Bunch length diagnostics with coherent relativistic electron radiation
One of the most important problems when designing resonance electron linacs with a high beam brightness is creation of an equipment for electron bunch length diagnostics. One of ways to solve this problem is based on analysis of radiation from relativistic electrons (transition, synchrotron etc.). T...
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| Zitieren: | Bunch length diagnostics with coherent relativistic electron radiation / M.I. Ayzatsky, I.V. Khodak, V.A. Kushnir, V.V. Mitrochenko, A.N. Opanasenko, D.L. Stepin // Вопросы атомной науки и техники. — 2001. — № 5. — С. 66-68. — Бібліогр.: 6 назв. — англ. |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine| _version_ | 1859818407937966080 |
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| author | Ayzatsky, M.I. Khodak, I.V. Kushnir, V.A. Mitrochenko, V.V. Opanasenko, A.N. Stepin, D.L. |
| author_facet | Ayzatsky, M.I. Khodak, I.V. Kushnir, V.A. Mitrochenko, V.V. Opanasenko, A.N. Stepin, D.L. |
| citation_txt | Bunch length diagnostics with coherent relativistic electron radiation / M.I. Ayzatsky, I.V. Khodak, V.A. Kushnir, V.V. Mitrochenko, A.N. Opanasenko, D.L. Stepin // Вопросы атомной науки и техники. — 2001. — № 5. — С. 66-68. — Бібліогр.: 6 назв. — англ. |
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| container_title | Вопросы атомной науки и техники |
| description | One of the most important problems when designing resonance electron linacs with a high beam brightness is creation of an equipment for electron bunch length diagnostics. One of ways to solve this problem is based on analysis of radiation from relativistic electrons (transition, synchrotron etc.). The paper presents results of calculations and experiments on studying the millimeter-band radiation that is a beam exited on the surface of the grating periodic structure and in a linac beam pipe. Experiments were carried out on the linac LIC with 13 MeV particle energy and 0.8 A pulse beam current. The possibility of observed radiation application for estimation of the bunch length value, monitoring its variation and for optimization of the accelerator operating mode was shown experimentally.
|
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BUNCH LENGTH DIAGNOSTICS WITH COHERENT
RELATIVISTIC ELECTRON RADIATION
M.I. Ayzatsky, I.V. Khodak, V.A. Kushnir,
V.V. Mitrochenko, A.N. Opanasenko, D.L. Stepin
National Science Center "Kharkov Institute of Physics & Technology"
1, Akademicheskaya St., NSC KIPT, 61108 Kharkov, Ukraine
e-mail: kushnir@kipt.kharkov.ua
One of the most important problems when designing resonance electron linacs with a high beam brightness is cre-
ation of an equipment for electron bunch length diagnostics. One of ways to solve this problem is based on analysis
of radiation from relativistic electrons (transition, synchrotron etc.). The paper presents results of calculations and
experiments on studying the millimeter-band radiation that is a beam exited on the surface of the grating periodic
structure and in a linac beam pipe. Experiments were carried out on the linac LIC with 13 MeV particle energy and
0.8 A pulse beam current. The possibility of observed radiation application for estimation of the bunch length value,
monitoring its variation and for optimization of the accelerator operating mode was shown experimentally.
PACS numbers: 29.27 Fh, 41.85.Qg
1 THEORETICAL CONSIDERATIONS
It is known that an electron beam moving near dis-
continuities generates electromagnetic radiation. If dis-
continuities are placed periodically along a beam trajec-
tory (for example a diffraction grating) the radiation has
specific characteristics and it was named as Smith-Pur-
cell radiation in honor of scientists who observed such
radiation for the first time [1]. Such radiation is a sub-
ject of investigation at many research centers (see for
example [2]). It was determined that the radiation wave-
length λ depends on the viewing angle θ as follows:
( ) ( )
−= θ
β
θλ cos1
n
d
, (1)
where β is the normalized electron velocity, n is the
diffraction order and d is the grating period.
