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IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 33, NO. 6, DECEMBER 2005
Laser Ion Sources
Boris Sharkov and Richard Scrivens
Invited Paper
Abstract—Laser ion sources (LIS) are capable of delivering short
pulses of highly charged ions of almost any element, with high intensity. In this paper, the basic processes of the laser interaction
with plasma generated from a solid target will be discussed. The application of the laser–plasma theory will be applied to the subsystems of the LIS, and some examples of ion beam properties given,
with a more detailed description of the implementation of a LIS at
ITEP, Moscow. Finally the parameters for a LIS for high-current,
low charge-state, long pulse operation will be discussed, along with
transverse magnetic confinement, and the possibilities for injecting
a laser plasma into an electron cyclotron resonance ion source.
Index Terms—Highly charged ions, ion beam applications, ion
beams, ion sources, laser applications, particle beams, plasma generation, plasma heating.
I. INTRODUCTION
T
HE LASER ion source (LIS) is based on plasma generation by a high-power laser beam focused by a mirror
or lens system onto a solid target made of the material to be
ionized. The first two detailed proposals to use laser-produced
plasma as a source of ions for particle accelerator injection were
made independently by Peacock and Pease [1] and by Byckovsky et al. [2], both in 1969. The high charge state ions were
found to expand as a jet emitted perpendicular to the target surface with a narrow conical angle of 20 –30 [3].
The first application of a laser-produced plasma as a source of
ions for injection into a high energy synchrotron was done at the
10-GeV synchrotron at the Joint Institute for Nuclear Research
(JINR), Dubna, Russia, in 1977 [4]. Most of this development
[5].
used light or medium mass ions up to chromium
LISs for Van-de-Graaf accelerators have been employed at
the Technical University of Munich [6], Munich, Germany,
and at the Institute for Theoretical and Experimental Physics
(ITEP), Moscow, Russia [7], since 1988. The first successful
attempt to match a LIS to the high-current MAXILAC radio
frequency quadrupole (RFQ) accelerator was done at GSI,
Darmstadt, Germany, in 1994 [8]. Direct injection of carbon
ions into an 80-MHz RFQ was done using a 4-J, 40-ns,
laser at Tokyo Institute of Technology, Tokyo, Japan, in 2002
[9]. At the present time, LISs based on the use of repetitively
lasers are used routinely at the Laboratory of High
pulsed
Manuscript received February 25, 2005; revised August 10, 2005.
B. Sharkov is with the Institute for Theoretical and Experimental Physics
(ITEP), 117259, Moscow, Russia (e-mail: [email protected]).
R. Scrivens is with the AB Department, European Organization for Nuclear
Research (CERN), CH-1211 Geneva 23, Switzerland.
Digital Object Identifier 10.1109/TPS.2005.860080
Energies, JINR, and at ITEP. A new generation of LISs has
been developed and tested for the large hadron collider (LHC)
at CERN [10], and for the ITEP-terawatt accumulator (TWAC)
accelerator facility at ITEP-Moscow [11]. Futhermore, commercial pulsed solid-state lasers have been used to produce
fully stripped carbon ions at relatively low power densities [12].
Resonant LISs, for the selective production of ions, are covered in [13].
II. BASICS OF LASER PLASMA PHYSICS
Plasma electrons are heated by the intense laser radiation up
to temperatures of several hundred electronvolts. Energy absorption is by the inverse Bremsstrahlung mechanism, a classical absorption process due to the scattering of plasma electrons, accelerated in the light wave, by plasma ions [14]. The
absorption coefficient (ratio of absorbed laser energy to the energy of the incident laser radiation) is determined by the electron–ion collision frequency in the underdense plasma corona,
the region of the plasma where the plasma frequency, , is less
than or equal to the frequency of the laser light, . The surface
is referred to as the critical surface and the denwhere
sity at that surface
as the critical density.
