Peculiarities of the bunch shape monitor operation for high-intensity electron beams
The simulation results of the Bunch Shape Monitor operation using coherent transformation of a time structure of an analyzed high-intensity electron beam into a spatial one of low-energy electrons emitted from a wire target will be presented. The electromagnetic field of an analyzed bunch disturbs t...
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Національний науковий центр «Харківський фізико-технічний інститут» НАН України
2001
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| Цитувати: | Peculiarities of the bunch shape monitor operation for high-intensity electron beams / V.A. Moiseev, A.V. Feschenko // Вопросы атомной науки и техники. — 2001. — № 3. — С. 131-133. — Бібліогр.: 7 назв. — англ. |
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irk-123456789-792552015-03-31T03:02:04Z Peculiarities of the bunch shape monitor operation for high-intensity electron beams Moiseev, V.A. Feschenko, A.V. The simulation results of the Bunch Shape Monitor operation using coherent transformation of a time structure of an analyzed high-intensity electron beam into a spatial one of low-energy electrons emitted from a wire target will be presented. The electromagnetic field of an analyzed bunch disturbs the trajectories of secondary electrons, thus resulting in a degradation of phase resolution and in errors of phase position reading. Moreover there is a perturbation of the target potential due to the current compensating emission of the secondary electrons. The accuracy analysis has been carried out. The confident result to achieve the phase resolution less then one degree was obtained. 2001 Article Peculiarities of the bunch shape monitor operation for high-intensity electron beams / V.A. Moiseev, A.V. Feschenko // Вопросы атомной науки и техники. — 2001. — № 3. — С. 131-133. — Бібліогр.: 7 назв. — англ. 1562-6016 PACS numbers: 29.27.Bd http://dspace.nbuv.gov.ua/handle/123456789/79255 en Вопросы атомной науки и техники Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
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The simulation results of the Bunch Shape Monitor operation using coherent transformation of a time structure of an analyzed high-intensity electron beam into a spatial one of low-energy electrons emitted from a wire target will be presented. The electromagnetic field of an analyzed bunch disturbs the trajectories of secondary electrons, thus resulting in a degradation of phase resolution and in errors of phase position reading. Moreover there is a perturbation of the target potential due to the current compensating emission of the secondary electrons. The accuracy analysis has been carried out. The confident result to achieve the phase resolution less then one degree was obtained. |
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Article |
| author |
Moiseev, V.A. Feschenko, A.V. |
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Moiseev, V.A. Feschenko, A.V. Peculiarities of the bunch shape monitor operation for high-intensity electron beams Вопросы атомной науки и техники |
| author_facet |
Moiseev, V.A. Feschenko, A.V. |
| author_sort |
Moiseev, V.A. |
| title |
Peculiarities of the bunch shape monitor operation for high-intensity electron beams |
| title_short |
Peculiarities of the bunch shape monitor operation for high-intensity electron beams |
| title_full |
Peculiarities of the bunch shape monitor operation for high-intensity electron beams |
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Peculiarities of the bunch shape monitor operation for high-intensity electron beams |
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Peculiarities of the bunch shape monitor operation for high-intensity electron beams |
| title_sort |
peculiarities of the bunch shape monitor operation for high-intensity electron beams |
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Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
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2001 |
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http://dspace.nbuv.gov.ua/handle/123456789/79255 |
| citation_txt |
Peculiarities of the bunch shape monitor operation for high-intensity electron beams / V.A. Moiseev, A.V. Feschenko // Вопросы атомной науки и техники. — 2001. — № 3. — С. 131-133. — Бібліогр.: 7 назв. — англ. |
| series |
Вопросы атомной науки и техники |
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AT moiseevva peculiaritiesofthebunchshapemonitoroperationforhighintensityelectronbeams AT feschenkoav peculiaritiesofthebunchshapemonitoroperationforhighintensityelectronbeams |
| first_indexed |
2025-07-06T03:17:54Z |
| last_indexed |
2025-07-06T03:17:54Z |
| _version_ |
1836865945855852544 |
| fulltext |
PECULIARITIES OF THE BUNCH SHAPE MONITOR OPERATION
FOR HIGH- INTENSITY ELECTRON BEAMS
V.A. Moiseev, A.V. Feschenko
Institute for Nuclear Research, Russian Academy of Sciences
60th October Anniversary Prospect, 7a
Moscow, 117312, Russia, e-mail: moiseev@al20.inr.troitsk.ru
The simulation results of the Bunch Shape Monitor operation using coherent transformation of a time structure of an
analyzed high-intensity electron beam into a spatial one of low-energy electrons emitted from a wire target will be
presented. The electromagnetic field of an analyzed bunch disturbs the trajectories of secondary electrons, thus re-
sulting in a degradation of phase resolution and in errors of phase position reading. Moreover there is a perturbation
of the target potential due to the current compensating emission of the secondary electrons. The accuracy analysis
has been carried out. The confident result to achieve the phase resolution less then one degree was obtained.