The spectral intensity depends exponentially on the
parameter b (distance between the beam axis and the
highest point on the grating surface) P(λ) ∼ exp(-4πb/(λ
γβ)) (where γ is the Lorenz factor) that is featured for
the given radiation type [3]. Regardless of a grating pro-
file the maximum of intensity of incoherent radiation of
relativistic particles is in a short-wave part of a spec-
trum at viewing angles close to θ ~ 1/γ. The radiation
coherent component is observed in the wavelength
range larger than bunch sizes when a grating is excited
by relativistic electron bunches with linear sizes less or
compared with the period d. This causes significant in-
creasing the long-wave radiation intensity and changing
its angular distribution [4]. In general case the radiation
intensity in a solid angle dΩ can be represented as fol-
lows:
( )
( ) ( ) ( )( )0 4exp 1 1
dPdP bN N F
d d
θ π λ θ
λ θ β γ
= − + − Ω Ω
, (2)
where dP0(θ)/dΩ is the angular intensity distribution of
the spontaneous radiation of a single electron moving
with zero parameter b over the grating; F(λ) is the
bunch form-factor (the spatial Fourier component in
particle density average distribution in a bunch). For the
bunch with Gauss distribution it follows
( ) ( )[ ]2/2 exp λπ σλ zF −= . In this work we investigate the
coherent radiation observed in the orthogonal direction
to the grating surface (θ = 90°, so λ = d as it follows
from Eq. (1)). Fig. 1 illustrates the dependence of the ra-
diation intensity density - to - incoherent radiation inten-
sity density ratio on the grating period for various val-
ues of a bunch length σz. Calculations were carried out
for the bunch with N ≅ 1.8 109 electrons and energy of
3 MeV.
1.E -01
1.E + 00
1.E + 01
1.E + 02
1.E + 03
1.E + 04
1.E + 05
1.E + 06
1.E + 07
1.E + 08
1.E + 09
1.E + 10
0 2 4 6 8 10 12 14 16 18
G rating period (m m )
dP
/d
Ω
/(N
dP
0/d
Ω
)
1
2
3
4
5
6
Fig. 1. Relative density of the diffraction radiation
intensity: 1 - σz=0, 2 - σz=1.5 mm, 3 - σz=2.5 mm,
4 - σz=3.5 mm, 5 - σz=4.5 mm, 6 - σz=6 mm.
The strong dependence of the coherent radiation in-
tensity on the bunch length can be used for the bunch
length monitoring. Thus, for σz/λ varying form 0.2 to
0.75 the radiation intensity is decreased by a factor of
109 down to incoherent radiation threshold. Estimations
showed that for the bunch with N ≅ 1.8 109 passing with
zero parameter b over the grating with 8 mm period the
power density is NdP0(θ)/dΩ ≈ 1.2 µW/sr.
2 MONITOR
Proposed monitor consists of a copper diffracting
grating equipped with the gear for the diffraction grating
movement and the secondary emission monitor for the
measurement of the beam transverse distribution. Radia-
ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 2001. №5.
Серия: Ядерно-физические исследования (39), с. 66-68.
66
mailto:kushnir@kipt.kharkov.ua
tion is extracted from a vacuum chamber through a sap-
phire window. The gear provides movement of the grat-
ing in perpendicular direction to the beam axis to
change the distance between the beam axis and the grat-
ing as well as the grating rotation to change an effective
grating period. The radiation is received by a horn aerial
placed at 1 m from the beam axis and detected by the
calibrated detector. The H-plane of the aerial coincides
with the flute plane of the diffraction grating. Pulse sig-
nals that are proportional to the number electrons falling
on the secondary emission monitor and the grating as
well as the grating position and a the pulse signal from
the detector are measured by the computerized control
system to be observed, stored and processed. The exper-
iment set-up layout is illustrated in Fig. 2 in a simplified
way.
Fig 2. Experiment layout.
To check ability of the proposed device to monitor
the bunch length a series of simulations and experiments
were done using the linear electron accelerator LIC [5]
as a base research facility. The accelerator is applied for
investigations in the field of high-brightness electron
beam shaping and accelerating, short-wave radiation
generation and interaction of a relativistic beam with
plasma. The accelerator operating frequency is
2797.15 MHz. The main accelerator feature is the small
beam emittance for a pulse current up to 1 A. Beam per-
formances at the accelerator exit are shown in Table 1.
Table 1. Beam parameters
Particle energy, MeV 13 – 18
Pulse current duration, µs 0.3 – 1.5
Pulse repetition rate, Hz 1 – 6.25
Bunch length, mm (70% of particles) 2.5 – 6
Bunch frequency, MHz 2797.15
Beam pulse current, A 0.2 – 1.0
Particle number per bunch to 2⋅109
RMS beam size, mm 1.5 – 2.5
Beam divergence, mrad < 1.0
The simulations of bunch characteristics at the linac
exit were carried out with the PARMELA code [6].