The inverse Bremsstrahlung absorption coefficient is given by
(1)
where
(2)
is the temperature
is the electron–ion collision frequency,
is the ion charge state, and
of the plasma electrons,
are the charge and mass of the electron, respectively,
is the
,
is the
Coulomb logarithm
critical electron density, is the speed of light,
is the
scale length of the underdense plasma region, is the plasma
is the laser pulse duration.
velocity, and
A detailed analysis of the inverse Bremsstrahlung absorption process shows that the absorption efficiency decreases as
the laser intensity and wavelength increase, and increases with
the duration of the laser pulse. For LIS designs, the laser in, and laser pulse
tensity (or power density) , wavelength
duration
of interest are
,
nm, and
ns. Experimental results
and theoretical predictions of absorption under these conditions
0093-3813/$20.00 © 2005 IEEE
SHARKOV AND SCRIVENS: LASER ION SOURCES
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have shown that absorption of the incident laser energy is typically between 70% and 95%.
Plasma ions are stepwise ionized due to electron–ion collisions in the dense, high temperature plasma. The temperature of
and the final ion charge state distribution depend
the plasma
strongly on the laser power density
at the target. The
ion pulse expands longitudinally due to the energy spread of
ions in the plasma, resulting in a plasma pulse duration that is
considerably longer than the laser pulse duration. The length of
the drift space between the target and the extraction system determines the ion pulse duration; it is in this drift space that the
plasma expands.
The ion charge state distribution does not remain “frozen”
during the plasma expansion. After the end of the laser pulse,
the plasma temperature rapidly drops and the ion charge state
decreases at a large range of distances from the target, due to a
dramatic increase in the three-body recombination through high
exited levels; this recombination rate is given by
Fig. 1. Layout of the LIS at CERN.
(3)
A. Laser Characteristics
is the electron density.
where
In its turn, this process is very sensitive to the energy balance during plasma expansion, in particular, to the quantity of
energy released in an individual act of the recombination. This
issue was examined theoretically in [15], where the asymptotic
behavior of plasma temperature and ion charge state was found.
It was shown that for most initial conditions, the ion charge
state freezes but only after some considerable changes [16]. The
higher the critical density and the shorter laser pulse duration,
the larger these changes.
For shorter wavelengths, higher laser intensities are possible.
This allows generation of plasmas with a higher electron temperature at higher critical density and, therefore, the creation of
higher charge states. The ions can be removed from the laserplasma interaction point by the electric field generated by the expulsion of suprathermal electrons. The escaping ions can have
charge-states in the region of 30+ to 50+ for heavy elements,
with several megaelectronvolts energy spread [17].
III. GENERAL DESCRIPTION
A typical LIS configuration is shown in Fig. 1. It consists of
a number of individual components or subsystems.
1) A repetitively pulsed laser of 0.1–100 J output energy,
pulses without
capable of producing more than
maintenance or adjustment.
2) A target illumination system.
3) A target holder and manipulator capable of delivering
pulses without maintenance or adjustment.
4) An extraction system for forming the ions from the expanding laser-produced plasma into an energetic ion beam
of typical energy 10–30 keV/u.
5) A low energy beam transport line (LEBT) for matching
the ion beam from the LIS to the subsequent preaccelerator.
In the following, we consider these five subsystems in more
detail.
The laser-plasma scaling laws for charge state distribution,
plasma density, and plasma velocity impose requirements on the
minimum laser energy necessary for producing the required ion
charge state and other beam parameters [15], [18]. The repetition rate of the accelerator (into which the ions are to be injected) and reliability requirements set strong constraints on the
maximum laser energy available from each active medium, due
to the present status of laser technology and the basic physics
involved. The advantages for the use of the Nd-glass lasers generating 1.06- m radiation have been discussed in [15]. But the
output energy of the glass lasers in repetition rate operation
mode is restricted by the cooling system. At present, it is still
below 5 J by commercially available devices. The development
of the diode-pumped laser technology might improve the performance of the glass lasers in the future [19].
laser
A transverse excitation atmospheric (TEA) type
has the best compromise of parameters for present requirements,
delivering energy to a solid target fabricated from the material
to be ionized. The pulse energy of the laser can vary up to 100
s, and with a repetition rate up
J, in a pulse length of
to about 50 Hz.
The resulting ion current (amount of plasma produced) is
mainly a function of laser energy. Detailed measurements show
that for a constant illumination angle between the laser and the
target, the ion current density is directly proportional to the
and to the laser power
square of the focal spot diameter
density , and hence is directly proportional to laser energy:
.