PACS numbers: 29.27.Bd
1 INTRODUCTION
There is a long-term experience of INR Accelerator
Division in development of Bunch Shape Monitors
(BSM), Bunch Length and Velocity Detectors (BLVD)
and Three-Dimensional Bunch Shape Monitor (3D-
BSM) [1]÷[5]. Operation of all these detectors is based
on the same principle: the longitudinal structure of a
beam under study is coherently transformed into a spa-
tial one of the low energy secondary electrons through
transverse RF modulation. The properties of low- ener-
gy secondary electrons are almost independent on the
type and the energy of primary particles and hence the
detectors look to be applicable for a large variety of ac-
celerated beams.
Typically the phase resolution of the detectors is
about 1o at the frequencies of hundreds MHz. The reso-
lution is determined by a number of parameters. The
most complicated effects are due to the influence of
electromagnetic field of the analyzed beam. The fields
disturb the trajectories of the electrons thus resulting in
degradation of the accuracy of measurements. Another
effect is the perturbation of the potential of the target
due to the current in the wire induced by a bunch as well
as to the current compensating emission of the sec-
ondary electrons.
In this paper the simulation results are presented for
studies of a possibility to create a detector of three-di-
mensional distribution for the DESY photo-injector
(PI). Below the parameters of the DESY photo-injector
beam are presented:
− type of particles: electrons;
− beam energy: 20 Mev;
− bunch charge: ~1nC;
− bunch dimensions: σx≈1mm, σy≈1mm, σz≈1mm;
− accelerating frequency: 1.3 GHz;
− period of bunch sequence: 80 ns;
− beam pulse duration: 800 µs;
− beam pulse repetition rate: up to 10 Hz.
From point of view of detector operation the follow-
ing parameters are extreme: high density of a charge in
bunches, small longitudinal dimensions, large pulse du-
ration and relatively large beam current.
The typical BSM geometry is presented in Fig. 1.
The electron motion is analyzed from target 1 to the
plane of electron collector 4.
Fig. 1. General configuration of the BSM for DESY
Photo-Injector (1 - target, 2 - collimator of sec-
ondary electrons, 3 - RF deflector, 4 - multi-channel
collector, 5 - horisontal collimators, 6 - vertical col-
limator, 7 - screen, 8 - target holders).
2 PECULIARITIES OF THE PROBLEM
Compared with the assumptions for ion beam simu-
lations [6] there are the specific features of the mea-
sured electron beam and its self fields:
As a rule, the bunch charge is essentially higher for
electron beam. It follows the self bunch electric and
magnetic fields are greater, that can lead to the
higher measurement errors.
A bunch velocity is higher for electron beams. As a
rule the electron bunch relativistic factor is tens
whereas it is in unit order for ion beams.
From above remark the electron bunch self field has
transverse polarization, its longitudinal component
is negligible and concentrates close to the bunch
ends. The longitudinal region of the electron bunch
self fields is approximately equal to the bunch
length.
The longitudinal bunch length is essentially smaller
for electron beams.
Due to the above remarks the time interval of inter-
action of the secondary electrons with bunch self
ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 2001. №3.
Серия: Ядерно-физические исследования (38), с. 131-133.
131
mailto:moiseev@al20.inr.troitsk.ru
field is drastically less for electron beams. For ion
beams the secondary electrons are under the action
of a self bunch fields over the full transport distance
target-collimator 2, whereas for electron beams the
interaction takes place approximately up to the mo-
ment when bunch has traversed the target wire. In
latter case the interaction travel distance for elec-
trons is less then 0.5 mm.
3 DESCIPTION OF THE MODELS
The motion of the electrons from the target to input
collimator 2 (Fig. 1) was analyzed for the real 3D geom-
etry. Downstream of input collimator a 2D model was
used. The field in the target-collimator region satisfies
the Poisson equation:
0/),(),( ερ trtrEdiv
= , ),Г(Г tf
=φ (1)
where ),( tr
ρ is a charge density in the bunch of the an-
alyzed beam at the moment of time t, φГ is a boundary
potential. One can split the problem (17) into two inde-
pendent problems to find the fields 1E
and 2E
(
21 EEE += ) with the boundary conditions φГ1 and φГ2
(φГ=φГ1+φГ2).
Problem 1: 0)(1 =rEdiv , )(11 Γ=Γ fφ (2)
The field 1E
can be found from a solution of the
Laplace equation for the potential )(1 rφ without a beam:
)()( 11 rgradrE φ−= .
