as small as laser spot, which means the beam spot radius is a few microns. There is no doubt .... [9] Roth M, Cowan T E, Key M H et al. Phys. Rev. Lett., 2001, 86: ...
Submitted to “Chinese Physics C”
Preliminary design of laser accelerator beam line Y.Shang, K.Zhu*, C.Cao, J.G.Zhu, Y.R.Lu, Z.Y.Guo, J.E.Chen, X.Q.Yan*
State Key Laboratory of Nuclear Physics and Technology & Key Lab of High Energy Density Physics Simulation, CAPT, Peking University, Beijing 100871, China
Abstract: A Compact laser plasma accelerator (CLAPA) is being built in Peking University, which is based on RPA-PSA mechanism or other acceleration mechanisms. According to the beam parameters from preparatory experiments and theoretical simulations, the beam line is preliminarily designed. The beam line is mainly constituted by common transport elements to deliver proton beam with the energy of 1~50MeV, energy spread of 0~±1% and current of 0~108 proton per pulse to satisfy the requirement of different experiments. The simulation result of 15MeV proton beam with an energy spread of ±1%, current of 1×108 proton per pulse and final spot radius of 9mm is presented in this paper. Key words: laser plasma accelerator, beam line, common transport elements PACS: 41.75.Jv, 52.59.-f, 41.85.Lc
stage [11-19]. The reason is that the beam produced
1 Introduction
by laser accelerator has the characteristic of High energy particles can be produced [1]
when laser interacts with the plasma . The laser 12
short duration, high pulse current and wide energy spectrum
[20]
. The beam pulse duration is
accelerator can acquire 10 V/m accelerating
only tens of picoseconds and 108-1010 ions are
electric field gradient which is at least 1000
produced in one pulse [21], so the peak current
[2]
.
can reach ampere scale. The initial beam spot is
The laser accelerator can accelerate ions more
as small as laser spot, which means the beam
effectively and greatly reduce the scale and cost.
spot radius is a few microns. There is no doubt
It has promising prospects in compact ultra high
that the space charge effect will be very strong.
times higher than conventional accelerators
[3]
energy ion accelerator and many applications .
Although the beam contains co-moving electrons
A compact laser accelerator (CLAPA),
which can neutralize the space charge effect at
[4-6]
the initial, these electrons will be moved out of
or other acceleration mechanisms , will be built
the beam under the effect of transport elements.
in Peking University. Many researches will be
So, the initial collection and collimation is a very
carried on CLAPA: basic research in physical
difficult and critical part of the beam line. Many
mechanism of laser-plasma acceleration, ultra
kinds of elements are tried, like permanent
which is according to RPA-PSA mechanism [7]
short
and
intense
pulse
beam
transport,
self-supporting ultra-thin target production; application research in medicine
[8]
, inertial con-
finement fusion (ICF) [9], astrophysics etc. At present, Laser accelerators are studied in [10]
magnet quadrupole lens [14] [15]
[12] [13]
, solenoid magnet
, laser triggered micro-lens
[16]
. Particle
selection is another critical part. Because the beam has wide energy spectrum, the chromatic aberration of collected beam is serious. The
, but the
beam must be selected with desired energy spec-
application research is still in an exploratory
tra. It usually depends on bending magnet [17] or
many institutes around the world
Submitted to “Chinese Physics C” a set of dipole magnets [18] [19].
First order simulation of transport is carried
To solve the problems above, we prelimi-
by program Trace-3D and high order simulation
narily design a beam line for CLAPA. The beam
is carried by program Track [22]. The simulation
line is mainly constituted by common transport
results are shown in Fig.2.
elements to deliver proton beam with the energy
BEAM AT NEL1= 1 H A= 0.00 B=0.100E-03 V A= 0.00 B=0.100E-03
I= 0.0 mA W= 15.0000 15.0000 MeV FREQ= 0.00 MHz WL=********* mm EMITI= 0.250 0.250 0.00 EMITO= 150.888 0.250 0.00 N1= 1 N2= 21
of 1~50MeV, energy spread of 0~±1% and cur-
MATCHING TYPE = 8 DESIRED VALUES (BEAMF) alpha beta x 0.0000 0.0001 y 0.0000 0.0001
8
rent of 0~10 proton per pulse
BEAM AT NEL2= 21 H A=0.315E-02B=0.556 V A= 170. B= 276.