B. Target Illumination System
The target illumination system provides guiding and high
quality focusing of the laser beam on the target surface. The
target illumination optical scheme employed at ITEP and
CERN [20], [21] is shown in Fig. 2.
This scheme has the following features.
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IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 33, NO. 6, DECEMBER 2005
remains approximately constant and equals 1.6
0.5 for the
power density region of interest. The experimental scalings obtained in this way are
(5)
s
Fig. 2.
Target chamber with target illumination system.
1) The laser beam is nearly perpendicular to the target
surface (while the target surface is perpendicular to the
target-extraction axis).
2) Geometrical protection of the final focusing mirror and
laser)
input window (KCl, NaCl, or ZnSe for a
against coating by deposition of ions and neutrals produced by the laser beam–target interaction.
Experiments with the illumination angle of incidence varied
have indicated the importance of minimizing the angle between
the laser beam and the target normal. Even when a small 6
angle from the normal of the target was used, the ion generation
efficiency was reduced by approximately 50%–70%.
laser systems, NaCl and KCl vacuum transition winFor
dows can be used, which have laser energy density damage
and can withstand more than
thresholds of more than 5
laser shots.
The entrance window should be inclined to the laser beam
so as to avoid reflection back into the laser system. The angle
depends on the geometry and is normally 3 –5 .
The stainless steel chamber is insulated from ground to a
potential of up to 100 kV. The chamber is pumped down to
after each laser shot with repetition rate 1 Hz.
This requires a pumping speed of about 1000 l/s, in turn necessitating installation of a pump system on the high-voltage
platform with power consumption about 5 kW, or alternatively,
to pump through an isolator.
Beside the optical focusing system, the chamber houses the
target mounted on a target manipulator mechanism. Any solid
material can be used for the target in the source.
C. Pulsewidth and Target-Extractor Separation
The pulse duration of the ion beam extracted from a LIS is determined by the drift space between the target and the extraction
plane (typically 70–300 cm), depending on the energy spread of
the ions (which can range from a few hundred electronvolts up
to mulit-megaelectronvolts).
The ion beam pulse duration full-width at half-maximum
(FWHM) for plasma generated by 10.6- m radiation, can be
estimated from [22]:
(4)
is the characteristic halfwhere is the drift length, and
width of the ion velocity spectrum
. The ratio
(6)
The drift distance between the laser-target interaction point,
and the ion extraction area is normally free of fields, in which
case, the extracted ion pulse length is proportional to the dis. The number of ions
tance between these two areas:
and the current extracted through a given aperture both deand
.
crease steeply with increasing :
Hence, the ion pulse duration can be adjusted by varying the
distance between the target and the extraction system, although
limitations on this distance exist.
The minimum distance is limited by the maximum plasma
density that can be tolerated in the extraction gap without highvoltage breakdown. The maximum distance is limited by the
current level required, the available laser energy, and the allowed
extraction area. Increasing the extraction area leads to a larger
ion beam emittance, limits for which are imposed by the following accelerator and its application.
The target is a rotatable cylinder with a surface area of more
m in two
than 400 cm , provided with fine adjustment
axes, with a lifetime of at least
shots before replacement is
needed. This will allow several days of uninterrupted operation
at 1-Hz repetition rate. The possibility exists to provide several
different elements on one target system, allowing fast switching
between ion types.
D. Extraction System
The plasma expands through the drift region, free of external
fields, before the ions are extracted. This is to allow the plasma
density to fall to a value at which ions can be extracted with
voltages of about 100 kV.
Before extraction, the plasma consists of multiply charged
mA/cm [11]. The potential
ions with a current density of
required for extraction can be estimated from the Child–Langmuir law
(7)
where is the current density,
is the voltage across the extractor gap, is the gap spacing, and and are the ion charge
and mass, respectively. For example, for
ions and a gap
kV is required
spacing of 30 mm, an extraction voltage
for optimal extraction. The current density can be increased by
a factor of 1.25–2 when the extraction electrodes have two circular apertures for beam extraction.
