scientific, technological and health care applications [2]. The LS regime of RPA is particularly promising for accelerating protons and ions more efficiently as it has a
favourable dependence on the laser fluence Ƒ [3–6], and produces similarly efficient acceleration for both protons and higher mass ions, as predicted by numerous computational and analytical studies [2].
1. INTRODUCTION
TNSA beam characteristics pose significant limitations for many of the envisaged applications [2-3]. By contrast, ion production by the radiation pressure of intense lasers has been predicted to be a
promising route for accelerating large numbers of ions quasi-monoenergetically to the GeV per nucleon range in a more efficient manner to TNSA [2-3].
Surface contaminants
Laser
accelerates
electrons (MeV)
•
eLASER
•
Fast electrons travel
through the target
Fast electrons create
a Debye sheath at
the rear of the
target.
Ions pile up
at the rear
surface of
the target
before the
end of the
laser pulse
The E-field at
discontinuity (~1012
V/m) accelerates the
ions to multi MeV
energies.
LE1
Pinhole
Magnets
LE2
RCF
B
E
C6+
V0
P+
Electric plates
O7+
Figure 3: Schematic of the experimental setup.
LB1
𝑺𝑩 = 𝒓𝑳 −
LB2
𝒓𝟐𝑳
−
𝑳𝟐𝑩𝟏
+
𝒒𝑬𝑳𝑬𝟏 𝑳𝑬𝟏
𝑺𝑬 =
+ 𝑳𝑬𝟐
𝟐
𝟐
𝒎𝒗𝟎
𝑳𝑩𝟏 𝑳𝑩𝟐
𝒓𝟐𝑳 − 𝑳𝟐𝑩𝟏
𝒓𝟐𝑳
4. RESULTS
•
TP0: Z/A = 1
TP0: Z/A = 0.5
1.E+13
The M and Z of the species to be traced are entered and the
code generates a trace line, according to the values of the E
and B fields. The data is obtained by selecting a region
enclosing the traced species, subtracting a background
region of the same size, then the signal and energy
spectrum are calculated. The distribution can be viewed
immediately, thus allowing a visualisation of the
experimental results before detailed analysis is carried out.
OAP, f/3
TCC to pinhole
TP3: 1855±5 mm
TP0: 1865±5 mm
TP-3: 1917±5 mm
Figure 6: Spectra obtained from 50nm Au
irradiated by a 𝐼0 ~3 × 1020 W cm-2, 1.00
ps pulse.
dN/dE (in MeV/nucleon/sr
•
If 𝑙 > evanescence
length of the
ponderomotive
force, the whole
layer is cyclically
accelerated
Figure 2: Radiation Pressure Light Sail Acceleration.
3. DATA ANALYSIS
The data from the TPs was analysed using a MATLAB GUI
TPspec, which reads in images scanned from the image
plates of the TPs.
Laser axis
Target normal
𝑙
Figure 1: Target Normal Sheath Acceleration.
•
The experiment was conducted employing the VULCAN laser system at the Rutherford Appleton Laboratory,
UK. The laser delivered ~53 to 367 J in pulses of 1.00-5.90 ps. The 𝐼𝑜 of the laser varied from 3 × 1019 to
3 × 1020 W cm-2 and targets of varying 𝑙 and composition were irradiated. The ions produced were diagnosed
by three Thompson Parabola (TP) spectrometers.
Figure 4: Schematic
of a TP and the
equation set used
by TPspec, rL is the
Larmour radius.
𝒓𝟐𝑳 − 𝑳𝟐𝑩𝟏
TP-3: Z/A = 1
TP-3: Z/A = 0.5
•
1.E+12
1.E+11
1.E+10
1
10
100
Figure 7: Spectra obtained from 50 nm Cu
irradiated with a 𝐼0 ~8.9 × 1019 W cm-2, 3.05
ps pulse.
Narrow band features can be clearly seen in the ion
spectra. The highest energy peak for protons and
carbon ions were calculated to be 18±2 and 7.4±0.5
MeV/nucleon respectively .
