Coherent Radiation Sources Based on Laser
Plasma Accelerators
D.A. Jaroszynski and G. Vieux
University ofStrathclyde, Department of Physics, John Anderson Building, 107 Rottenrow, Glasgow
G4 ONG, Scotland, UK
Abstract. Laser-driven plasma wakefield accelerators (LWFAs) based on table-top terawatt
lasers have the potential of producing high brightness ultra-short electron bunches that are ideal
for driving free-electron lasers (FELs). These sources are excellent candidates for reaching the xray spectral region. However, the creation of a compact radiation source based on this
technology requires a number of difficult challenges to be met. Currently, LWFAs produce
beams with excellent transverse emittance but very large energy spectra. To meet the
requirement that the fractional energy spread should be less than the universal PEL gain
parameter, p, the electron bunch injected into the accelerator must occupy a small region of
phase space. We will discuss a new project that has recently been set up in the UK to develop
LWFA technology and apply to the creation of a compact PEL. To meet the stringent injection
requirements, 10 MeV ultra-shot injection electron bunches, with durations a fraction of the
plasma wake period, will be produced in a photoinjector. A fully ionized hydrogen filled
capillary, with plasma densities up to 1019 cm"3, will have a dual function of acting as a
preformed plasma waveguide for guiding the laser pulse while providing the medium for the
LWFA. Table-top terawatt Ti:sapphire lasers will be utilized as drive lasers. As a demonstration
of the utility of the compact accelerator, electron bunches from the LWFA will be used to create
coherent electromagnetic radiation in a PEL. Progress on the development of the plasma
capillary channel and diagnostic systems based on terahertz time domain spectroscopic
techniques are presented.
INTRODUCTION
Intense electromagnetic radiation with a wide range of temporal, spectral, coherence
and spatial characteristics is most likely one of the most important tools of
contemporary research. A large scientific and industrial community currently utilizes
tunable incoherent x-ray radiation from synchrotron facilities. As a result of recent
advances in laser technology users are now demanding ultra-short x-ray pulses that are
both coherent and intense, for time-resolved studies. To satisfy this need several large
programmes to develop XUV self-amplification of spontaneous emission (SASE) freeelectron lasers (FELs) using GeV electron beams have been instigated. A great deal of
progress has been made on these projects towards achieving these goals and SASE
FELs are now beginning to produce radiation in the VUV with far greater brightness
and very much shorter pulse durations than has been hitherto been possible with
synchrotron sources. However, the high cost of SASE FELs and the need to extend
their wavelengths into the "water-window" and beyond, is stimulating a drive to make
accelerators more compact and higher brightness. One candidate that could fulfill both
CP647, Advanced Accelerator Concepts: Tenth Workshop, edited by C. E. Clayton and P. Muggli
© 2002 American Institute of Physics 0-7354-0102-0/02/$19.00
902
of these requirements is the laser-driven wakefield accelerator (LWFA), which could,
by making ultra-short x-ray pulses available to medium sized laboratories,
revolutionize the application of x-ray radiation, and make significant changes to the
way science is carried out. LWFAs have one excellent quality in that acceleration
gradients of more than 100 GeV/m, 3-4 orders of magnitude greater than conventional
accelerators, are possible. Thus they have the potential of producing extremely bright
electron beams with bunches only a few femtoseconds in duration, which makes them
ideal candidates for driving compact x-ray FELs or as sources for pulse radiolysis
studies and other time resolved applications. Their compactness would also lead to
very large cost savings in shielding and accelerator equipment, making tuneable x-ray
lasers affordable for medium-sized university or industrial laboratories. However,
many challenges remain ahead to make the LWFA a useful electron source and
construct an x-ray SASE PEL based on these accelerators. Their potential as a useful
compact accelerator still needs to be demonstrated. One of the largest challenges
facing developers of LWFAs as SASE PEL drivers is to reduce their electron energy
spread. To meet this challenge a consortium of major UK research groups at Imperial
College, Daresbury Laboratory, Rutherford Appleton Laboratory, and the Universities
of Oxford, Abertay-Dundee, St. Andrews, and Strathclyde, and many collaborators
(listed in the acknowledgements) has been set up. The project has been named
Advanced Laser-Plasma High-brightness Accelerators towards X-rays, ALPHA-X, or
cc-£, to underline the long-term objective of creating a compact x-ray source. A
specific goal of the ALPHA-X project will be to utilize the output of a LWFA to drive
a PEL as a demonstration of the basic technology.