In order to separate the ions from the plasma electrons, a positive electric field is required. The field created by the extraction
electrodes is depressed by the space charge of the beam ions,
which is very strong due to the low ion velocity just outside
the plasma sheath. The additional streaming energy of the ions
from a laser plasma reduces this space-charge depressions by
the ions, and allows a higher current density to be extracted. An
increase in the current density of a factor 1.4–1.6 is possible for
SHARKOV AND SCRIVENS: LASER ION SOURCES
Fig. 3. Average Pb ion beam current density, showing also the pulse-to-pulse
3 10 W=cm .
statistical spread. P
2
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Fig. 5. Measured ion charge state distribution of a Pb beam for ion analyzer
registration energy setting E = 300Z eV. Laser power density on the target
was P
3 10 W=cm .
2
Fig. 4. Example of early-time, high-charge-state phase, pulse shape of a Pb
ion beam current pulse. P
3 10 W=cm .
2
a streaming energy of 2 keV per charge at an extraction potential of 60 kV [23].
E. LEBT
The LEBT plays an important role as a matching section
to the subsequent accelerator, normally a radio-frequency
quadrupole (RFQ) accelerator. It is a key element of the LIS in
that it must transport and match the space charge dominated
ion beam; the total current of the ion beam extracted from the
laser-produced plasma can easily reach 50–100 mA.
Space-charge compensation of the ion beam by electrons (either from the rest-gas or secondary emission from surfaces due
either to beam losses or ultraviolet (UV) and X-ray radiation
from the plasma) cannot be used for high-charge-state, high-current beams, due to the large recombination rates of the beam ions
[24].
IV. BEAM PARAMETERS
Fig. 6. Charge state distribution of a lead ion beam generated by a CO laser
at a power density P
3 10 W=cm [21].
2
Typical lead ion pulses generated by a 95-J
two very different power density levels
Figs. 3. and Fig. 4.
laser at
is given in
B. Charge-State Distribution
The charge-state distribution of the extracted ion beam is
determined by the laser power density P. The power density
should be chosen according to the ionization potential of the
principal charge state required from the LIS. For a given the
plasma consists of a mixture of charge states in the vicinity
of the principal charge state, as shown in Fig. 5, for the case
. An example of the charge-state distribution for a lead beam formed at a laser power density
is shown in Fig. 6. for the time window during
ions are dominant. Typically, the principal ion
which
charge state does not exceed 20% of the total beam current.
A. Current Profile
C. Beam Emittance
The laser produced plasma delivers a mixture of ions with
a charge-state distribution that changes with time. Normally,
the higher charge states have higher velocities. As a result, the
charge states of interest are only present for a few microseconds
during the first spike of the extracted ion pulse.
High charge states of interest are only present for a few microseconds during the first spike of the extracted ion pulse. Thus,
to measure the emittance, it is necessary to use a device which
can be activated for that specific part of the beam pulse only.
Due to variation from pulse to pulse and also because of the low
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IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 33, NO. 6, DECEMBER 2005
(100 kJ, 100 ns) of intense ion beams, is in progress at the ITEP
TWAC project [11], [20], [26]. The ITEP LIS aims to produce
high-current pulses (5 mA, 5 s) of highly charged (preferably
He-like) ions of medium atomic mass elements (Co, Cu, Zn) for
single-turn injection into a booster synchrotron ring.
According to the ITEP-TWAC acceleration-accumulation
scenario, the required parameters of the LIS are the followinga.
1)
2)
3)
Fig. 7. Phase-space ellipse for a Pb ion beam. Extraction voltage 50-kV,
middle electrode voltage 10 kV, PCO camera gate width 10-s, gate opening
time. 40 s (circles), 60 s (squares), and 80 s (triangles).
0
repetition rate, it is desirable to perform the measurement during
a single pulse.
At ITEP, an array of 100- m slits with 3-mm spacing and
a CsI crystal as scintillator, recorded by a gated PCO camerabased readout system, are used for measurement of the angular
distribution of a Pb ion beam [25] at each of the slit points.
Emittance measurements for lead ion beam of about 10 mA
total current and ion pulse length of 75 s result in about 400
(root mean square), as shown in Fig. 7.
Other measurements of the emittance have shown a strong
shot-to-shot variation of the emittance, as well as a changes of
the phase-space area and associated Twiss parameters during the
ion pulse [23].
