Download Observation of a collimated bunch of high

Observation of a collimated bunch of high-energy electrons
produced by a 35-fs laser pulse propagating in an exploding-foil
plasma
D. Giulietti, M. Galimberti, A. Giulietti, L. A. Gizzi and P. Tomassini
Intense Laser Irradiation Laboratory, IFAM, Area della Ricerca CNR, Via Moruzzi 1, 56124
Pisa, Italy
M. Borghesi
Department of Pure and Applied Physics, The Queen's University, Belfast, BT7 1NN, UK
V. Malka and S. Fritzler
Laboratoire pour l'Utilisation des Lasers Intenses, Palaiseau, Cedex, France
M. Pittman and K. Taphouc
Laboratoire d'Optique Appliquee, ENSTA, Chemin de la Huniere F - 91761 Palaiseau Cedex,
France
(April 17, 2001)
Abstract
A very collimated bunch of high-energy electrons was emitted along the laser
axis by focusing a super-intense femtosecond pulse in an under-dense plasma.
The density of the plasma, pre-formed with the exploding foil technique by
the amplied spontaneous emission preceding the 35 fs pulse, was mapped
also
at Dipartimento di Fisica Universita di Pisa, Unita INFM, Via Buonarroti 2, 56100 Pisa
Italy
1
by Nomarski interferometry. The experimental data strongly suggest that
resonant conditions for laser wakeeld acceleration have been achieved. These
conditions were found to depend critically on the experimental parameters.
52.38.Kd 52.38.Ph 52.50.Jm
Typeset using REVTEX
2
The recent availability of relativistic intensities from femtosecond lasers based on Chirped
Pulse Amplication (CPA) has opened new horizons in laser-matter interaction studies [1].
At such intensities the laser electric eld overcomes by several orders of magnitude the atomic
one so that ionization occurs in a fraction of the laser wave cycle [2]. In this laser-matter
interaction regime several new phenomena have been discovered and promising applications
foreseen [3] [4]. In particular the Laser Wakeeld Acceleration (LWA) of charged particles in
plasmas, as rst predicted by T. Tajima and J.M. Dawson as early as 1979, became possible.
In that paper [5] two physical mechanisms for electron acceleration in plasmas were identied.
Both mechanisms were based on the enormous longitudinal electric elds of the plasma
waves excited either by two lasers beating resonantly with the plasma frequency (Plasma
Beat Wave Accelerator) or by propagation of a short pulse in an appropriate density plasma
(LWA). More recently, physical mechanisms have been proposed in which the electrons are
directly accelerated by the super-intense laser electric elds in the presence of auto-generated
quasi-static magnetic elds (Direct Laser Accelerators) [6] [7].
The LWA mechanism becomes possible now with powerful CPA pulses. However CPA
pulses are typically aected by a prepulse due to Amplied Spontaneous Emission (ASE)
in the nanosecond time scale, which can play an important role in laser-matter interaction
experiments [8]. In the experiment presented in this letter, as in a previous, recent one
[9] [10], we used the ASE in order to produce, by means of the exploding-foil technique, a
pre-formed plasma suitable for achieving LWA condition with a 35 fs pulse. The exploding
foil-technique driven by ASE irradiation allows the controlled interaction of ultra-short laser
pulses with pre-formed plasmas to be achieved.
The control of the plasma density, size and homogeneity is basic for the LWA mechanism.
In fact the near-resonant condition for the growth of the plasma wave in the wake of the
ultra-short pulse requires c ' p =2 [5], or n[cm 3] ' 3 10 9 = [s]2 where is the laser pulse
duration, p is the wavelength of the plasma wave and n is the electron density. In the case
of our experimental conditions, being ' 35 fs, we need n ' 2:5 1018 el=cm3.
In general, the study of laser acceleration of plasma electrons can be performed also with
3
a dierent experimental approach. In particular, the interaction of ultrashort pulses with
gas-jets provided recently evidence of forward electron acceleration. In one case the energetic
electrons were produced with pulses of hundreds of femtoseconds interacting with a subsonic
gas jet [7] and the eect was attributed to direct laser acceleration [6]. More recently,
energetic electrons were obtained in the interaction of 35 fs pulses with a supersonic gas jet
and attributed to self-modulated laser wakeeld instability [11]. In the latter experiment
the laser parameters were very similar to the one of the present experiment. Nevertheless,
we point out that the dierent approach based on the exploding-foil technique allowed for
the rst time the plasma conditions of the CPA interaction region to be fully characterised.