Problem2: In this problem the electromagnetic field of a
moving bunch for zero boundary condition is calculated.
Generally the bunch generates both electric and magnet-
ic fields and a complete system of Maxwell equations
must be solved. To simplify the problem we consider
the field to be electrostatic in the reference frame mov-
ing with the bunch. In this frame we assume all the par-
ticles of the bunch to be at rest and the geometry of the
boundaries to be defined according to Lorenz transfor-
mations. The field in a beam frame is defined by charge
distribution in bunches and by zero boundary conditions
and can be found as a solution of Poisson equation:
0000002 /),(),( ερ trtrEdiv
= , 0),Г( 0002Г02 == tf
φ (3)
The subscript “0” indicates that the beam frame is
considered. After solving the equation (3) the electric
and the magnetic fields in the laboratory frame can be
found with the help of Lorenz transformations for elec-
tromagnetic fields. The equations (2) and (3) were
solved numerically for a 3D uniform mesh.
The boundary condition (3) means that the boundary
charge distribution is exactly tracking a charge distribu-
tion of the bunch as it passes through the detector. How-
ever this assumption is valid for relatively slow process-
es. The criterion of slowness of the process can be for-
mulated as Dz/Vz > L0/c, where Dz and Vz are typical lon-
gitudinal dimension of the bunch and its velocity corre-
spondingly and 0L - typical dimension of the boundary
elements. In our case Vz=c and the longitudinal rms
bunch size σz can be used in the capacity of Dz. One half
of the target length can be treated as L0. For σz =1 mm
and a target wire length of 45 mm the above condition is
not satisfied. Nevertheless in this case Problem 2 can be
used for an extreme estimation.
Another extreme estimation can be done by assum-
ing that the distribution of the charge in the target does
not change at all. In this case the bunch “does not see”
the target and the boundary conditions for Problem 2
should be modified: the target should be excluded from
the geometry.
One can expect that in reality the effects are con-
fined within the limits of the above two extremities.
Hence we did the simulations for the two extreme cases.
We will refer to the extremities as to Model #1 and
Model #2.
Influence of the effects of space charge in Bunch
Shape Monitor comes through in two ways: degrading
of phase resolution and arising of phase errors.
For Model #1 changing of the charge distribution
along the target q(y,t) is related with the current along
the wire. In its turn, the current results in some extra
voltage on the target. This voltage can be estimated if
the target is considered as a transmission line [6]. In this
case the current in the wire and the voltage satisfy the
equations:
y
tyi
t
tyq
∂
∂=
∂
∂ ),(),(
; ),(),(),( tyRi
t
tyiL
y
tyU +
∂
∂=
∂
∂
(4)
Here L, C and R are inductance, capacitance and resis-
tance of the transmission line per unit length. These pa-
rameters have been considered to be the same as those
for a coaxial transmission line with the outer and inner
diameters of 50 mm and 0.1 mm, respectively, and a
frequency of 300 GHz. One can show that a reasonable
variation of the parameters does not strongly influence
the final results. The line was assumed to be grounded
at the ends of the target because the dimensions of the
target holders are much larger than the transverse di-
mension of the target.
The model of a transmission line can also be used to
calculate a voltage on the target because of the currents
due to emission of secondary electrons. In this case the
voltage can be described by the equation:
t
ILRI
t
URC
t
ULC
y
U
∂
∂+=
∂
∂−
∂
∂−
∂
∂
2
2
2
2
, (5)
Here I(y,t) is a distributed current generator due to sec-
ondary emission. It can be written as
( )
yt
yy
Tt
ty
e
TI
tyI σσ
σπ σ
22
2
0
2
0
2
2
),(
−
−
−
−
= (6)
The value of the average electron current I0 was esti-
mated to be about 120 µA [7].
4 RESULTS OF SIMULATIONS
Some results of simulations are presented in
Fig. 2, 3. One should note that the electrons
corresponding to the head of the bunch (left part of the
curves) are influenced by the bunch fields much
stronger than those corresponding to the tail. Fig. 2
shows deviation of energy of secondary electrons
passing through the input collimator 2. The behaviour of
the curves is rather complicated and is different for
different parameters and models. For Model #2 (empty
132
signs) the secondary electrons are always decelerated,
the biggest energy deviation corresponding to the head
of the bunch. For the Model #1 (dark signs) the
behaviour of the curves is more complicated. Due to the
charge located close to the target the decelerating effect
is prevailing.
The behaviour of a phase error generally follows the
behaviour of energy deviation. To decrease the phase
error one should collimate the beam, increase the target
potential and locate the deflector 3 as close as possible
to the beam axis.
Fig.2. Deviation of energy of electrons at the input
collimator.
Fig. 3. Behaviour of phase resolutions along the bunch.
