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ELECTRON BEAM PROBE FOR BEAM POSITION AND BEAM SPACECHARGE MEASUREMENTS#
L.Mishra*, P. Roychowdhury, H. Kewlani, Dr. K.C Mittal,
Accelerator and Pulse Power Division, Bhabha Atomic Research Centre, Trombay, Mumbai
400085, India.
Abstract
Space charge (SC) compensation facilitates the
transport of low energy intense Ion Beam. Thus
determination of the radial distributions of the beam
potential, compensating electrons (CE) and particle
densities in the proton ECR Ion Source [1] which has
been developed for low energy and high intensity beam
for ADS [2] application is necessary. The source also
finds other applications in various areas of science and
technology. Such a high power beam system has to
perform as per the required design parameters and thus
the performance has to be monitored with reference to the
designed parameters. The ECR Ion Source designed
delivers a 40 mA Hydrogen beam current at 50 KeV. The
beam from the source enters in the LEBT section
consisting of the RFQ, Linac and finally the target where
neutrons will be generated to control the power
generation in a Nuclear Reactor. The complete system
integration involves regulated and stable beam. The beam
emittance has been measured and found to be less than
0.19 pi mm mrad rms normalized at 25 keV beam energy
and proton fraction has been found to be better than 80
percent of the total beam current. While the beam
propagates the emittance growth due to nonlinear radial
distribution of the residual space charge forces of the
system beam ions and CE. EBP offers a possibility to
measure preciously the SC neutralization of the ion beam
and also beam position could be inferred with this
technique. The EBP measurement does not perturb or
disturb the Ion beam and hence EBP gives advantage
over conventional ways to measure the beam current
including Faraday cup, Langmuir probe, slit wire scanners
and often sometimes with scintillating screens.
INTRODUCTION
The SC of Low-energy (25 - 50 keV) beam transport
(LEBT) of high-current (25 - 40 mA) positive-ion beams
in background gas pressures of order 10-5 mbar is
predicted to be effectively neutralized by electrons in a
beam plasma [3]. The degree of SC neutralization is an
important parameter in beam transport calculations and
simulations. Various techniques are developed for the
experimental investigation of SC compensation factor for
positive ion beams. It involves the measurement of the
slow ion energy distribution using a Four Grid Energy
Analyzer [4] which measures the secondary-ion energy
distribution to determine SC compensation of the Ion
_____________________________________
*Department of Atomic Energy, India.
# [email protected]
Beam. In this paper we describe an Electron Beam Probe
(EBP) to measure the Ion Beam Position and Space
Charge Compensation. This non-disturbing and nondestructing determination of SC neutralization and beam
positioning has become of common interest rather than
the other methods. RF Langmuir Probe has been used to
perform such diagnostics but the measurement differ from
the actual charge distribution as it perturbs the actual
space charge compensation by emitting secondary
electrons and even capturing the CE’s. Rowgoski coil and
similar other such inductive probes could only measure
total beam charge but cannot measure the beam crosssectional charge distribution. Hence EBP finds the best
place for beam space charge neutralization and beam
position monitoring. The beam availability during the
measurement process is what differentiates the
measurement using EBP than the other techniques, though
the apparatus and experimental set-up has resulted from
high advancement in involved electronics and
components.
MEASURING TECHNIQUE
The electron beam of 1µA and of 1 keV energy will
traverse the ion beam transversally. Assuming the EBP
field does not perturb the ion beam, in fact which is true
as far as electron probe current is less than the ion beam
current, the EB gets deflected due to the space charge
field of the ion beam, without influencing the ion beam.
The deflection of the EB is then used to determine the
space charge neutralization of the ion beam and its
position.
Under the assumption that the Ion Beam particles are
confined within an infinite long cylinder extending
symmetrically on both of the EBP, the analysis is simple.