To assess the influence of RP effects on the spectral
profiles, a simple theoretical model was used [2],
which estimates the peak energy of the ion will scale
as 𝜀 2 , for thin targets, where 𝜀 is the dimensionless
fluence parameter ≈ 𝑎02 𝜏𝑝 𝜒 [2]. This was calculated
for each laser shot where 𝐼0 is the peak laser
intensity, 𝜆; laser wavelength (1.054 μm), 𝑡𝑝 ; laser
pulse duration, 𝑙, 𝜌; target thickness and density, 𝑚𝑝;
proton mass, and 𝑛𝑐 ; critical electron density
(1 × 1021 cm-3).
4.E+13
dN/dE (in MeV/nucleon/sr)
Pre-plasma
Metal foil
e-
Z+
Metal foil
Detector
•
•
2. EXPERIMENTAL SETUP
TP-3: Z/A = 1
TP0: Z/A = 1
4.E+12
4.E+11
2.50
Energy/nucleon (MeV)
Energy/nucleon (MeV)
𝒂𝟎 = 𝟎. 𝟖𝟓 𝑰𝟎 𝝀𝟐 𝟏𝟎𝟏𝟖 𝑾𝒄𝒎−𝟐
20
O7+ O6+
C4+
Z/A = 1
Z/A = 0.5
Z/A = 0.33
Previous Data
Theoretical Model
18
16
14
𝝌 = 𝝆′ 𝒍 𝝀
𝝆′ = 𝝆 𝒎𝒑 𝒏𝒄
10
•
2
8
6
4
2
0
100
a02τp/χ
1000
The peak energy scaled as 𝜀 2 as expected. Numerous
data points lie to the left hand side of the graph,
which may have been caused by self-induced
transparency.
Figure 8: Peak energy against 𝑎02 𝜏𝑝 𝜒 for TP0 (a), TP-3 (b).
The black line is the theoretical fit and the green data points
are from Henig et al [7].
Z/A = 1
Z/A = 0.5
Z/A = 0.33
Previous Data
Theoretical Model
18
16
14
1
(b)
12
10
2
8
6
4
2
0
10
100
a0
2τ
p/χ
1000
5. CONCLUSIONS
Figure 5: MATLAB GUI TPspec with an example of an image plate showing traced proton, carbon and oxygen tracks.
•
[5] A. Macchi, S. Veghini, T. V Liseykina, and F. Pegoraro, New Journal of Physics 12, 045013 (2010).
[6] A. P. L. Robinson, M. Zepf, S. Kar, R. G. Evans, and C. Bellei, New Journal of Physics 10, 013021 (2008).
[7] A. Henig et al., Physical Review Letters 103, 245003 (2009).
1
12
10
[1] M. Borghesi et al., Physical Review Letters 92, 055003 (2004).
[2] S. Kar et al., Physical Review Letters 109, 185006 (2012).
[3] K. F. Kakolee et al., Central Laser Facility Annual Report 2010-2011 (2011).
[4] A. Macchi, S. Veghini, and F. Pegoraro, Physical Review Letters 103, 085003 (2009).
𝝉𝒑 = 𝒄𝒕𝒑 𝝀
(a)
20
Energy per Nucleon (MeV per nucleon)
C5+
Energy per Nucleon (MeV per nucleon)
P+
C6+
25.00
Narrow-band features in the ion spectra consistent with a regime in which RPA-LS overcomes TNSA have been observed. The ion energy peaks achieved are encouraging, yet they
are still a long way from the crucial milestone of 100 MeV/nucleon for the envisaged applications. The ion energy can be enhanced by increasing the fluence of the laser and/or
reducing the target areal density; with current development in laser and target fabrication technology this does not seem entirely out of reach in the near future. The authors
acknowledge funding from EPSRC (EP/E048668/1, EP/E035728/1 - LIBRA consortium, EP/J500094/1 and EP/J002550/1 - Career Acceleration Fellowship held by S.K.)