From a Seminar Presented February 7, 1996
Don E George
University of Toledo
Department of Physics and Astronomy
INTRODUCTION
The technology associated with laser ion sources is fairly well established and has been
used successfully in conjunction with many different experimental apparatuses requiring
positive ion generation. While there are many successful applications, the use of a laser
is not common among accelerator facilities sized similarly to those at the University of
Toledo. Currently the Toledo Heavy Ion Accelerator (THIA) at UT is undergoing an upgrade
to install a laser ion source in place of its current ion source. This paper is based on a
seminar presentation of the same title given on February 7, 1996 for the Physics
Department at the University of Toledo.
Included herein are a few topics relevant to the upgrade of the THIA facility. These
include: an overview of THIA hardware, a description of 'Your Basic Everyday' laser ion
source, why was a laser ion source selected for THIA, plasma plume collimation via a
magnetic field, some specifics about the laser that was selected, and a brief look at one
conceptual design of the source that is being developed. This paper does include
corrections made to the original presentation.
OVERVIEW OF THIA HARDWARE
The THIA facility is designed to generate and accelerate positively charged ions with
charge states primarily of charge one. The major components of the current THIA
configuration, as described here, will remain the same after the laser source upgrade. The
major functional units of the THIA apparatus are: the Ion Source, an Ion Extraction
Potential with focusing lens, a Mass Analyzing Magnet, the Main Accelerator Column,
various steering and focusing electronics, and the Experimental Apparatus. The upgrade
consists only of a removal of the current ion source and its replacement with a laser
ablation type ion source. Figure one depicts the major
components of the THIA apparatus.
An ion source is the functional unit which takes some sort of target material and applies
energy to remove electrons from its atoms thus ionizing them. We can then use an
electrical potential to accelerate the ions. The current Ion Source is a sputtering type
source which 'boils' away electrons to produce positive ions which are extracted from the
source chamber by an electrical potential of around 35 kV.
The ions then travel into a large Mass Analyzing Magnet. This magnet produces a magnetic
field which directs any ion from the source along a curved path. Each different ion
species, with its own specific mass and charge, travels along a different curved path. The
species desired for acceleration is selected by adjusting the analyzing magnet strength
such that only one curved path is directed down the beam tube to the Main Acceleration
Column.
The Main Acceleration Column uses progressively increasing electrical potentials to raise
the energy of the selected species. This increase continues until the total potential
reaches approximately 330 kV.
At this point the highly energetic ions undergo focusing and steering performed by static
or dynamic electric fields in the beam tube. Now the ions are allow to travel along one of
three beam lines to targets situated at the ends. These targets and their associated
instrumentation comprise the Experimental Apparatus which is the heart of the physics
research taking place.
'YOUR BASIC EVERYDAY' LASER ION SOURCE
Before going into the reasons for choosing a laser for the ion source, it is appropriate
to describe the basic components of such a device. A laser ion source consists of a
pulsed-beam laser which is delivered to a solid target by an optical array. The laser beam
is applied to the solid target under vacuum and results in laser ablation of the target
material. Laser ablation is the evaporation of electrons from a solid surface using laser
light photons. This results in their ejection from that surface. The ejection of positive
ions and electrons results in a charged plasma which, if not confined, would freely expand
mainly normal to the surface but with some parallel component. The ions are then collected
and accelerated to be used in the experimental apparatus in a manner which depends on the
particular use of the source.
Why a LASER Ion Source?
The most obvious question to ask is "Why do we want a laser ion source?" There
are a number of reasons for selecting a laser ion source as well as additional benefits of
laser ablation as it relates to an ion source for an accelerator. A primary desire in
modern ion accelerators is the production of ions with high energies and/or high charge
states. In addition to this, a laser ion source will produce a large population of ions
per pulse. Additional benefits include a directional plasma plume, the flexibility of
using any solid source target material, and high system reliability.
High Ion Energy
Current levels of collision research require ions with very high energies. The present
THIA ion source produces ions with energies in the hundreds of keV range. It is expected
that the laser ion source will produce ions in excess of 1 MeV for many species and up to
4 or 5 MeV for some species. Since the THIA facility has a limited acceleration potential
(330 kV) the only way which we can increase the energy of accelerated ions is to increase
their charge state. The upgrade of the THIA facility into this range of ion energies will
provide for a significant increase in the usefulness of the accelerator in target
collision research. Among other benefits are addition of new types of collision
experiments that can be performed as well as increased run time due to an increased
lifetime of target foils due to the higher energies.
Charge states that can be produced are highly dependent on the laser power density applied
to the target material as well as the specific material that is used. Figure three shows a typical plot of mean charge state versus
power density. This figure basically shows the mean charge state increasing with
increasing power density, a fairly predictable relationship. It is expected that the THIA
Laser will produce a power density in the >1010 watts/cm2 range.