0.010mm x 100.000mrad Z A= 0.00
15.000mm x 100.000mra
MATCH VARIABLES (NC=4) MPP MPE VALUE 1 2 50.25767 1 4 -46.89536 1 6 27.09857 1 18 -2.78246 1 20 3.68215
B=0.242E-08
2 Beam line
Z A= 1.42
B=0.417E-06
P B O Lab T R A C E DATE: 11-27-2013 TIME: 14:44:32 0.000Degx 500.000KeV NP1= 1 50.00 mm(Horizontal)
In this section, we mainly talk about the
Q Q Q 123456
7
beam line on request of biomedical irradiation.
Q Q 8 9 10 11
0.000Degx 500.000KeV NP2= 21
0.0 Deg.(Longitudinal)
12
E B E 13 14 15
16
Q Q 17 18 19 20
50.00 mm(Vertical)
21
Length=
8921.21 mm
The beam parameters are shown in Tab.1. The simulation of the higher energy beam transport, like 50MeV proton beam, is shown in the end. Tab.1 beam parameters of CLAPA ion
proton
energy
15MeV
current
1×108proton/pulse
Fig.2 Simulation results of proton beam transport
initial energy spread
±15%
The upper is the result of Trace-3D and the lower is the result
final energy spread
±1%
of Track
initial transverse radius
0.005mm
Comparing the results of two programs, the
initial longitudinal length
1.06mm
beam envelopes are almost the same except the
Final transverse radius
9mm
size of final spots. The difference is caused by
Final longitudinal length
70mm
chromatic aberration. That is to say, the big en-
The beam line is constituted by two parts:
ergy spread causes the difference. There is no
collection part and analysis part. In collection
good way to deal with the chromatic aberration,
part, the aperture is used to remove big diver-
but reducing the energy spread after analyzing.
gence angle protons as more as possible at the
The detail of beam line structure is presented
initial. Then, the beam is collected by a
below.
quadrupole-triplet lens and a quadrupole-doublet
Aperture: Due to the laser acceleration
lens. In analysis part, the beam is analyzed by a
mechanism and high space charge effect, the
bending magnet. Finally, the beam is focused by
beam has big divergence angle. It is impossible
another quadrupole-doublet lens and delivered to
to deliver all of the protons to the end. To avoid
experiment platform. The structure of beam line
the influence of big divergence angle ions and
is shown in Fig.1.
reduce the space charge effect, the beam is screened by an aperture at the beginning of transport. The radius of aperture is 3mm. The distance is 50mm away from laser target. The proton beam passes through the aperture with divergence
angle
of
±50mrad,
transverse
emittance of 0.25π mm.mrad and current of 1×108 proton/pulse. Fig.1 Schematic diagram of beam line
Collecting lens: After passing the aperture,
Submitted to “Chinese Physics C” the beam will expand fast in transverse direction.
is 300mm and the distance is 20mm between
It is necessary to focus the beam as early as pos-
each other. When the magnet fields of lens are
sible to avoid the unnecessary losses. A
-0.278, 0.368 kG/cm, the radius of beam focus is
quadrupole-triplet lens is designed to focus the
9mm.
beam which is adjoined to the aperture. The inner radius of lens is 20mm, the length of lens is
3 Efficiency of transport
100mm and the distance is 80mm between each other. When the magnet fields of lenses are 5.00,
From the simulation of 15MeV proton
-4.71 and 2.75 kG/cm, the proton beam can be
beam by program Track, we can get the effi-
perfectly collected.
ciency of transport. The result is shown in Fig.4.
Assistant collecting lens: The collecting lens can collect the protons with the energy lower than 30MeV. But it is difficult to collect the protons with higher energy because of the limit of magnet field. So, a quadrupole-doublet lens is added to assist collection. The inner radius of lens is 40mm, the length of lens is 250mm and the distance is 150mm between each other. Bending magnet: The proton beam produced by laser accelerator has wide energy spectrum and a lot of different ions. Although the different ions may be screened partly in the collecting stage due to the chromatic aberration, the proton beam still contains different ions. To get the high quality proton beam, a 45° bending magnet is used to analyze beam. The radius of bending magnet is 650mm. The analyzing ability of bending magnet is simulated by program Track (Fig.3).
Fig.4 The efficiency simulation of 15MeV proton beam A: The energy spread is ±15%, B: The energy spread is ±1%.
Most of protons with the energy spread of ±15% can be delivered to the bending magnet after being focused by collecting lens. Some protons with big energy spread impact the vacuum tube of deflection magnet in vertical direction, the others impact the vacuum tube and analysis aperture outside of bending magnet in Fig.3 Simulation of analyzing ability of bending magnet The energy of center beam is 15MeV. The other beams have
horizontal direction. Finally, we can get the proton beam within the energy spread of ±1%.