D. Pulse Stability and Source Lifetime
By pulse stability, we mean the pulse-to-pulse repeatability
in pulse shape and energy. The required pulse stability of the
beam current is mainly determined by the stability of the laser
impinging the target. The stability typically demonstrated by
routine LIS operation at JINR and at ITEP is about 20% (for
shots.
the pulse amplitude) over long-term runs of about
Stability test runs have been undertaken at CERN for a 1-Hz
repetition rate, 95-J laser in MOPA configuration. The best value
achieved was 1 h 15 min, nonstop 1-Hz operation [21]. Shot-toshot statistical variation in the output laser pulse energy was less
than 15%. Upgrading of the prototype, using more radiationresistant optical materials, and lowering the energy density at
optical surfaces will lead to substantial improvement in lifetime.
For an experimental campaign lasting two weeks on the
LIS nonstop operation
TWAC facility (ITEP), about
cycles are required. The overall lifetime of the source is mainly
limited by the optical elements of the laser system.
V. EXAMPLES OF LASER ION SOURCES
Modification and upgrade of the existing heavy ion accelerator chain at ITEP, for the production of terawatt power level
Element: as heavy as possible.
Ion charge state: in the range C-like to He-like ions.
Ion pulse length (for 95% of ions with desirable charge
state): 10–15 s.
4)
Number of ions with the required charge state:
ions/pulse.
.
5)
Emittance of extracted beam:
6)
Repetition rate: 1 Hz.
7)
Number of source operation cycles between mainte.
nance periods:
The LIS will be used for the ITEP accelerator–accumulator
complex in two stages. In the first stage of the project, the exlaser generates an intense
isting 5-J, 0.5-Hz repetition rate
ions.
beam of
laser will be built and
In the second stage, a 100-J, 1-Hz
used as a driver for the LIS, generating intense beams of highly
charged ions (Z/A 0.3–0.4) with atomic masses of up to 60
[26], [27].
ions (charge-to-mass ratio 1/3) are being used in
Carbon
a “running-in” phase of the TWAC facility for testing of complicated beam gymnastics in the accelerator chain [25], [27]. The
ion beam current density was measured by a Faraday cup placed
just behind the extraction system. The drift region distance from
the target to the extraction system was 130 cm. The peak value
of ion current density reaches 8–9 mA/cm .
The charge state distribution of the ion beam was measured
at the outlet of the bending magnet. The most abundant charge
, with 35% of the total number of
state is the He-like ion
ions in the beam.
Time resolved ion beam emittance measurements were made
for different geometries of the three-electrode accel–decel extraction system. Phase space ellipses of the charge integrated
(containing all charge states) ion beam for an extraction voltage
of 50 kV and an extraction aperture of 20 mm were measured.
was obtained at 4 rms. The
An emittance of
emittance values do not change significantly with time during
the ion pulse.
Different extraction system geometries have been tested to
ions at the exit of the I-3
obtain as high as possible yield of
injector (Fig. 8). First electrode apertures of diameter 20, 40, and
80 mm were used. A 90% transparent grid was placed in the first
electrode to stabilize plasma boundary variations in time. The
ions at the injector exit was found for the
highest yield of
largest extraction aperture, 80 mm, and using the first electrode
grid. The carbon ion beam peak total current, measured by a
current transformer at the inlet of the I-3 injector, reaches about
90 mA. The LIS was routinely operated with a rep-rate of 0.25
ions into the ITEP-TWAC ion accumulator
Hz to inject
facility for more than three years.
SHARKOV AND SCRIVENS: LASER ION SOURCES
Fig. 8.
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ITEP-TWAC LIS injection line.
The investigation of a LIS as the preinjector for heavy ions
for the large hadron collider (LHC) at CERN, used a 100 J, 25
ns
laser system comprising a master oscillator and power
amplifier, designed for 1-Hz operation.
Measurements of plasma production were made at a laser
. The ion current density and
power density of
the charge-state distribution showed that a configuration was
in a window of 3.5 s, of
possible to deliver 7.9 mA of
which this charge-state accounted for 16% of the total ion current. Emittance measurements on a reduced intensity beam of
20-mA total current, resulted in a value of 0.2 mm mrad (rms
normalized) [21].