LASER-PLASMA WAKEFIELD ACCELERATOR
The potential to accelerate charge particles using the large electrostatic fields of laser
driven plasma waves has been known since the 1970s [1]. In these accelerators the
plasma wave is formed as a wake produced by the pondermotive force of an intense
laser pulse. Recent experiments have demonstrated wakefield acceleration over short
distances to energies in excess of 100 MeV using laser pulses with intensities, /,
greater than 1018 W/cm2. The large energy spread and high cut-off energy of their
spectra give a measure of the electrostatic field potential of the plasma wake. In these
demonstrations the laser pulses are longer than the plasma wavelength, Ap, and selfmodulation of the laser pulse occurs through a Raman forward scattering instability.
The plasma waves are produced with a phase velocity equal to the group velocity of
the laser pulse, which is less than the speed of light, c, and background electrons are
accelerated when the plasma wave breaks thus giving rise to a large energy spectrum.
The electric field strength of the plasma wave, E ~ (dneln^mec(Op, depends on its
relative density amplitude (Snelne). An electron can be trapped in the plasma wave
and gain an energy AW ~ 2 (dnelne)^ mec2 , where % « atflcOp2 is the relativistic
Lorentz factor associated with the plasma wave phase velocity and the laser group
velocity. Several processes limit the length of the accelerator: i) slippage between the
electrons and accelerating wave, analogous to slippage in a PEL; ii) diffraction of the
laser pulse (to less than the Rayleigh range); and iii) depletion of the laser energy
903
(which also governs the maximum charge that can be accelerated). In most of the
experiments to date the main length limitation has been diffraction of the driving laser
pulse. Several schemes for overcoming these limitations have been suggested e.g.
guiding the laser pulse through relativistic self-channeling or guiding in a pre-formed
plasma waveguide. Tapering of the plasma waveguide has been suggested as a method
of extending the phase matched length [17]. Another novel method that has been
suggested involves colliding a short duration high-intensity probe pulse with a modest
intensity pump pulse detuned by the plasma frequency [2]. Interference between the
counter-propagating waves results in a slow beat wave. This has the advantage over
standard LWFA in that: i) it enhances the wake, so that the required pump laser
intensity is reduced by several orders of magnitude, and ii) detuning allows the phase
of the accelerating wave to be controlled, thus extending the length over which phase
matched acceleration can occur. To obtain a small energy spread the injected electron
bunch length must be much smaller than Ap.
ALPHA-X PROJECT
The ALPHA-X project we be focussed on LWFA in preformed plasma channels
driven by 50 fs, 0.25 - 1 J, 800 nm laser pulses, and injected by 100 fs, 100 pC
electron bunches derived from a conventional 10 MeV photoinjector being constructed
at Strathclyde. Alternative all-optical injection schemes will also be developed at RAL
and Strathclyde. The goal will be to accelerate the short duration electron bunches to
an energy greater than 100 MeV using the lasers (TOPS and ASTRA) which are
available at the Consortium laboratories. For these conservative estimates we hope to
obtain an emittance en < 1 mm mrad. The main challenge will be to realise an energy
spread Sy/y< p ~ 0.01, where p is the PEL gain parameter [3], which is necessary to
achieve a high single-pass gain in the proposed PEL amplifier.
Free-Electron Laser: a Coherent Electromagnetic Source
The PEL [3] is a unique source of coherent electromagnetic radiation because of its
simplicity: the amplifying medium consists of an electron beam in vacuum subject to a
spatially periodic magneto-static field (undulator) which enables transfer of energy
between electrons and electromagnetic wave. The ponderomotive force arising from
the Lorentz force of the combined magnetic fields of electromagnetic wave and
undulator gives rise to bunching of the electron beam, which results in coherent
radiation at a Doppler up-shifted frequency. This is schematically shown in FIG. 1.
