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During a recent experiment
published in the American journal Science, researchers from the
“Sources de particules produites par laser” (Laser-Generated
Particle Sources) team, led by Victor Malka at the “Laboratoire
d’optique appliquée” (Applied Optics Laboratory, LOA(1))
, brought to light a very effective new mechanism for producing high energy
electron source using compact lasers. Their work was conducted with French
researchers from the CEA (French Atomic Energy Authority), and from the
“Centre d'études nucléaires de Bordeaux Gradignan”
(Center for Nuclear Studies of Bordeaux Gradignan, CENBG(2))
as well as British researchers from Imperial College. It has enabled electrons
to be accelerated from 0 to 200 MeV(3)
in 1 mm. The particle beams(4) generated
in this way have particularly advantageous properties (shortness, energy,
emittance, charge).
In order to study the components of matter and the interactions between
them, particle physicists accelerate particles to very high energy levels.
This is achieved in accelerators by means of electric fields, whose amplitude
is limited by breakdown phenomena to a few tens of MeV per meter. In order
to reach the required energy levels, the accelerating lengths must be
enormous: of the order of a few kilometers, or even a few dozens of kilometers.
By contrast, a plasma, which is an ionized medium, can withstand electric
fields of several hundreds of GV/m(5) , i.e. over 10,000
times higher than those with conventional accelerators. The length required
to obtain a given energy gain is reduced in the same proportions: electrons
can thus be accelerated to several hundreds of MeV over distances of the
order of a few millimeters!
The experiments conducted at LOA used a Titanium Sapphire laser which
delivers pulses of one joule in 30 femtoseconds at the wavelength =0.82
micrometers, thus reaching a power level of 30 TW and an intensity on
the target of 2 x 1018W/cm2. The high shot rate
(10 shots per second), and the optical quality of the facility have made
it possible to achieve good characterization and optimization of the electron
beams whose total charge reaches a few nanocoulombs. By focusing the laser
beam onto a jet of helium gas, electrostatic waves of high amplitudes
have been excited to levels such that the plasma electrons are accelerated
to up to 200 MeV. The electron beam, which is high quality and of very
short in duration (emittance < 3πmm-mrad), is energy-spread (Maxwellian
spectrum up to 150 MeV). The electron source produced in this new forced-wake
regime is perfectly suitable for synchronization with an accessory laser.
It opens up possibilities for applications in the very near future, and
marks a watershed in the field of pulsed, ultra-short electron sources.
Such sources should make it possible to study new phenomena on ultra-short
time scales of the order of 100 fs(6) , i.e. 100 millionths
of a billionth of a second.
Beyond the field of plasma physics, these sources open up possibilities
for numerous applications in medicine (radiotherapy, proton therapy, production
of radio-isotopes), accelerator physics, nuclear physics, chemistry, and
biology. Today it is possible, also using a compact laser, to generate
promising proton beams. Using the same laser, LOA has already obtained
a proton source at 10 MeV. Encouraged by this initial result, the team
has set itself a new challenge: to achieve a proton source in the 200
MeV to 240 MeV range, in particular as part of a European project called
“PROPULSE,” which explores ways to treat cancer.
(1)The
LOA, a joint research unit run by CNRS, the Ecole nationale supérieure
de techniques avancées (ENSTA), and the Ecole polytechnique (X)
studies femtosecond laser sources; solid state physics; solid state physics
and optics; laser-matter interaction; lasers, plasmas, and X-rays; molecular
biology and femto-chemistry. http://wwwy.ensta.fr/loa/
(2)The "Centre d'études nucléaires"
(Nuclear Research Center), a joint research unit run by CNRS and Université
Bordeaux I, conducts research into the theoretical study of the structures
of nuclei and hadrons; very high spin nuclei; "exotic" nuclei;
neutrino physics; high-energy gamma astronomy; and innovative electro-nuclear
power generating systems: http://wwwcenbg.in2p3.fr
(3)MeV = megaelectronvolt
(4)beams of electrons, protons and other types of ion.
(5)GV/m = gigavolts per meter
(6)fs = femtosecond
PROPULSE:
a European consortium project for treating cancer using proton therapy
PROPULSE, Proton Therapy Assisted By Ultra-Intense Laser, brings together
36 European institutions and laboratories and partners from industry under
the auspices of the "Laboratoire d’optique appliquée"
(ENSTA/X/CNRS), the scientific coordinator. Proposed to Brussels under the
6th RTD-FP* , the project is aimed at producing a proton
beam from a laser beam, in order to treat cancer using proton therapy. The
project will require a new, higher-energy (>20J) laser to be built in
order to produce protons with energy of over 200 MeV for treating cancer
tumors (including intra-cranial tumors).
Ultimately, PROPULSE will lead to the design of machines that are highly
advantageous in terms of cost, weight, compactness, and flexibility, compared
with existing accelerators. The new machines can be installed in numerous
hospitals. Similar projects are being developed in the United States and
Japan. As an interdisciplinary program, PROPULSE combines physics, chemistry,
biology, and oncology, and gives considerable importance to technology transfer,
from basic research toward industry and small businesses.
*The European Union's 6th RTD-FP, Research
and Technological Development Framework Program covers the years from
2002-2006.
Researcher
contact:
Victor Malka
Laboratoire d’optique appliquée
Tel : +33 1 69 31 99 03
e-mail : victor.malka@ensta.fr
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