A new high intensity electron beam for wakefield acceleration studies

Abstract

A new RF photocathode electron gun and beamline have been built for the study of electron beam driven wakefield acceleration. The one and a half cell L-band gun operates with an electric field on the cathode surface of 80 MV/m, and generates electron bunches with tens of nanocoulombs of charge and rms bunch lengths of a few picoseconds. The beam diagnostics include a Cherenkov radiator and streak-camera for bunch length measurements, YAG screens for beam profile, integrating charge transformers (ICTs) for bunch charge, an energy spectrometer, and a pepper-pot plate for measurement of the transverse emittance. Measurements of the beam properties at various bunch charges are presented.

Full text

Page 1

A NEW HIGH INTENSITY ELECTRON BEAM FOR WAKEFIELD
ACCELERATION STUDIES*
M.E. Conde#, W. Gai, C. Jing, R. Konecny, W. Liu, J.G. Power, H. Wang, Z. Yusof
ANL, Argonne, IL 60439, USA
Abstract
A new RF photocathode electron gun and beamline
have been built for the study of electron beam driven
wakefield acceleration. The one and a half cell L-band
gun operates with an electric field on the cathode surface
of 80 MV/m, and generates electron bunches with tens of
nanocoulombs of charge and rms bunch lengths of a few
picoseconds. The beam diagnostics include a Cherenkov
radiator and streak-camera
measurements, YAG screens for beam profile, integrating
charge transformers (ICTs) for bunch charge, an energy
spectrometer, and a pepper-pot plate for measurement of
the transverse emittance. Measurements of the beam
properties at various bunch charges are presented.
for bunch length
INTRODUCTION
The Argonne Wakefield Accelerator (AWA) has been
successfully used for conducting wakefield experiments
in dielectric loaded structures [1] and plasmas [2].
Although the initial wakefield experiments were
successful, higher drive beam quality would substantially
improve the wakefield accelerating gradients. For this
reason we have built a new L-band photocathode RF gun
[3]. This gun will generate high charge bunch trains
which will be used in high gradient wakefield acceleration
experiments and other high intensity electron beam
applications.
FACILITY UPGRADE
The new AWA photocathode RF gun produces high
charge electron bunches with shorter bunch length and
lower emittance, in comparison with the previous AWA
drive gun [4]. The new AWA laser system also presents
superior performance in terms of beam profile, energy per
pulse and stability.
New Electron Gun and Beamline
The new one and a half cell RF gun operates with a
focusing solenoid and a bucking solenoid to cancel the
magnetic field on the plane of the cathode. These two
solenoids are exactly next to each other, with the
photocathode plane as their plane of symmetry. A third
solenoid is located at the exit of the gun. There is a
vacuum pumping port in the full cell, located
diametrically opposite to the RF coupler, both being at the
equator line of the full cell. The initial photocathode
consisted of a small disc of copper inserted through an
opening on the back wall of the half cell. Recently, the
photocathode material was replaced by a magnesium disc,
which has a higher quantum efficiency. The cathode
holder can also function as a tuning plunger, allowing us,
in conjunction with the gun temperature, to adjust the
parameters of the two cells, in order to achieve the right
resonance frequency for the π mode and field balance in
the cavity.
The measured value of the unloaded quality factor (Q0)
of the gun is 20300. The gun cavity is somewhat
overcoupled (S11 = −10 dB), but the installation of a
tuning post in the waveguide will improve the coupling.
The new gun has been conditioned up to 13 MW of
power, with a corresponding accelerating gradient on the
cathode surface of 80 MV/m.
The beamline (Fig. 1) has an ICT for measurement of
bunch charge at the exit of the gun, and another one
downstream of the future wakefield structures to diagnose
possible beam scraping. The beamline includes several
insertable YAG screens for observation of the beam
profile, a pepper-pot plate for measurement of the
transverse emittance, and a quartz plate as a Cherenkov
radiator for bunch length measurement in conjunction
with a streak camera. A quadrupole triplet and an energy
spectrometer are also installed in the beamline.
New Laser System
The new laser system consists of a Spectra Physics
Tsunami oscillator followed by a Spitfire regenerative
amplifier and two Ti:Sapphire amplifiers (TSA 50). It
produces 1.5 mJ pulses at 248 nm, with a pulse length of
6 to 8 ps FWHM and a repetition rate of up to 10 pps. A
final KrF Excimer amplifier is optionally used to increase
the energy per pulse to 14 mJ.
BEAM MEASUREMENTS
Measurements of beam parameters are presented in this
section. Some of the parameters are plotted as a function
of the accelerating gradient on the cathode. This number
was calculated from the value of the input RF power into
the gun, using the measured value of the Q0 of the cavity.
In the near future a field probe will be installed in the gun
cavity allowing for a more direct measurement of the field
intensity.



___________________________________________
* Work supported by DOE, High Energy Physics Division, Advanced
Technology Branch, under Contract No. W-31-109-ENG-38.
#conde@anl.gov
0-7803-7739-9 ©2003 IEEE 2032
Proceedings of the 2003 Particle Accelerator Conference

Page 2

Q
G
S
I
T
WF
SP
QQ
T
I
S
S
D
D
D
D
D
L



Figure 1: Schematic and picture of the new AWA beamline.