Small influence of the space charge on the phase
resolution and on the contrary strong influence on the
phase error is due to relativistic shrinking of the fields in
the laboratory frame: for the secondary electrons the
field is practically longitudinal.
Perturbation of the target voltage U(y,t) (4) due to
the current induced by the bunch is shown in Fig. 4. The
solution of equation (4) gives a bipolar shape of pertur-
bation. Evidently the changing of the voltage within the
range –58 kV….+25 kV is impossible: even the poten-
tial inside the bunch is equal to –3.4 kV. This result
confirms that the Model #1 gives only extreme estima-
tion of the errors.
-22.5
-13.2
-4.0
5.3
14.6
0.
00
E
+0
0
3.
33
E-
12
6.
67
E
-1
2
1.
00
E
-1
1
1.
33
E
-1
1
1.
67
E
-1
1
2.
00
E-
11
2.
33
E-
11
-6.00E+04
-5.00E+04
-4.00E+04
-3.00E+04
-2.00E+04
-1.00E+04
0.00E+00
1.00E+04
2.00E+04
3.00E+04
Vo
lta
ge
, V
Y, mm
Time, s
Fig. 4. Voltage on the target due to the charge induced
by the electron bunch.
The solution of the equation (5) is shown in Fig. 5.
The bunch produces the waves, propagating from the
target center. The waves are reflected from target hold-
ers (grounded ends of the transmission line) and propa-
gate in opposite directions. At the center of the target
the waves interfere producing a spike and continue
propagating towards the target holders etc. The magni-
tude of the waves in our case is negligible. The above
considerations should be taken into account when se-
lecting the dimensions and especially the material of the
target. Thus the improper choice of the target length can
result in a resonant interaction of the waves induced by
a series of bunches. Using the targets with bad conduc-
tivity, for example carbon wires, also can result in unde-
sirable effects.
3.
68
E-
10
3.
89
E-
10
4.
11
E
-1
0
4.
32
E-
10
4.
54
E-
10
4.
75
E-
10
4.
97
E
-1
0
5.
18
E-
10
5.
40
E-
10
5.
62
E
-1
0-22.5
-11.7
-0.9
9.9
20.7
-1.00E-03
-8.00E-04
-6.00E-04
-4.00E-04
-2.00E-04
0.00E+00
2.00E-04
4.00E-04
6.00E-04
8.00E-04
Vo
lta
ge
, V
Time, s
Y, mm
Fig. 5. Voltage on the target due to emission of sec-
ondary electrons.
5 CONCLUSIONS
The possibility to create the 3D-BSM with trans-
verse RF scanning of low energy secondary electrons
for the DESY Photo-Injector has been investigated. The
detector can provide an accuracy of longitudinal mea-
surements of 0.5°÷1°. For the vertical coordinate the ac-
curacy is defined by the size of the horizontal collimator
and is about 0.4 mm. For the horizontal coordinate the
accuracy is extremely high as the only electrons emitted
from a very small portion of the target surface are used
as an information-carrying medium. Though the prob-
lems of space charge and high density of the beam are
of extreme importance nevertheless they can be over-
come by collimating the analyzed beam. The increasing
of the target wire potential evidently decreases all the
space charge effects. The longitudinal distribution can
be measured per one beam pulse. The accuracy of both
longitudinal and vertical measurements can be improved
by decreasing the size of the collimating slits for both
primary and secondary electrons.
REFERENCES
1. A.V.Feschenko et al. Bunch shape measuring Tech-
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Proc. of the 1986 Linac Conf., Stanford, 1986, p.
323-327.
2. S.K.Esin et al. Bunch shape monitor for the SSCL
Linac // Proc. of the 1993 PAC, Washington, 1993,
p. 2426-2428.
3. Yu.V.Bylinsky et al. Bunch length and Velocity
Detector and Its Application in the CERN Heavy
Ion Linac // Proc. of the EPAC-94, London, 1994,
v. 1, p. 1702-1704.
4. S.K.Esin et al. Three Dimensional Bunch Shape
Monitor for CERN Proton Linac // Proc. of the
XVIII Int. Linac Conf., Geneva, 1996, p. 193-195.
5. A.V.Feschenko et al. Bunch Shape Monitor for the
DESY H- Linac // Proc. of the 1997 PAC, Vancou-
ver, 1997, p. 2078-2080.
6. V.A.Moiseev, A.V.Feschenko. Space Charge Ef-
fects in Bunch Shape Monitors // Proc. of the XX
133
Int. Linac Conf., Monterey, 2000, v.1, p. 178-180.
7. A.V.Feschenko et al. Three Dimensional Bunch
Shape Monitor for DESY Photo-Injector. INR inter-
nal report, Moscow, 2001.
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