The absence of a solid or gaseous amplifying medium allows the EEL to attain
extremely high powers and broad tuneability. Tuning of the EEL wavelength, which is
given by /I =/Lu/2'j?(l+au2), can be achieved by varying either the electron energy (y=
El me2) or the undulator parameters (au and /^, respectively). Several x-ray FELs are
being developed as 4th generation light sources at centres throughout the world. The
ultimate goal of these projects is to reach the water window and beyond using a selfamplification of spontaneous emission (SASE) FEL amplifier driven by a GeV
electron beam. When complete, these x-ray sources will produce bright and coherent
904
x-ray pulses with durations of the order of the electron bunch duration. One drawback
of SASE amplifiers is that they are essentially noise amplifiers and have spiky and
fluctuating outputs [3] and it is not yet known whether they will have good spectral
and temporal properties.
probe
electron
bunch
laboratory frame
undulator: periodic magnetic field
electron rest frame
undulator field: seen
as propagating field
electron bunch
probe
«_
u
backscattered field
FIGURE 1. The free-electron lasers amplifier
However, superradiance [4], self-amplification of coherent spontaneous emission
(SACSE) [5] and amplification of an injected signal are ways of improving the
temporal characteristics of the x-ray pulses. As x-ray SASE FELs are extremely
expensive devices it is very important that they produce useful output. If ways are
found to produce an electron bunch microstructure with Fourier components at the
resonance frequency, a large stable "spontaneous" coherent signal will act as an
(intrinsic) injection source in the EEL amplifier (i.e. SACSE EEL) [6]. This may be
achievable using future laser-plasma accelerators because their predicted electron
bunch durations can approach one femtosecond or less.
The growth in intensity of an injected or spontaneous field in a EEL amplifier is
given by I=Io exp(gz), where z is the propagation distance, g = 4np3mlku , is the small
signal gain and p is the EEL gain parameter [3] which is a function of the beam
energy, current and emittance. For a matched electron beam, the PEL parameter is
given by p = 1.1 flBuAu4/3Ipk/3£n1/S, where Bu (~ 1 T) is the undulator magnetic field,
Ipk is the peak electron beam current and en the normalised emittance. The matched
electron beam radius for electron beams from laser-plasma accelerators (en < 1 mm
mrad) is of the order of the plasma wake wavelength, giving p ~ 0.01 to 0.02, for the
electron beam parameters expected from a laser-plasma accelerator, and a gain length
of less than 10 undulator periods, which is sufficient to obtain saturation in a 200period, Au ~ 1.5 cm, undulator should be achievable over a wide wavelength range.
905
PEL sources at x-ray wavelengths are less compact because the dependence of the
gain on electron energy, p oc f1, leads to a lower gain and therefore the requirement
of a longer undulator to achieve saturation. To significantly shorten the undulator
length SACSE could be used to enhance the start-up power. This has the additional
benefit that the nonlinear regime is entered promptly and the superradiant pulses
should evolve self-similarly [4] leading to very high efficiencies and extremely short,
smooth and stable pulses. Pulses as short as several attoseconds should be feasible in
future x-ray PEL sources because the gain bandwidth is automatically increased in this
regime. Superradiance and SACSE will be examined in the proposed research
programme.
ALPHA-X Programme
The ALPHA-X programme will involve the development of conventional electron
source with suitable characteristics to allow injection into the laser-plasma
accelerating channel. One of the main challenges will be to create an injection source
with a bunch duration less than the plasma period. A combined DC/RF gun, based on
a system developed at Eindhoven, will be constructed. This will consist of a 1 MeV
low emittance DC photoinjector followed by a high quality RF accelerating structure
to provide 10 MeV electron bunches, sufficient to minimize space-charge effects. An
RF system existing at Strathclyde [5] will be upgraded to provide 10 MW of pulsed
power at 3 GHz, which is sufficient to drive a 100 MV/m RF accelerating structure.
The upgrade will include the development of a photocathode and a 1 nanosecond, 1
MV pulsed power supply to drive the high gradient DC gun.
IRtoVUV
SASE or
SACSE
FIGURE 2. ALPHA-X layout
A plasmon enhanced back-illuminated metal photocathode driven by 800 nm
radiation will be used as an electron source [7]. A design study is being undertaken to
evaluate these cathodes and compare them with emission from thick front-illuminated
metal cathodes. One of the challenges in achieving short duration electron bunches
will be to minimize the deleterious effect of space charge. Electron beam diagnostic
systems, based on terahertz time-domain spectroscopic techniques, are being
developed for measuring the phase-space distribution (emittance, energy spread, etc.),
electron bunch duration and charge, and for monitoring and controlling the
transportation of the beam to the interaction region.