±1% and ±2% different energy compare with the center
The efficiency of all protons is nearly 14%.
beam. The energy spread of every beam is ±0.000001%.
The efficiency of needed protons, which means
Back focus lens: A quadrupole-doublet lens
the protons with initial energy spread smaller
is used to focus the proton beam to the end. The
than ±1%, is bigger than 99%. Considering the
inner radius of lens is 50mm, the length of lens
magnet field distortion of quadrupole lens, the
Submitted to “Chinese Physics C” efficiency of needed protons is nearly 94%.
Rep. Prog. Phys., 2012, 75: 056401 [4] Yan X Q, Wu H C, Sheng Z M et al. Phys.
4 Transport of higher energy beam
Rev. Lett., 2009, 103: 135001 [5] Wang H Y, Yan X Q, Chen J E et al. Phys.
The proton beam produced by the laser ac-
Plasmas, 2013, 20: 013101
celerator has wide energy spectrum. In order to
[6] Wang H Y, Lin C, Sheng Z M et al. Phys.
widen the scope of application, the transport of
Rev. Lett., 2011, 107: 265002
higher energy proton beam is also taken into
[7] Mackinnon A J, Sentoku Y, Patel P K et al.
consideration. The transport of higher energy
Phys. Rev. Lett., 2002, 88: 215006
beam is achieved just by changing the magnet
[8] Fuchs J, Cecchetti C A, Borghesi M et al.
fields of lenses and bending magnet. The
Phys. Rev. Lett., 2007, 99: 015002
transport of 50MeV proton beam is shown in
[9] Roth M, Cowan T E, Key M H et al. Phys.
Fig.5.
Rev. Lett., 2001, 86: 436
BEAM AT NEL1= 1 H A= 0.00 B=0.143E-03 V A= 0.00 B=0.143E-03
I= 0.0 mA W= 50.0000 50.0000 MeV FREQ= 0.00 MHz WL=********* mm EMITI= 0.175 0.175 0.00 EMITO= 262.443 0.175 0.01 N1= 1 N2= 21
BEAM AT NEL2= 21 H A=-.864E-01B=0.379 V A= 21.6 B= 102.
Mod. Phys., 2013, 85: 751
MATCHING TYPE = 5 DESIRED VALUES (BEAMF) alpha beta x 0.0000 0.0001 MATCH VARIABLES (NC=2) MPP MPE VALUE 1 8 9.34028 1 10 -5.81397 1 18 -4.11176 1 20 6.82970
0.010mm x 100.000mrad Z A= 0.00
B=0.416E-09
15.000mm x 100.000mra Z A=0.787
B=0.637E-07
P B O Lab T R A C E DATE: 11-04-2013 TIME: 14:47:00
0.000Degx 500.000KeV NP1= 1 70.00 mm(Horizontal)
Q Q Q 123456 7
Q Q 8 9 10 11
12
[11] Ter-avetisyan S, Schnurer M, Polster R et al. Laser and Particle Beams, 2008, 26: 637 [12] Schollmeier M, Becker S and Geißel M,
0.000Degx 500.000KeV NP2= 21
0.0 Deg.(Longitudinal)
E B E 13 14 15
[10] Macchi A, Borghesi M and Passoni M, Rev.
16
Q Q 17 18 19 20
70.00 mm(Vertical)
21
Length=
8821.21 mm
Fig.5 The transport of 50MeV proton beam
Phys. Rev. Lett., 2008, 101: 055004 [13] Nishiuchi M, Daito I, Ikegami M et al. Appl. Phys. Lett., 2009, 94: 061107 [14] Roth M, Alber I, Bagnoud V et al. Plasma Phys. Control. Fusion, 2009, 51: 124039
5 Summary
[15] Harres K, Alber I, Tauschwitz A et al. Phys. Plasmas, 2010, 17: 023107
The beam line of CLAPA is preliminary de-
[16] Toncian T, Amin M, Borghesi M et al. AIP
signed by common transport elements. From the
Advances, 2011, 1: 022142
simulations of program Trace-3D and Track, the
[17] Nishiuchi M, Sakaki H, Hori T et al. Phys.
beam line is able to deliver the proton beam with
Rev. ST Accel. Beams, 2010, 13: 071304
energy of 1~50MeV and energy spread within
[18] Yogo A, Maeda T, Hori T et al. Appl. Phys.
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Lett., 2011, 98: 053701
we get more accurate beam parameters.
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