This kind of power density has been shown to produce charge states as high as (+16) for
some species. Figure four shows a charge state spectrum
for Iron as determined by Time-of-Flight methods. This type of method simply involves the
application of an extraction potential to a laser ablated plasma plume followed by a
collection of ions after some drift distance. Ions with higher charge states are
accelerated faster by the extraction potential and thus traverse the distance to the
collection point sooner. Thus the ordinate axis of figure four is in micro-seconds and
indicates relative flight times of different charge states. The vertical axis is simply
relative charge collected at any given time.
Large Ion Concentrations
In addition to high charge states, laser ablation of a solid target produces a large
number of ions. The ions produced have a charge state distribution which depends on not
only the physics of plume expansion, but also on the laser incidence angle and any applied
fields.
A freely expanding plasma plume tends to expand perpendicular to the surface of the
target. Figure five depicts the overall charge state
distribution inherent to the ablation process for various materials. This figure
represents the average concentrations for various charge states. There is also an angular
distribution dependence within the plume of charge states. While lower charge states will
expand out at as much as 45 degrees off perpendicular, higher charge states tend to have a
smaller cone of expansion which can be as small as 20 degrees depending on the species.
In addition to this natural angular dependence, there is an additional dependence based on
laser angle of incidence. While the plume still expands normal to the target surface, the
charge state distribution is skewed away from the laser. Thus it is desired to minimize
the laser incidence angle (with the target normal). One problem with minimizing this angle
is that the distance between the source target and the extraction point must be increased.
During this distance, the plasma plume would expand further than is desired. This is
discussed in a later section.
In an effort to minimize expansion losses of ions an external magnetic field can be
applied to the plasma plume to both contain and collimate it. The fact that the plume
expands with a velocity normal to the target and that it can be collimated make it easily
directable to a small extraction point. Figure six depicts
this directability. Note that, in the configuration of the THIA source, the extraction
point will be at the end of the plasma plume, not to the side as depicted in the figure.
This topic is discussed further in a later section.
PLUME COLLIMATION VIA A B-FIELD
As mentioned previously, the source target material must be far enough from the extraction
point so as to allow for the minimization the laser incidence angle. This, along with the
tendency of the plasma plume to expand transverse to the desired direction of motion,
makes it necessary to confine the plasma plume during this drift distance. If the plume
were not confined, it would expand beyond the physical size of the accelerator beam tube
and a large percentage of ions would be lost.
In order to confine the plasma plume, an external magnetic field is applied to the source
chamber. This field consists of equipotential lines which are basically parallel to the
normal of the target (the desired motion of the plume) and begins prior to the point of
ablation. This field translates any motion perpendicular to the field lines in to a spiral
behavior referred to as gyromotion. Gyromotion is caused by a force exerted on the ion due
to its motion in the magnetic field. This force is the vector-product of the motion,
perpendicular to the field. This force produces the spiraling motion which is
characterized by a radius of rotation referred to as the gyro-radius which is essential in
the physical design of the magnetic field used to contain the expansion of the plasma
plume. In addition to simply confining the expansion of the plasma plume, it is desired to
collimate it as well. The reduction in radial area of the plume will contribute to a
higher number of ions being collected and extracted by the extraction potential.
Generally, coiled electro-magnets are used for plume confinement. For the THIA source this
is to be accomplished by using a rare-earth permanent magnet which can be custom designed
and built to have the necessary shape and field strengths to achieve collimation of the
plasma plume. The custom design will produce equipotential field lines that converge along
the plume path. In addition, a permanent magnet will facilitate a port being drilled in it
such that the laser may pass through to the source target.
SPECIFICS ABOUT THE LASER
The Laser we have selected for our Ion Source upgrade at THIA is a Spectra-Physics
Quanta-Ray GCR-100 Nd:YAG Laser. This type of Laser is commonly referred to simply as a
YAG Laser, the GCR-100 is a solid state Neodymium-Doped: Yttrium Aluminum Garnet based
Laser. A YAG Laser is based on a four-Level Transition Scheme which provides a lasing
transition wavelength of 1064 nm (just IR range). The laser will be pulsed at a 50 Hz
cycle time with a power output of 600 mJ per pulse. Unfocused, the generated beam is
approximately 9mm in diameter. The basic pulses are modified with the use of a Q-switch to
provide smaller pulse widths of 8-9 nsec. This configuration provides a power density of
109 W/cm2. Additional focusing will bring the beam diameter down to
provide a power density of >1011 W/cm2 at the target surface.
REFERENCES
Wolf, B., Handbook of Ion Sources, 1995, CRC Press, pp 1-22, pp149-155
Gray, L.G, et. al., Heavy-ion source using a laser-generated plasma transported through an
axial magnetic field, 1982, Journal of Applied Physics, 53(10) pp 6628-6633
Brown, I.G., The Physics and Technology of Ion Sources,1989, John Wiley and Sons, pp 1-52,
pp 299-311
Phaneuf, R.A., Production of High-Q Ions by LASER Bombardment Method, October 1987, IEEE
Transactions on Nuclear Science, pp 1182-1185
Hughes, R.H., et.al., Ion Beams from LASER-generated Plasmas, August 1980, Journal of
Applied Physics 51(8), pp 4088-4093
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