Since the middle of the 1970s, a LIS has been in operation
at JINR [28], producing a number of light ion species, from
ions to He-like
and
ions, for
fully stripped
injection into the Dubna synchrophasotron and more recently
into the Nuclotron [29]. After preacceleration up to 5 MeV/u in
the LU-20 linac, He-like ions are fully stripped. A two-module
laser with 1-Hz repetition rate and output energy
TEA
of several Joules has been successfully used for more than 20
years, but a growing demand for increased ion beam current has
led to the decision to upgrade the laser to an output energy of
20 J, but still in free-running laser generator configuration and
with refracting focusing optics.
VI. OTHER OPERATING OPTIONS
A. High-Current, Low-Charge State Mode
There is a need for an ion source of heavy elements of relatively low charge state, e.g.,
,
,
, etc., but with
rather long pulse duration, 60–80 s. A long pulse corresponds
to the multiturn injection mode of a synchrotron ring. This mode
of source operation can be achieved using a nitrogen-rich laser
gas mixture for a free-running laser (for instance
) and/or by defocusing the laser beam at the target
surface, i.e., by decreasing the laser power density P. The power
density level required for efficient production of low-charge.
state ions is typically
ion yield from laser-proExperiments to optimize the
duced plasma by variation of focal spot size and laser pulse dulaser with
ration have been carried out using a 100 J
laser power density on the target surface in the range
. The emittance of the lead ion beam extracted
from the laser produced plasma was measured as a function of
time with plasma parameters (electron temperature, ion velocity,
production. Simulaand ion charge states) optimized for
tions of an existing GSI matching channel between source and
RFQ have been carried out to provide information about transport of the LIS ion beam. The main results are as follows.
ions was obtained for a laser
1) The highest yield of
power density on the target surface in the range
for a laser pulse duration 15 ns.
ions for a laser pulse duration of 15
2) The yield of
ns (optimized conditions) is greater than for a laser pulse
duration of 40 ns, over the range of target power densities
.
explored,
ion current of 10 mA with pulse length 80 s was
3) A
obtained for optimized conditions. This corresponds to a
ions in the pulse of about
.
total number of
of the different charge states
4) The momentum spread
of lead ions was measured in the expanding plasma. Using
ions in
the data obtained, the momentum spread of
the beam extracted by dc potential 50 kV can be estimated
. This value can be decreased to a few
as
percent by ramping the extraction voltage.
5) The emittance of the lead ion beam extracted under conyield was measured as a
ditions optimized for the
function of time for 30 and 50 kV extraction voltage. The
emittance of a beam with total current about 10 mA and
(for
ion pulse length 75 s is about 400
about 75% of ions in the beam).
B. Influence of Magnetic Field on the LIS Plasma
Investigations at the U-200 cyclotron at JINR Dubna and at
the Moscow Engineering Physics Institute have shown some
specific aspects of the influence of longitudinal and transverse
magnetic fields on the LIS plasma [5], [30].
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A longitudinal magnetic field mainly confines the plasma,
narrowing the expansion angle of the plasma plume, and
forming cylindrical plasma along the field axis with increasing
magnetic field. Increased pulse length is a result of particle
rotation and collective effects in this plasma cylinder. The
ion charge states remained the same, for power densities
and a field length of about 10 cm.
A transverse magnetic field has less influence on the plasma
expansion angle but can lead to instability of the expanding
plasma for higher power densities and longer plasma plume drift
space. The plasma expansion rate is decreased by the transverse
magnetic field and in this way changes the ions energy spectrum.
A more detailed description of the influence of longitudinal and
transverse magnetic fields on the LIS plasma can be found in
[30].
volume, plasma density, and average charge-state in the ECR
plasma.
Such a scheme is best suited to the production of pulsed
beams, with a delay from the injection of the laser plasma to
the production of highly charged ions by step-wise ionization.
For higher charge-states, the long breeding times result in the
high-charge-state ions being present in the plasma for many
milliseconds. The use of a high repetition rate laser ( 100 Hz)
can therefore result in quasi-direct current extraction of high
charge-state ions [33].