The deflection of the electron trajectories is described in
terms of deflection angle α (𝑎) [5], (Fig.1) defined as
𝛼(𝑎) =
𝑒.𝑎
𝑈
𝑟
𝐸(𝑟)𝑑𝑟
∫𝑎 𝑑 (𝑟 2−𝑎2 )1/2
(1)
Here 𝑎 is the impact parameter, 𝐸(𝑟) is the
electric filed at distance ‘𝑟’ from the beam axis, 𝑟𝑑 is the
distance of e-gun from the ion beam axis which in the
designed apparatus is the beam pipe radius, 𝑒 is the
charge of the electron. The charge of the ion beam which
we have assumed is cylindrical gives rise to the
field 𝐸(𝑟). The magnitude 𝐸(𝑟) depends on the charge of
the ion beam and the distance from the beam axis. Around
Fig.1 Trajectory of electrons due to the space charge of the Ion beam Halo
the Ion beam the field only depends on the charge seen
at that point.
The charge of the ion beam is neutralized by the
electrons produced during the residual gas ionizations
by the ion beam. These electrons shield the electric
field of the beam particles from each other and
prevents the beam from blowing up due to the its space
charge forces. The beam space charge increases with
the ion beam current. In this paper we have simulated
the deflection of the probe electrons as a function of
the impact parameter 𝑎, for fixed energy of the Ion
beam. In Fig 2, the deflection 𝛼 is plotted as a function
of impact parameter 𝑎 for various levels of space
charge neutralization.
Fig. 2 Simulated deflection characteristics at various level of charge
compensation.
Fig. 2 shows the simulated characteristics for different
degrees of space charge compensation for a 40mA H+
50 KeV ion beam. Practical measurements of the
deflection angles are difficult to make rather than the
deflection parameter 𝜆 which is easily measured
experimentally. For small deflection angles, the
deflection parameter is related with the deflection
angle.
𝜆(𝑎)
𝛼(𝑎) ≈
𝑟𝑑
⟹ 𝜆(𝑎) = 𝛼(𝑎)𝑟𝑑
The radial space charge potential distribution 𝑉(𝑟) can
be obtained from the deflection characteristics with an
Abel inversion of the integral equation (eq.1), as the
space charge potential.
𝑉(𝑟) =
𝑟
2𝑈
𝜆(𝑎)𝑑𝑎
∫𝑑
𝑒𝜋𝑟𝑑 𝑟 (𝑎2 −𝑟 2 )1/2
(2)
The distribution determines the charge compensation
of the Ion beam by the ionized electrons of the residual
gas. Thus one can measure experimentally the
deflection parameter of e-beam and from above
analysis determine the degree of space charge
compensation of beam. The EBP could be used to
measure s.c compensation at various residual gas
pressures which then directly determines the density of
ionized gas molecules/ions to compensate for the ion
beam charge.
As EBP gives the potential distribution from which
charge density profile of the ion beam could be found,
the technique would also give the beam location with
the beam tube and its transverse profile [6]. In principle
EBP could also be used to perform time of flight
measurement to determine the time frame in which the
charge of the ion beam is space neutralized. The time
resolution would be of ns order which is the time of
traversal of the electrons from the e-gun to the edetector scintillator screen, while the beam s.c
neutralization event is of ms duration [7].
EXPERIMENTAL TECHNIQUE
In spite of the straight forward analysis, experimental
performance of the EBP is challenging. The set-up
designed is shown in Fig. 3. The test setup requires an
compact e-gun, emitting Gaussian profile electron
beam with energy 1keV and very low thermal energy
spread (~3eV), the e-beam should be stable within the
experimental time frame. As the energy of the
electrons from the probe is low and being very low
mass particles, electrons deviate from straight
trajectories due to background weak electric and
magnetic fields apart from the deflection due to the
space charge force of the ion beam. The effect of
geomagnetic field responsible for producing electron
Fig.3 The e-beam diagnostic experimental setup.
deflection is corrected by the electrostatic deflecting
magnets such that the offset due to earth’s magnetic
field is compensated by the field of the chicane
system. The electrons after traversing the ion beam are
detected at the low energy scintillator screen. The
electron position on the screen is measured at various
electron beam offsets and stored for data analysis.
Thus at different impact parameters the deflection
parameter is measured and by Abel inversion potential
distribution of the ion beam is determined. Also the
position could be inferred from the analyzed data.
ACKNOWLEDGEMENTS
The author would like to thank P. Roychowdhury for
helpful discussions, Dr. K.C Mittal for support and
guidance
during
the
work.
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