906
One particular advantage of laser-based particle acceleration schemes is that it is
possible to use "all-optical" methods of electron injection - which would significantly
simplify the technology of future particle accelerators. There have been four
suggestions for how relativistic electrons could be injected into a plasma wave using
lasers: i) injection of electrons using an adjacent solid target interaction [8], ii)
electron injection using "standing" plasma waves set up by two colliding collinear
pulses [9], iii) injection using electrons from laser-ionised, ponderomotively
accelerated noble gas atoms [10], iv) electron injection using the ponderomotive force
from an intense transversely propagating laser pulse [11]. Recent results from the
University of Michigan suggest that (iv) has the capability of dramatically increasing
the total electron charge from these experiments - as well as reducing the emittance of
the injection beams [12]. We will examine the possibilities of using each of these
techniques.
The expected emittance of laser-plasma accelerators is excellent, and the peak
current high (~ kA). However, the gain of the PEL is very sensitive to the energy
spread for 8y/y > p. We expect p in the range 0.1 - 0.02 (depending on wavelength)
and therefore need to produce beams with an energy spread better than a few percent,
this will ultimately limit the shortest wavelength that can be reached and remains one
of the greatest challenges of the project. With low energy spread the gain length of the
PEL should be short and saturation within the length of the 200 period undulator.
According to theoretical studies, properly phased matched acceleration with injected
bunches shorter than the plasma period should provide suitable electron bunches.
Plasma Channel Waveguide
The development of plasma waveguides capable of guiding the high-intensity laser
pulses over many centimeters will be key to a successful research programme. We are
developing a gas-filled capillary discharge waveguide [13], to tailor the properties of
the plasma channel to achieve efficient laser-based acceleration. Examples of a
capillary preformed plasma channel and a schematic of the driving circuits developed
at Strathclyde are shown in FIG. 3 and FIG. 4.
FIGURE 3. 5 cm long, 300 urn diameter preformed plasma waveguide capillary at Strathclyde.
907
1 - 5 cm long 300 urn diameter capillary
OOOfflffiD
switch
ll-^L.
HV = 35 kV
FIGURE 4. Schematic of capillary discharge circuit
Guiding of laser pulses with peak input intensities of up to 10,17-Wcm" in pi;
•lasma
channels of up to 5 cm long and densities between 1.5 x 1018 cm"3 to 1.5 x 1019 cm"3
has already been demonstrated by the Oxford group [14,15], and more recently at
Strathclyde. Guiding laser pulses with a constant spot size requires the mode of the
plasma channel to be matched to the input pulse cross-section by adjusting the radius
rc of the capillary. Recent magnetohydrodynamic simulations [16] have predicting that
the matched spot size is proportional to rc1/2. To achieve the necessary conditions for
acceleration it will be necessary to guide high power femtosecond laser pulses through
relatively long channels with high efficiency without exciting higher order transverse
modes or cavitation. The transverse laser profile transmitted through a preformed
hydrogen plasma channel in a 4 cm long, 300 jim diameter alumina capillary, as
shown in FIG. 3, has been measured in guiding experiments at Strathclyde.
Preliminary measurements of far field radiation patterns for 2xl0 1 8 cm" 3 plasma
densities at low intensities (~ 5 x 1016 Wcm"2) are shown in FIG. 5. The radiation
patterns are contrasted with that for waveguiding with gas filled capillary and no
discharge and waveguiding in vacuum. In these preliminary studies shown here the
laser spot size of 75 jim (1/e2) is not perfectly matched to the 300 jim diameter
capillary because of poorer than expected beam quality of the laser. A matched beam
should have a ~ 40|im spot size.
FIGURE 5. (a) Waveguiding with discharge, (b) waveguiding with gas filled capillary (no discharge)
and (c) waveguiding in vacuum.
908
In common with conventional accelerators, the maximum length over which
acceleration can occur in a uniform channel is limited by the phase slippage between
electrons and wave. As the group velocity of the laser pulse in the plasma determines
the wake phase velocity, phase matching can be achieved by suitably tapering the
plasma density along the channel [17]. Effective matching allows larger final energies
to be achieved and also offers control over the properties of the electron beam (i.e.
charge, emittance, energy spread and chirp).
The development of suitable plasma channels for acceleration will be followed by a
series of studies of acceleration at Strathclyde and RAL using their existing multiterawatt lasers. Optical and electron diagnostic systems are being developed to
measure the plasma wake and examine the influence of space-charge effects. The
biggest challenges will be the injection and synchronization of the electron beam from
the photoinjector, and preserving the bunch temporal structure and shape while
transporting to and through the plasma-channel.