3 SIMULATION RESULTS
We are going to use the Smith-Purcell radiation ob-
served at the fixed angle for bunch monitoring. There-
fore accordance of the bunch form-factor maximum at
the specified wavelength to the bunch length minimum
was the main question needed to be cleared. It was also
necessary to determine the suitable harmonic of the
linac operation frequency to be used for bunch length
monitoring.
The simulations allow to determine the dependence
of the bunch length at the grating position on the phase
shift between the RF gun and the accelerating section.
At the same time dependencies of the maximal instant
bunch current and form-factors for some harmonics on
the phase shift were investigated. Results of the simula-
tions are represented in Fig. 3.
Fig. 3. Maximal instant current (1), the bunch form-
factor for the 15th harmonic of bunch repetition fre-
quency f0 (2) and normalized bunch phase length (3)
versus phase shift.
One can see in Fig. 2 that the form-factor of the 15th
harmonic (λ = 7.145 mm) is rather high and its maxi-
mum is in a good agreement with the minimum of
bunch phase length. The simulations also show that the
maximum of a sum of bunch form- factors for a large
amount of harmonics (for example from 14 to 34) corre-
sponds to the minimum of the bunch length more pre-
cisely. Nevertheless it is clear that the 15th harmonic can
be used for adjustment of the linac.
4 EXPERIMENTAL RESULTS
For preliminary experiments a copper grating with a
period of 8 mm, a groove depth of 4 mm and period
number of 10 was used. The experiments were done to
investigate characteristics of the Smith – Purcell radia-
tion in the perpendicular direction to the grating.
The radiation power depending on the parameter b
for the pulse current of 0.75 A (N ≅ 1.8 109) and elec-
tron energy of 13 MeV was measured during the first
stage of experiments. Transversal size of a beam
(FWHM) at a grating position was 2.5 mm. Fig. 4
shows the detector voltage dependence on the parameter
b. The detected radiation power reached 10 mW that
corresponds to the rf power flow density more than
6 W/sr. This value is enhanced more than by six order
of magnitude of the power flow density of the incoher-
ent beam radiation. It is necessary to note that the nar-
ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 2001. №5.
Серия: Ядерно-физические исследования (39), с. 67-68.
67
row peak of the curve in Fig. 4 can not be explained by
Eq. (2). One can see that for γ=26 dependence of radiat-
ing intensity on the b is almost linear. We carried out
additional experiments to study this feature.
0
20
40
60
80
100
120
0 2 4 6 8 10 12 14 16 18 20
b, mm
U
, a
rb
. u
n
it
s
Fig. 4. Detector voltage versus parameter b.
The experiments showed existing of the millimeter
radiation in the linac beam pipe regardless of the grating
presence. The radiation power measured at 1 m far from
the beam axis (Fig. 2) was about 1 mW, in case when a
metallic screen covered the grating. In our opinion the
observed radiation is the coherent radiation from rela-
tivistic electron bunches in places where the beam pipe
has discontinuities. To approve this we investigated ra-
diation from a glass gap placed in the beam pipe up-
stream of the diffraction grating. When a wave-guide
section with cut-off wavelength of 7.2 mm was installed
between the aerial and the detector we observed strong
dependence of the radiation intensity on phase shift be-
tween the injector and the accelerating structure (see
Fig. 5).
Fig. 5. Linac output current (1) and detector volt-
age (2) versus phase shift.
It is obvious that behavior of a detector voltage does
not correspond with behavior of an output pulse current
that is evidence of bunch length changing.
Thus, the radiation observed in experiments consists
of the radiation generated in the beam pipe and of the
radiation from the grating. The carried out experiment
confirmed that bunches at the LIC exit are enough short
so a diffracting grating with a smaller period
(d=7.145 mm) was used at the next stage. Fig. 6 shows
dependence of radiation intensity at the front of grating
on phase shift between the injector and the accelerating
section. Output current of the linac was some high than
in the previous experiment.
Fig. 6. Llinac output current (1) and radiation inten-
sity at the front of grating (2) versus phase shift.
It can be seen that simulation and experimental re-
sults are in a good accordance (see Fig. 3, 5 and 6).
Therefore an optimal tuning of the accelerator can be
made using a detected signal that is proportional to the
radiation intensity as an indicator.