The first experimental results at the SERSE ECR source in
Catania were recently published in [34], where tantalum and
gold targets in the superconducting ECR source were irradiated,
and the metal ions could be extracted and analyzed. The proand
could be increased by 50% over
duction of
the oven evaporation technique, even though the source extraction aperture area was reduced by a factor of 3, however, the
charge-state distribution was not shifted to higher charge-states,
possibly due to outgassing affecting the gas mixture. The stability of the output beam is not as high as from ECR ion sources
with loading by conventional evaporation. Further investigations may use ECR ion sources with a higher electron density,
and will move the laser target interaction point into a separate
chamber.
C. Laser Plasma Loading in the ECR
The performance of electron–cyclotron resonance ion sources
(ECRIS) may be enhanced by the loading of laser plasmas.
Using different laser power density regimes, both negative and
positive ions, as well as neutral atoms can be created and injected into an ECRIS where they are charge-bred to produce
higher charge states.
The following advantages for an ECRIS can be achieved by
loading a laser plasma [31].
1) Production of atoms of solid material that is difficult to
produce by thermal ovens (e.g., refractory metals).
2) Enhancement of the ion content of the ECR plasma
without increasing the neutral content (reducing
charge-exchange processes with highly charged ions).
The first technique is typically performed by the laser ablation of solid materials using a low laser fluence (energy per unit
area), and will not be discussed further. The production of ions
for injection requires power densities above
.
The ions are slowed by ion–ion plasma collisions, with a loss
rate for an ion of charge, atomic mass, and energy , , and
of
(8)
where , , and
are the average plasma ion charge, atomic
, , , and
are the electron mass and
number, and density.
classical radius, the speed of light, and the Coloumb logarithm,
respectively [32]. In order to trap the laser produced ions inside
an ECRIS plasma (with a typical length of 10 cm), ions must be
injected with an energy per charge no larger than a few hundred
electronvolts. The production of low ion kinetic energies limits
, resulting in
the laser power to the range below
low-charge-state ions (1+ to 5+ of heavy ions).
The total kinetic energy of the laser plasma ions should be
only a fraction of the ECR plasma ion energy, in order not to
strongly disturb the ECR plasma, i.e.,
(9)
where and
lengths and
are the laser plasma partial ion current and pulse
,
,
, and
are the ion temperature,
VII. CONCLUSION
The LIS is well suited for the generation of intense, high
brightness, pulsed ion beams from any solid material, with
high, medium, and low ion charge states, with low power
consumption of the high-voltage platform. The high intensity,
s , and pulse repetition rate ( 1–10
short pulse length
Hz) are very well matched to the requirements needed for
filling of synchrotron rings in single-turn injection mode. The
long-term operation of high-energy lasers, and the shot-to-shot
reproducibility of the ion beam delivered by the source, must
still be improved for some accelerator applications.
The basic properties of the laser plasma interaction and the
high charge state production is reasonably well understood for
the range of laser intensities of interest for LIS, due to many
years of comparisons between theory, simulation, and experiment.
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Boris Sharkov received the Ph.D. degree from the
Moscow Engineering Physical Institute, Moscow,
U.S.S.R., in 1978.
After receiving the Ph.D. degree, he joined the
Institute of Theoretical and Experimental Physics
(ITEP), Moscow, where his research interests were in
the areas of high-density plasmas containing highly
charged ions, for applications to particle accelerators, and inertial confinement fusion. Since 2005, he
has been the Executive Director of ITEP-Moscow.
Prof. Sharkov is a member of the EPS Board of the
Plasma Physics Division, the Scientific Council of Minatom of the Russian Federation, the Experimentausschuss of GSI, Darmstadt, Germany, and a member
of Nuclear Society of Russia (Soviet Union). In 2001, he was awarded the Presidential Stipendium for outstanding scientists of Russia.
Richard Scrivens received the Ph.D. degree on the
extraction of an ion beam from a laser ion source from
the University of Wales, Swansea, U.K., in 1999.
He joined the European Organization for Nuclear
Research (CERN), Geneva, Switzerland, in 1991 to
work on LHC super-conducting cavity models, and,
in 1994, assisted in the installation and commissioning of CERN’s Lead Ion Linac. Since 2005, he
has been the Head of Hadron Sources and Linac Supervision for CERN’s Hadron Linear Accelerators,
with continued research interests on high-current
low-energy beam transport, as well as electron–cyclotron resonance and
electron beam ion sources.