Laser Sources
The Strathclyde Electron and Terahertz to Optical Pulse Source (TOPS) [5] is a
Scottish user facility comprising a femtosecond oscillator and three synchronized
high-power Tiisapphire amplifier chains. The lasers are used to generate new
frequencies from the XUV to the IR using parametric and harmonic generation and
nonlinear mixing. The lasers include a 10 Hz, 5 TW, 50 fs Tiisapphire laser system,
two further 1 kHz amplifiers capable of delivering up to 3 ml in 40 fs, nonlinear
crystals for 2nd and 3rd harmonic generation, an infrared OPA and optical diagnostic
equipment. A Fourier plane filter is being constructed to provide optical pulses with
arbitrary temporal structure. The terawatt laser has been set up as a driver of the
LWFA, advanced laser-plasma studies and XUV and hard x-ray radiation production.
The Central Laser Facility at RAL maintains and operates two powerful and
versatile lasers systems - the Ndiglass laser Vulcan and the Tiisapphire laser ASTRA
- for investigations by researchers. The Vulcan laser has been upgraded to provide
pulses with peak powers of up to 1 PW on target. The ASTRA laser operates at 1 Hz
and can deliver 50 fs, 10 TW laser pulses capable for producing fields with an
intensity of up to 1019 Wcm"2.
Electron Beam and Plasma Diagnostic Systems
There are a number of significant challenges in determining the electron beam
parameters and the PEL output beam characteristics. The Coulomb field of the
electron bunch will be measured using electro-optic techniques developed for THz
time-domain spectroscopy. Several other diagnostic tools, including a pepper-pot
emittance diagnostic system, with a resolution sufficient to measure small emittance,
and a high-resolution energy spectrometer, for the measurement of electron energy
distribution, are being developed.
A 200 period 1.5 cm focussing undulator, alignment targets, optical system and a
vacuum chamber will be used to create a EEL amplifier. A matched electron beam
909
transport system will be designed to guide the electron beam through the laser-plasma
transport system
will be designed
to guide
electronstudy
beam of
through
the laser-plasma
accelerator
and undulator.
This will
allowthe
a detailed
amplification
up to the
accelerator
and
undulator.
This
will
allow
a
detailed
study
of
amplification
to the
non-linear superradiant regime to be evaluated. These studies should up
show
the
non-linear
superradiant
regime
to
be
evaluated.
These
studies
should
show
thexfeasibility of using a possible future 1 GeV LWFA to realize an ultra-short pulsed
feasibility of using a possible future 1 GeV LWFA to realize an ultra-short pulsed xray source in the water window.
ray source in the water window.
Microwave techniques are well established as diagnostic tools for characterizing
Microwave techniques are well established as diagnostic tools for characterizing
low density plasma [18]. The advent of the laser has extended these interferometric
low density plasma [18]. The advent of the laser19has extended
these interferometric
techniques to plasmas with densities as high as 1019 cm3-3. Alternatives to direct use of
techniques to plasmas with densities as high as 10 cm" . Alternatives to direct use of
lasers are new methods based on terahertz time domain spectroscopy (THz-TDS),
lasers are new methods based on terahertz time domain spectroscopy (THz-TDS),
which
techniques to
to be
be used
usedtotofully
fullycharacterizing
characterizingplasma
plasmaover
over
which enables
enables time
time resolved
resolved techniques
aa wide
range
of
densities.
This
new
method
may
find
application
in
the
wide range of densities. This new method may find application in the
characterization
and
monitoring
of
industrial
plasma,
tokomaks,
laser
and
beam
driven
characterization and monitoring of industrial plasma, tokomaks, laser and beam driven
wakefield
radiation sources
sources based
based laser-plasma
laser-plasma interactions
interactions and
and
wakefield accelerators,
accelerators, radiation
conventional
plasma.
conventional gas
gas lasers
lasers plasma.