Dark Current
A Faraday-cup consisting of a ceramic DC break and an
aluminum block was installed in the beam line for the
measurement of dark current. An RC circuit with a time
constant of 5 seconds connected to the Faraday-cup
allowed an ammeter to measure the average dark current
when the machine was operating at a rate of five pulses
per second. Figure 2 shows a plot of the charge collected
by the Faraday-cup per RF pulse as a function of the
accelerating gradient on the cathode surface. We observe
the expected rapid increase of dark current as the
accelerating gradient in the gun is increased.
Bunch Charge
Using an ICT (Bergoz ICT-178-070-20:1), we have
measured the bunch charge as a function of the laser pulse
energy (Fig. 3a) and also as a function of the injection
phase at the gun (Fig. 3b). The quantum efficiency of the
magnesium cathode (presently about
improve when we implement the procedure for the laser
cleaning of the cathode surface [5]. At the highest laser
beam energies there is clear indication of space charge
effects on the cathode, preventing the extracted charge
from reaching much beyond 100 nC (Fig. 3a).

4
101
−
×
) will
E (MV/m)
5060 7080
charge (nC)
0
2
4
6
8

Figure 2: Dark current measurement. Average charge
collected by the Faraday-cup during one RF pulse (about
6 µs long), as a function of the accelerating gradient on
the cathode surface.
Beam Energy
The spectrometer at the end of the beamline was used to
measure the energy of the electron beam. Figure 4 shows
a plot of the beam energy as a function of the accelerating
gradient on the cathode surface. This is in good agreement
with numerical simulations (PARMELA) [6].
S
G
I
L
T
D
Q
WF
SP
= Solenoid
= RF Gun
= ICT
= Laser Input Cross
= Trim Coils
= Diagnostic Cross
= Quadrupole
= Wakefield Structure
= Spectrometer
2033
Proceedings of the 2003 Particle Accelerator Conference

Page 3

laser energy (mJ)
02468
charge (nC)
0
20
40
60
80
100
120
(a)
injection phase (degrees)
040 80120160
charge (nC)
0.0
0.5
1.0
1.5
2.0
(b)

Figure 3: Measured bunch charge: (a) as a function of the
laser pulse energy; (b) as a function of the injection phase.
E (MV/m)
50 607080 90
beam energy (MeV)
4
5
6
7
8

Figure 4: Electron Beam energy as a function of the
accelerating gradient on the cathode surface.
Bunch Length
A 1.5 mm thick quartz plate was used as a Cherenkov
radiator for bunch length measurements with a
Hamamatsu C1587 streak camera. Initial measurements
showed bunch lengths spanning from about 13 to 17 ps
FWHM, for bunch charges up to 70 nC.
Transverse Emittance
A tungsten pepper-pot plate was used for transverse
emittance measurements. The plate was 0.5 mm thick
with 0.2 mm diameter holes spaced by 5.6 mm in a cross
pattern. The profiles of the resulting beamlets are
analyzed on a YAG screen. Preliminary data indicate a
normalized emittance of 40 mm-mrad for 20 nC bunches.
IMMEDIATE NEXT STEPS
The splitting of the laser pulses into pulse trains, and
the subsequent generation of electron bunch trains will be
implemented right away.
A dielectric loaded wakefield structure has been built
and will be installed in the new beamline very shortly. It
will allow us to test the high power handling capability of
the dielectric material.
CONCLUSION
The initial measurements of the beam parameters
indicate good agreement with the design values, and
confirm the tremendous improvement that the new AWA
drive beam represents in comparison with the old drive
beam. Some of the beam diagnostics will be further
refined, allowing for more precise beam characterization
and optimization of the operating parameters. The present
magnesium photocathode will soon be replaced by a high
quantum efficiency cesium telluride cathode, enabling the
generation of long high charge bunch trains (16 or more
bunches of 40 nC). A one meter long linac structure will
be added to the beamline to increase the beam energy to
about 18 MeV, lowering the physical emittance of the
beam and facilitating studies of higher gradient wakefield
structures.

REFERENCES
[1] Gai, W. et al., “Experimental Demonstration of Two
Beam Acceleration Using Dielectric Step-Up
Transformer” in Particle Accelerator Conference-
2001, edited by P. Lucas and S. Webber, Chicago,
2001, pp. 1880-1882.
[2] Barov, N. et al., Phys. Rev. ST Accel. Beams 3,
011301 (2000).
[3] M.E. Conde et al., Proceedings of Particle
Accelerator Conference, p.3957, 2001.
[4] M.E. Conde et al., Phys. Rev. ST Accel. Beams 1,
041302 (1998); M.E. Conde et al., Proceedings of
Particle Accelerator Conference, p.1996, 1997.
[5] X.J. Wang and T.
communication.
[6] W. Gai et al., Nucl. Instr. and Meth. A 410, p.431,
1998; W. Gai et al., Proceedings of Advanced
Acceleration Concepts Workshop, Baltimore, 1998.
Srinivasan-Rao, private
2034
Proceedings of the 2003 Particle Accelerator Conference