In this letter we present experimental data concerning a very collimated bunch of highenergy electrons generated under LWA condition in an exploding-foil plasma which, to our
knowledge are the rst to be reported. A detailed plasma density mapping of the pre-formed
plasma was obtained with an unprecedented temporal resolution [12]. These measurements
allowed us to verify that the experimental conditions required for the growth of high amplitude relativistic plasma waves in the wake of a super-intense 35 fs laser pulse were indeed achieved. We measured the angular distribution and energy spectrum of accelerated
electrons. We also found that, moving away from the LWA near-resonant conditions, the
collimated emission of energetic electrons disappeared.
The experimental set up is shown in g.1. The 1 J in 35 fs, 815 nm linear-polarised (p
on target) beam of the Ti:Sapphire LOA laser was focused by an o-axis f=5 parabola on
a 5 m FWHM Gaussian focal spot, at an intensity up to I 1020 W=cm2. The
CPA/ASE intensity contrast ratio was 106 . A second beam (100 mJ) originating from the
same Ti:Sapphire oscillator was frequency doubled using a 2 mm thick KDP crystal and was
used as a probe beam for plasma Nomarsky interferometry. The probe timing was synchronized with the main CPA pulse within a fraction of a ps. Details of this interferometric
measurements will be described in a forthcoming paper [12].
Energetic electrons have been detected using an electron beam analyzer (SHEEBA) consisting of a stack of GAFCHROMICtm lms [13] [14](see g.1). A laser beam stop consisting
4
of a 10 m Aluminium foil was used in front of the rst lm to prevent direct laser exposure
or damage of the lms. A 1:5 mm thick Aluminium lter was placed between the 6th and
7th layer of GAFCHROMICtm to identify electron components at very high energy. These
lms provide a straightforward measurement of the angular distribution of the electrons.
Fig.2 shows the result obtained by focussing the CPA pulse at an intensity of 1020 W=cm2
on a 1 m thick formvar foil. In our conditions, foils of such a thickness are expected to
give, at the time of arrival of the CPA pulse, a pre-formed plasma of a maximum density
of approximately 1019 el=cm3. According to g.2 only the small central feature is visible
up to 7th layer, while the surrounding ring-like feature was basically stopped by the rst
layer. These images immediately show that besides an intense electron ux of relatively
lower energy in the ring-like structure of ring 15Æ aperture, there is a bunch of much
more collimated (divergence bunch < 1Æ) and energetic electrons inside the ring, close to
the laser propagation axis.
In our experimental conditions, due to the temporal and radial prole of the short laser
pulse, the longitudinal and radial contributions to the electron density perturbation in the
wake of the pulse are comparable [15] [16]. Therefore, the ring-like structure could be
explained by the premature escape of part of the electrons from the acceleration region,
while the remaining electrons are accelerated over a longer distance by the longitudinal eld
along the beam axis. Moreover, we point out that ring is very close to the aperture of the
transmitted laser radiation cone. This suggests that the spotty structure visible in the ring
may be correlated to scattered laser light and second harmonic emission observed in similar
experimental conditions [9]. This scattering can be attributed to instabilities growing in
marginal regions of the pre-formed plasma [17]. It is important to observe that, without
target, no signal was detected on any of the GAFCHROMICtm layers. This test conrms
that the electrons are not generated by the laser radiation impinging on the laser beam stop
foil (see g.1).
Fig.3(left) shows an interferogram of the preformed plasma obtained 30 ps after the
arrival of the CPA pulse. By changing the probe timing it was veried [12] that no eect
5
of the CPA pulse propagation could be detected by the interferometry within a few tens of
ps after the CPA interaction. Therefore, the interferogram of g.3 is representative of the
preformed plasma conditions at the time of arrival of the CPA. The interferogram shows
that a quite homogeneous and well under-dense plasma exists at the time of the interaction,
while in the plasma marginal regions the second harmonic emission indicates the presence
of important density gradients as well as of a substantially higher electron density [9] [18].
The laser transmission was found to be rather high, with a spectrum of the transmitted light
very similar to the laser spectrum. These observations show the same interaction scenario
as in previous works [9] [10], indicating that the CPA pulse propagates through the plasma
without signicant perturbations.