The
involves measurement
measurementof
ofthe
thedispersion
dispersionand
andattenuation
attenuationofof
The THz-TDS
THz-TDS technique
technique involves
broadband
pulses of
of coherent
coherent THz
THz radiation
radiation [5]
[5] on
on transmission
transmission
broadband subpicosecond
subpicosecond pulses
through
the complex
complex refractive
refractive index
index and
and hence
hencethe
thedensity
densityand
and
through plasma
plasma to
to measure
measure the
collisional
plasma. To
To demonstrate
demonstrate the
the utility
utilityofofthe
thetechnique
techniquewe
we
collisional frequency
frequency of
of the
the plasma.
have
measured
the
plasma
characteristics
of
a
non-magnetized
He
discharge
plasma
have measured the plasma characteristics of a11non-magnetized
He discharge plasma11
1 and a density in the range of 10
[19]
frequency of
of 10
1011 s"s-1
[19] with
with aa typical
typical collisional frequency
and a density in the range of 1011
-3
13
-3
cm
phase shifts
shifts over
over most
mostof
ofthe
thespectrum
spectrumofofthe
theTHz
THzprobe
probe(50
(50
cm"3 and
and 10
1013 cm
cm"3. Large phase
GHz
to
3
THz)
used
in
the
measurements
has
enabled
lower
plasma
density
to
GHz to 3 THz)
measurements has enabled lower plasma density to bebe
measured
than
is
possible
using
interferometric techniques
techniques with
with mid-IR
mid-IRtotovisible
visible
measured than
interferometric
wavelength
lasers.
The
large
absorption
coefficient
at
low
frequencies
makes
these
wavelength lasers.
absorption coefficient at low frequencies makes these
THz-time
domain
methods
a
particularly
direct
and
practical
method
of
measuring
THz-time domain
particularly direct and practical method of measuring
collisional
plasma.
collisional properties
properties of the plasma.
For Interact Generator!
;by laser pulse
'Defection via .-electro-optic
effect an loser pulse
FIGURE 6. Schematic of THz time domain diagnostic system
FIGURE 6. Schematic of THz time domain diagnostic system
In the experiments described here, a biased large area GaAs wafer Auston switch is
In the experiments described here, a biased large area GaAs wafer Auston switch is
used as a source of quasi-unipolar THz radiation pulses [20]. The electric field of the
used as a source of quasi-unipolar THz radiation pulses [20]. The electric field of the
910
free space
THz pulse
pulse transmitted
transmitted through
through the
the plasma
plasma is sampled
by aa ZnTe
ZnTe electroelectrofree
space THz
THz
sampled by
by
free space
pulse transmitted
through the
plasma isis sampled
a ZnTe electrooptic detector
detector [21],
as shown
shown in
in FIG.
FIG. 6.
6. A
A Tiisapphire
Ti:sapphire laser
laser [5]
[5] provides
provides ~~ 11 ml,
mJ, 800
800
optic
[21],
as
optic detector [21], as shown in FIG. 6. A Ti:sapphire laser [5] provides ~ 1 mJ, 800
nm,
80
fs
long
pulses
for
initiating
the
THz
emission
from
the
GaAs
emitter,
and
nm,
80
fs
long
pulses
for
initiating
the
THz
emission
from
the
GaAs
emitter,
and
nm, 80 fs long pulses for initiating the THz emission from the GaAs emitter, and
sampling
the THz
THz pulse
pulse in
in aaa 111 mm
mm thick
thick <<
< 110>
110>
ZnTe crystal.
crystal. The
Theemitter
emitterand
and
sampling the
the
THz
pulse
in
mm
thick
110> ZnTe
ZnTe
crystal.
The
emitter
and
sampling
detector
systems
have
a
bandwidth
of
several
terahertz.
FIG.
7
shows
a
typical
detector
systems
have
a
bandwidth
of
several
terahertz.
FIG.
7
shows
a
typical
detector systems have a bandwidth of several terahertz. FIG. 7 shows a typical
spectrum
and temporal
temporal profile
profile of
of the
the THz
THz pulse
pulse emitted
emitted from
from the
theGaAs
GaAswafer,
wafer,
spectrum and
and
temporal
profile
of
the
THz
pulse
emitted
from
the
GaAs
wafer,
spectrum
measured
using
the
THz-TDS
techniques.
measured
using
the
THz-TDS
techniques.
measured using the THz-TDS techniques.
1
1
0
0
loglog
|E(ω)|
10 10
|E(ω)|
-1
-1
E(t)
E(t)
-2
-2
-3
-3 0
0
1
1
ν (THz)
ν (THz)
2
2
3
3
4
4
00
-5
-5
000
5
55
delay
(ps)
delay (ps)
(DS)
delay
10
10
10
FIGURE 7.
