Design and Experiment of 200MeV Energy Spectrum
Analysis System of Linac in NSRL
Ping Lu, Yuan Ji Pel, Baogen Sun, Weimin Li, Hongliang Xu, Zuping Liu
Jun Hong, Junhua Wang, Duohui He
NSRL, University of Science and Technology of China, P.O.Box 6022, Hefei, 230029, China
Abstract. Energy Spectrum Analytic System of 200MeV Linear Accelerator in NSRL is used to measure
the beam energy and energy spread. This paper shows the status of the system, and analyses a few of its
shortcomings. In the NSRL phase-II Project, the system is required to improve. The results of simulation
calculation and some preliminary experiment are described.
INTRODUCTION
National Synchrotron Radiation Laboratory (NSRL) uses a 200MeV linear accelerator
as the injector of Hefei Light Source (HLS). The LINAC construction began in 1984,
and commissioning was successful in 1987[1]. The beam energy spectrum analysis
system[2"5] was installed in a beam switch yard area at the end of the LINAC as is
shown in Fig.l. There are three ways for an electron beam to go by means of the
switch magnet, one way is to an electron storage ring of SOOMeV, one to the beam
dump and nuclear physics experimental hall, and another to the energy analysis system.
The energy analysis system is composed of an analysis magnet of 60°, a fluorescent
ceramic plate to image the beam spot, a reflection mirror, and a CCD. The system as
mentioned above has run well since 1988 and provided important information about
the energy spectrum of the electron beam from the 200 Mev LINAC . But the system
is not perfect, because the tunnel width where the analysis magnet was located, is not
enough so that the beam target is not located at the focus point.
Fig.l. general layout of the transport line after switch magnet. 1. switch magnet,
2. analysis magnet, 3.target, 4.to ring, 5.to beam dump, 6.to nuclear physics
experiment hall.
CP648, Beam Instrumentation Workshop 2002: Tenth Workshop, edited by G. A. Smith and T. Russo
© 2002 American Institute of Physics 0-7354-0103-9/02/$19.00
283
With the NSRL Phase II Project, our machine's performance will be upgraded, this
demands that we know the beam energy spectrum accurately, so we plan to redesign
the linac's spectrum analysis system.
Spectrum Analysis Principle
The beam motion in the energy analysis system will be described by Beam
Transformation Matrix[6]. When we have the initial conditions of the beam and the
transfer matrix between the start point and the target, we can get the terminal
information. For example, the horizontal motion is as the following[7]:
•*/
X
xf
m
x21
m
x22
(1)
x
m
x23
Ap/
V *> J
\P
Then we get:
(2)
We usually use a CCD to get the fluorescent image on the target. The intensity
distribution indicates the particle distribution. If we make mxll or mxU equal to 0 and
jc. or x't can be limited to a certain range, then we can get the energy spectrum by
means of the intensity distribution of fluorescent image on the target. According to
the principle, we designed the energy analysis system in 1984. The sketch map of the
beam line arrangement and its parameters are shown in Fig.2.
Switch Magnet Drift Space
Analytic
Drift Space
Beam
-[[Target
3500.29mm
2085.2mm
Analytic Magnet
Fig. 2 sketch map of the beam line arrangement.
Fig. 3 sketch map of the switch
magnet and the analytic
magnet.
The transfer matrix of the horizontal motion from the switch magnet entrance to the
target is the following:
0
2.683 0.9403"
-0.3727 -0.7541 0.1506
,
°
0
(3)
1 ,
In the computation, we use meter, radian as x and x 's units respectively. The half
width of the fluorescent target is 5cm=0.05m, the centre energy of HLS Linac electron
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beam is 200MeV, the half energy offset AE displayed on the target corresponds to
—xmxl3 =005^1, thus we get A£ = Q-Q5x2QQ = IQ 535 MeV's. In order to make the
E
'
0.9403
beam dispersion function less than 1mm on the target, the aperture of the scraper
should limit the dispersion angle to +
2.683
rad = ±0.186 mrad.
From the above analysis, we have the following conclusions:
1. The resolution of this system is very low. The fluorescent material granule and
the CCD have a definite resolution, the detailed spectrum will be blurred.
2. The demand on the aperture of scraper is high; it will greatly influence the
machine operation and storage injection.
As the building's construction didn't match the design well, as mentioned above, the
vacuum chamber had to be shortened so that the real length of the drift space
downstream of the analytic magnet was 1.422m(see Fig.l). Using this length, the
horizontal transfer matrix will be
0.2469
3.183 0.8405
-0.3727 -0.7541 0.1506
1
0
0
(4)
As the matrix shows, mn ^ 0, so both jt. and x\ will influence the target image.
NEW ENERGY ANALYSIS SYSTEM
According to the design requirements, we have tried and computed many schemes
and determined a layout for the new energy analysis system as shown in Fig.4.
Whereas the useable space in the existing linac tunnel is so limited, a rational idea is
Fig.4 Layout of the new energy analytic system. 1.switch magnet,
2. analysis magnet, 3. target, 4. to ring, 5. to beam dump.