Fig.3(right) shows a contour plot of the plasma density obtained by Abel inversion of the
probe de-phasing map measured from the interferogram of g.3, using a technique based on
Continuous Wavelet Transform [19]. According to this density map we can see the plasma
density in the interaction region ranges from 1018 el=cm3 to 1019 el=cm3. As discussed above,
the optimum density for LWA falls within this range. The contour plot also shows a low
density (ne =nc 10 3 ) channel-like feature along the laser axis. This is a typical feature of
exploding-foil plasmas that is always seen experimentally and in 2D hydro-simulations [20].
We can not exclude that this density depression along the laser axis could play an important role in the acceleration process of the most energetic and collimated electrons (see g,2).
In fact several experimental observations [7] [21] [22] [23] and sophisticated 3D simulations
[24] indicate that the presence of a channel could enhance the acceleration mechanisms.
The energy of the electrons accelerated in the forward direction was measured using
an electron spectrometer based on an electro-magnet coupled with a set of photodiodes
(Surface Barrier Detectors). This spectrometer has been used in a previous experiment and
is described elsewhere [11] [25]. Although the spectrometer allows to analyse electrons up
to 200 MeV, our measurements were limited to 36 MeV due to the photodiodes detection
threshold. Fig.4 shows the energy spectrum of the forward emitted electrons in an angle
of spec 0:6Æ along the propagation axis of the laser. The spectrum of g.4 shows that
6
there is a sizeable number of electrons up to energies of several tens of MeV. The energy
distribution is strongly depressed at the lower energies side due to the electron absorption
by the thin quartz plate 100 m thick, used to collect the transmitted laser radiation (see
g.1). Due to the very small aperture of the entrance collimator, these data do not exclude
the possibility that much more energetic electrons could be produced out of such an angle,
as those strongly collimated we detect in the small central spot of GAFCHROMICtm lm
(see g.2).
The maximum energy of the electrons detected by the spectrometer was Uexp 40 MeV.
This value is consistent with the value obtained simply by multiplying the maximum electric
eld of the plasma wave Emax = (m c !p )=e [26] by the electron charge e and the actual
acceleration length L: Umax = m c !p L, i. e. Umax[eV] n[cm 3]1=2 L[cm]. In fact, with
our experimental parameters: n 5 1018 el=cm3 and L 200 m, one nds Umax 45 MeV.
In our conditions L is denitely shorter than the \dephasing length" Ldeph ' p2 p, being
p = !=!p the Lorentz factor of the wave [5]. Consequently much higher electron energy
could be in principle attained with a longer acceleration length.
We also measured the charge of the electrons emitted forward in a cone of coll 7Æ
aperture along the laser axis, using a charge collector placed behind the target (and behind
the quartz thin plate). We found a total charge of 0:2 nC, corresponding to 109 electrons.
Since the electrons in the ring ( ring 15Æ) were not collected and only the more energetic
electrons could cross the thin plate, this number can be considered as a representative value
of the number of electrons accelerated along the laser axis (bunch < 1Æ ).
The whole of the experimental results and in particular those reported in g.2 and g.4
strongly suggest that we are in the LWA regime. In fact, when near-resonance conditions
for the LWA where disrupted ad hoc, electron emission in the forward direction as described
above was no longer observed. In particular, when the plasma density, even if lower than
the critical one, was substantially higher than that required for the LWA, the collimated
bunch of energetic electrons was not detected. Also when the laser pulse was stretched to
150 fs, again we observed no collimated energetic electrons.
7
In conclusion, we have shown experimentally for the rst time that the propagation of a
few-tens of femtosecond laser pulse in a pre-formed plasma can accelerate to ultra-relativistic
energies a signicant number of electrons in an extremely collimated bunch propagating
forward. Our results consistently suggest that LWA is the leading mechanism for such a
process.
We acknowledge fruitful discussions with Emilio Picasso, Vincenzo Flaminio, Vincenzo
Cavasinni and Francois Amirano on the dierent mechanisms of particle acceleration in
plasmas potentially interesting for conventional accelerators. We are very grateful to the
technical sta of IFAM, in particular to Alessandro Barbini for assistance in data acquisition,
and to Antonella Rossi, from the IFAM Target Preparation Laboratory, for the special thin
foil targets, essential to the success of this experiment. We acknowledge also the eectiveness
of the Laboratoire d'Optique Appliquee laser sta. This experiment has been partially
supported by European Commission in the frame of Human Potential Programme, Access
to Research Infrastructures.