7. Probe
Probe quasi-unipolar
quasi-unipolar THz
THz pulse
pulsetemporal
temporal and
andspectral
spectralprofiles
profiles
quasi-unipolar
THz
pulse
temporal
and
spectral
profiles
FIGURE
uniform plasma was created in a 15
15 cm long,
tubefilled
filled with
with24
24
A uniform
long, 22 cm
cm diameter,
diameter, tube
tube
filled
with
24
of helium.
helium. A
A transient
transient electrical
electrical discharge
discharge was
provided by
byaaa 11 kHz,
kHz,666 kV,
kV,50
50ns
ns
mbar of
was provided
by
kHz,
kV,
50
ns
solid-state high
high voltage
voltage pulse
pulse power
power supply,
which was
was synchronized
synchronized with
with
rise time solid-state
supply, which
which
was
synchronized
with
Ti:sapphire laser.
laser. A
A variable
variable delay
delay between
between the
the laser
laser and
power supply
supplyallowed
allowedthe
the
the Ti:sapphire
Tiisapphire
and power
power
supply
allowed
the
to be
be measured at
at different
different times
times thus
thus enabling
enabling the
the properties
properties to
to be
be sampled
sampled
plasma to
the
properties
to
be
sampled
over the
the ~~ 300
300 ns
ns duration
duration of
of the
the discharge
discharge current.
current. FIG.
FIG. 88 shows
shows the
the measured
measured THz
THz
FIG.
shows
the
measured
THz
over
probe
pulse
after
propagation
through
different
density
plasmas.
A
THz
reference
density plasmas.
plasmas. A
A THz
THz reference
reference
probe pulse after propagation through different density
pulse is
is obtained
obtained from
from aa measurement
measurement of
of the
the transmitted
transmitted THz
THz pulse
pulsewithout
withoutdischarge.
discharge.
pulse
reference
~8
5
10
tima [ps]
15
20
Measurement of
of plasma
plasma density.
FIGURE 8. Measurement
density.
911
Standard THz-TDS techniques are used to make sub-picosecond resolution
measurements of the phase shifts and amplitude changes experienced by the THz
probe on transmission through the plasma over the full spectral bandwidth available.
This allows the frequency dependence of the absorption coefficient and refractive
indice to be determined from a single measurement with a spectral resolution less than
10 GHz.
We plan to measure the electron beam profile by sampling the Coulomb field of the
electron beam using an electroptic crystal, such as ZnTe, and a chirped sampling pulse
to make a time to frequency transformation in a spectrometer, as shown in FIG. 9.
This techniques, which is very similar to those described above for measuring the
plasma properties, can also be used to measure coherent transition radiation pulse
shapes to infer the electron bunch shape.
transition
radiation
/"""\
/
\
time-to-frequency
transformation
Pulse
or
direct
chirped sampling pulse
measurement
of Coulomb!
field
Coulomb field of
electron bunch
spectrometer
electro-optic
crystal
Lorentz contraction
7
FIGURE 9. Chirped pulse measurement of electron bunch shape using THz timedomain spectroscopic techniques.
In summary, we have presented an outline of the ALPHA-X laser-wakefield
experiment that has recently been set up in the UK.
ACKNOWLEDGMENTS
The following people, who form the ALPHA-X consortium and those who are
collaborating in one from or another, are gratefully acknowledged: Bob Bingham
(RAL), Alan Cairns (St Andrews), Keith Burnett (Oxford), Mike Poole (Daresbury),
Terry Garvey (IN2P3, CNRS, France), Simon Hooker (Oxford), Padma Shukla (RuhrUniversitaet Bochum, Germany), Henry Hutchinson (RAL), Peter Norris (RAL), Ken
912
Ledingham (Strathclyde), Klaas Wynne (Strathclyde), Gennady Shvets (Fermilab),
Antonio Ting (NRL), Karl Krushenlick (Imperial College), Tom Katsouleas (USC),
Bucker Dangor (Imperial College), Allan Gillespie (Abertay-Dundee), Allan McCloud
(Abertay-Dundee), Justin Wark (Oxford), lan Walmsley (Oxford), Warren Mori
(UCLA), Chan Joshi (UCLA), Tito Mendonca (Instituto Superio Technico, Lisbon),
David Jones (Strathclyde), Bernhard Ersfeld (Strathclyde), David Clark (Strathclyde),
Steven Jamison (Strathclyde), Riju Issac (Strathclyde), Ken Muir (Strathclyde) and
Marnix van der Wiel (Eindhoven), Kees van der Geer (Holland), Bas van der Geer
(PULSAR, Holland) and Marek Loos (PULSAR, Holland). The EPSRC is gratefully
acknowledged for supporting the project.
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