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to move the analytic magnet and expand the transfer line to the spacious corner near
the nuclear physics experimental hall (see Fig.4). There is a quadrupole magnet both
downstream and upstream of the analytic magnet respectively.
Obviously, we can select an appropriate length so that the image of the object can be
displayed on the target (make the transfer matrix's element mxl2 equal zero from the
object to the target).
We can adjust the resolution of the spectrum analysis by setting the corresponding
exciting current of the quadrupoles. After a simulation calculation, we found a
problem. The py function is so big that it is difficult to transfer the beam to the target.
To solve the possible problem, we add a quadrupole magnet in the middle of the
Switch Magnet
Object k
0.689m
pi Ql
1 1
4.864m
LJ
Q2
4.976m
tAnalytic Magnel
1— 1 0.4417m
B
0.697nl
Q3
Tar
<————————— *\-^
1.818m
1
FIG.5. sketch of new energy analytic system. Ql, Q2, Q3 are the quadrupoles magnets, the dashed Ql is
supplemented, the effective lengths of Ql, Q2, Q3 are all 0.28m.
transfer line between the switch magnet and the analytic magnet, then set its K<0, to
decrease the py, so it is limited to an acceptable range. The dimension parameters of
the new energy analytic system are shown in Fig.5.
Calculated Result of Twiss Parameter
The initial Twiss Parameter at the entrance of the switch magnet is: px = 48.32974
m, py = 1.35255 m, ax = - 2.56926, ay = - 0.09582, rj = 0, TJ' =0, the emittance is
0.5mm.mrad(the horizontal and the vertical are the same) [8]. We calculated the P
function along the transport line of energy spectrum analysis system with and without
Ql. Fig.6 shows the P function when Ql is not added, the different curves present
different resolutions on the target. Fig.7 shows the curves where Ql is added (KQi=0.65m"2). It is obvious that the py is depressed, but the px is increased.
——— +4.00MeV
—-----±3.33MeV
——— +2.78MeV
——— +2.50MeV
——— +2.00MeV
.......... ±1.67MeV
—+4.00MeV
—+3.33MeV
—+2.78MeV
+2.50MeV
+2.00MeV
+1.67MeV
4
6
8
10
4
12
Distance from the switch magnet /m
6
8
10
12
Distance from the switch magnet /m
Fig.6. the (3 function curve when the Ql is not added
286
600500400,300200100-
2
4
6
8
10
12
14
2
16
4
6
8
10
12
14
Distance from the switch magnet /m
Distance from the switch magnet /m
Fig.7. the (3 function curve when the Ql is added
Table 1 is shows some parameters of the system according to the simulation
calculation. In the table, K values of Ql, Q2, Q3 are for a different energy range on
the target, the matrix element mxll is of the horizontal motion, and the element myll,
myl2 of the vertical motion from the object to the target.
From the table we can find a stirring advantage. The horizontal matrix element
mxll is very small, in the range about 0.1-0.36. We have also observed the beam spot
at the flag detector. The diameter is about 4mm, so the blurring effect by the beam
size on the target is aproximately 1mm. In the future, the system will usually run at
±2.78 MeV resolution(no current supplied to Q3), so the beam size influence is small
and we need not add a gap in the transfer line upstream of the switch magnet. The
nearest flag detector can be used for measuring the approximate beam spot size, so we
can use it as the object.
TABLE 1. Numerical Values.
AE
4.00
3.33
2.86
(±MeV)
KqiCn
0
0
0
m
**?J
2.50
2.00
1.67
4.00
3.33
2.78
2.50
2.00
1.67
0
0
0
-0.65
-0.65
-0.65
-0.65
-0.65
-0.65
-0.19
-0.884 -0.323
0
0.2073 0.4615 0.6109 -0.751
^m
1.258
0.6255
0
-0.611 -1.813 -2.981 1.2512 0.6163
mxll
-0.15
-0.207 -0.265 -0.322 -0.437 -0.552 -0.098 -0.136 -0.174 -0.212 -0.288 -0.364
K<
0.1275 0.3395 0.5939 0.7428
0
-0.625 -1.831 -3.004
m>n
-1.216 -0.716 -0.534 -0.499 -0.634 -0.887 -0.688 -0.589 -0.476 -0.354 -0.111 0.1266
myl2
-11.029 -5.266 -3.684 -4.041 -7.383 -12.22 -7.410 -4.178 -3.089 -2.979 -4.137 -6.084
EXPERIMENT RESULT
The new system was installed in August of 2001, some preliminary experiments
were performed to test the analysis system in October and November last year. Fig. 8
287
shows some photos. The upper picture was taken by setting the parameters at
±2.86MeV and no exciting current in Ql and the lower picture was taken by setting
the parameters at ±4.0MeV with no exciting current in Ql. We found the system
worked well, and all the quadrupoles did not need to be energized during normal
machine operation. Now the linac and storage ring have a long shutdown for
reconstruction, and further experiments will be conducted in the near future.
FIG.8. Photograph on the target. Upper picture was taken by setting the parameter as ±2.86MeV and no exciting
current in Ql; the lower was taken by setting the parameter as ±4.0MeV when no exciting current in Ql.
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