8
REFERENCES
[1] M. Perry and G. Morou, Science 264, 917 (1994).
[2] S.Augst et al., Physical Review Letters 63, 2212 (1989).
[3] C. Joshi and P. Corkum, Physics Today 36, (1995).
[4] T. Dimitre et al., Nature 386, 54 (1997).
[5] T. Tajima and J. M. Dawson, Physical Review Letters 43, 267 (1979).
[6] A. Pukhov, Z. Sheng, and J. Meyer ter Vehn, Physics of Plasmas 6, 2847 (1999).
[7] C. Gahn et al., Physical Review Letters 83, 4772 (1999).
[8] D. Giulietti et al., Physical Review Letters 79, 3194 (1997).
[9] M. Galimberti et al., Laser and particle beams (2001), in press.
[10] D. Giulietti et al., Physical Review E (2001), sudmitted.
[11] V. Malka et al., Physical of Plasmas (2001), accepted.
[12] M. Borghesi et al., in preparation.
[13] W. L. McLaughlin et al., Radiation Protection Dosimetry, Nuclear Technology Publishing 6, 263 (1996).
[14] N. V. Klassen and L. Van Der Zwan, Medical Physics 24, 1924 (1997).
[15] J. R. Marques et al., Physics of Plasmas 5, 1162 (1998).
[16] F. Amirano et al., Physical Review Letters 81, 995 (1998).
[17] S. V. Bulanov et al., Plasma Physics Report 22, 600 (1995).
[18] A. Giulietti et al., Physical Review Letters 63, 524 (1989).
[19] P. Tomassini et al., Applied Optics (2001), submitted.
9
[20] M. Borghesi et al., Physical Review E 54, 6769 (1996).
[21] X. Wang et al., Physical Review Letters 84, 5324 (2000).
[22] L. A. Gizzi et al., Laser and particle beams (2001), in press.
[23] S. Y. Chen et al., Physics of Plasmas 6, 4739 (1999).
[24] A. Pukhov and J. Meyer ter Vehn, Physical Review Letters 76, 3975 (1996).
[25] V. Malka, Utilisation du spectrometre a electrons a 200 MeV, Note LULI - LOA, (1998).
[26] J. M. Dawson, Physical Review Letters 113, 383 (1959).
10
FIGURES
FIG. 1. Experimental set up. Also shown in detail is the electron beam analyzer (SHEEBA)
based on a stack GAFCHROMICtm lms and Al lters. A 10 m Aluminium foil is used in front
of the rst lm to prevent direct laser exposure/damage of the lm stack. A 1:5 mm Al lter is
placed between the 6th and 7th layer. As shown in the gure, the electron beam analysis using
SHEEBA was alternative to the electron magnetic spectrometer and to the optical spectrometer.
FIG. 2. Typical GAFCHROMICtm result obtained from the laser-plasma interaction at
I 1020 W=cm2 . The reference graph indicates the half-aperture angles as measured from the
axis. The rst lm is placed at a distance of 26 mm from the interaction region. The divergence
of the ring-like feature is ring 15Æ, while that of the central spot is bunch < 1Æ.
FIG. 3. left) Plasma interferogram taken 30 ps after the arrival of the CPA pulse. The arrows
indicate the target position. The replica of the image is typical of the Nomarsky interferometry
technique. The white spot is produced by second harmonic emission at 90Æ, from marginal regions
of the plasma. right) Electron density map corresponding to the dotted region of the interferogram
of g.2a.
FIG. 4. Energy spectrum of the electrons emitted forward in a cone of spectro 0:6Æ aperture
along the laser axis. The dashed band refers to the detection threshold.
11
to optical spectrometer
100µm quartz window
off−axis parabola
target
charge detector
to electron
0.6°
e−
GAFCHROMICtm layers
1 2 3 4 5 6 7
SHEEBA
prob
e
lase
r
spectrometer
e−
26mm
laser beam stop (Al 10µm)
Al 1.5 mm
1
8°
4°
2
3
4
5
6
7
Laser
50 µm
T
T
9
Number of electrons/MeV/sr
10
8
10
Detection threshold
7
10
0
5
10
15
20
25
30
Electron energy (MeV)
35
40