5 CONCLUSION
On the basis of results obtained the following con-
clusions can be made. The intensity of the observed ra-
diation is significantly higher than the incoherent radia-
tion intensity and is determined by a bunch length. The
radiation observed consists of a coherent radiation from
the grating and of a coherent radiation from discontinu-
ities of a beam pipe. The technique described can be
used as a bunch length monitor to choose the optimal
conditions of a relativistic bunch shaping and accelerat-
ing.
REFERENCES
1. S.J.Smith and E.M.Purcell. Visible Light from Lo-
calized Surface Charges Moving across a Grating //
Phys. Rev. 1953, v. 2, No. 4, p. 1069.
2. Y.Shibata et. al. Coherent Smith-Purcell Radiation
in the Millimeter-Wave Region from a Short-Bunch
Beam of Relativistic Electrons // Phys. Rev. E.
1998, v. 57, p. 1061-1074.
3. B.M.Bolotovsky, G.V.Voskresensky. Diffraction
radiation // Uspekhi Phys. Nauk. 1966, v. 88, (2),
p. 209-229. (in Russian).
4. D.C.Nguyen. Electron Bunch Length Diagnostic
with Coherent Smith-Purcell Radiation // Proc. of
the 1997 PAC Conf. p. 1990–1992.
5. M.I.Ayzatsky et al. Operating Performances and
Current Status of the Laser Injector Complex Facil-
ity (LIC) // Proc. of the XVIII LINAC Conf., Gene-
va, Switzerland, 1996, p. 116-118.
6. L.M.Young. PARMELA. Los Alamos National Lab-
oratory, LA-UR-96-1835, 1996.
68
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| id | nasplib_isofts_kiev_ua-123456789-78986 |
| institution | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| issn | 1562-6016 |
| language | English |
| last_indexed | 2025-12-07T15:23:25Z |
| publishDate | 2001 |
| publisher | Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
| record_format | dspace |
| spelling | Ayzatsky, M.I. Khodak, I.V. Kushnir, V.A. Mitrochenko, V.V. Opanasenko, A.N. Stepin, D.L. 2015-03-24T15:58:00Z 2015-03-24T15:58:00Z 2001 Bunch length diagnostics with coherent relativistic electron radiation / M.I. Ayzatsky, I.V. Khodak, V.A. Kushnir, V.V. Mitrochenko, A.N. Opanasenko, D.L. Stepin // Вопросы атомной науки и техники. — 2001. — № 5. — С. 66-68. — Бібліогр.: 6 назв. — англ. 1562-6016 PACS numbers: 29.27 Fh, 41.85.Qg https://nasplib.isofts.kiev.ua/handle/123456789/78986 One of the most important problems when designing resonance electron linacs with a high beam brightness is creation of an equipment for electron bunch length diagnostics. One of ways to solve this problem is based on analysis of radiation from relativistic electrons (transition, synchrotron etc.). The paper presents results of calculations and experiments on studying the millimeter-band radiation that is a beam exited on the surface of the grating periodic structure and in a linac beam pipe. Experiments were carried out on the linac LIC with 13 MeV particle energy and 0.8 A pulse beam current. The possibility of observed radiation application for estimation of the bunch length value, monitoring its variation and for optimization of the accelerator operating mode was shown experimentally. en Національний науковий центр «Харківський фізико-технічний інститут» НАН України Вопросы атомной науки и техники Bunch length diagnostics with coherent relativistic electron radiation Диагностика длительности сгустка с использованием когерентного излучения релятивистских электронов Article published earlier |
| spellingShingle | Bunch length diagnostics with coherent relativistic electron radiation Ayzatsky, M.I. Khodak, I.V. Kushnir, V.A. Mitrochenko, V.V. Opanasenko, A.N. Stepin, D.L. |
| title | Bunch length diagnostics with coherent relativistic electron radiation |
| title_alt | Диагностика длительности сгустка с использованием когерентного излучения релятивистских электронов |
| title_full | Bunch length diagnostics with coherent relativistic electron radiation |
| title_fullStr | Bunch length diagnostics with coherent relativistic electron radiation |
| title_full_unstemmed | Bunch length diagnostics with coherent relativistic electron radiation |
| title_short | Bunch length diagnostics with coherent relativistic electron radiation |
| title_sort | bunch length diagnostics with coherent relativistic electron radiation |
| url | https://nasplib.isofts.kiev.ua/handle/123456789/78986 |
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