свадебное платье русалка

Multicharged ion formation in plasma of electron cyclotron resonance discharge

S. Golubev, A. Bokhanov, I. Izotov, S. Razin, A. Sidorov, V. Skalyga, A.
Vodopyanov and V. Zorin
Institute of Applied Physics, RAS, 46 Ulyanov st., 603950 Nizhny Novgorod, Russia
Abstract. Great demand for multicharged ion (MCI) sources stimulated investigation of new methods of plasma
producing by means of ECR gas discharge in magnetic traps. The authors of this research propose a new type of pulse
sources of multicharged ions, namely, a quasi-gas-dynamic ECR source. Its main difference from the classical ECR ion
sources is a different, quasi-gas-dynamic regime of plasma confinement in a magnetic trap [1]. A zero-dimensional model
was constructed [2] that describes gas breakdown, formation of charge state distribution in a plasma, and plasma flux
through the plugs of the trap. A wide spectrum of model experimental studies was covered. Plasma was produced and
heated by a pulse (1 ms) gyrotron at the frequency of 37.5 GHz and power of 100 kW in a cusp magnetic trap with
magnetic field in plugs up to 2.5 T. Such a trap has axisymmetric configuration and allows one to realize a quasi-gasdynamic
regime of confinement with reliable stabilization of MHD perturbations. It was demonstrated that with such a
confinement regime it is possible to generate multicharged ions and create intense (more than 1 А/cm2) ion fluxes
through the trap plugs. The flux could be easy regulated and extracted with high efficacy. Comparison of results of
calculations and data of experiments shows that they are in a good agreement, which allows us to predict with a high
degree of certainty creation of an ECR source of a new generation. The data obtained were used to design a pulse quasigas-
dynamic ECR ion source with pumping at the frequency of 100 GHz, effective trap size 1 m, average ion charge in
plasma comparable with that in the best classical MCI ECR sources but with an order of magnitude higher flux density
and absolute magnitude of plasma flux through trap plugs. Creation of intense plasma fluxes allows one to extract highcurrent
MCI beams of high brightness. Transverse homogeneity of a plasma flux makes it possible to use multi-aperture
extraction system for formation on broad intense MCI beams.
Keywords: ECR ion source, ECR discharge, multicharged ions, Cusp magnetic trap, ion beam formation
PACS: 48.85.Ar, 52.25.Jm, 52.50.Sw, 52.55.Jd
INTRODUCTION
ECR ion sources are widely used for the production of high quality multiply charged ion beams for accelerators,
atomic physics research and industrial applications. One of modern trends in the development of ECR sources of
multicharged ions is enhancement of the power and frequency of microwave pumping. Therefore, gyrotrons –
powerful sources of radiation in the millimeter wavelength range – are now used for creation and heating of plasma
[3, 4, 5]. These generators of microwave radiation are capable of producing and confining plasma of very high
density (for example 1013 cm-3 and higher in [6]), thus providing conditions for a substantial increase of extracted
ion beam current. Most ECR sources of MCI use for plasma confinement mirror magnetic traps with “min B”
configuration that suppress MHD instability of plasma. However, it is extremely difficult to construct such systems
as they are intended for pumping frequencies higher than 30 GHz and demand strong magnetic fields of complicated
configuration. In addition, MHD stabilization of plasma by means of sextupoles makes the magnetic trap nonaxisymmetric,
which results in neoclassical transverse plasma losses. Apparently, it was the reason for strong
overheating of side walls of the ion source chamber in experiments with high-density plasma pumped at the
frequency of 28 GHz [3, 4, and 5]. All this makes search for simpler axisymmetric MHD-stable systems for plasma
confinement very topical.
A cusp trap produced by two coils with opposite currents is the simplest MHD-stable magnetic trap. Magnetic
lines at any point of plasma in such a trap have a curvature that suppresses plasma MHD perturbations. The authors
of this research propose a new type of pulse sources of multicharged ions, namely, a quasi-gas-dynamic ECR source
based on the cusp magnetic trap. Its main difference from the classical ECR ion sources is a different, quasi-gasdynamic
regime of plasma confinement in a magnetic trap [7]. Previous attempts of creation of an ECR ion source
based on a cusp magnetic trap (for example Ref. 8) were not very successful. Sufficiently high currents and high
average ion charge could not be attained in earlier researches on creation of ECR plasma sources using a cusp trap
because of large plasma losses along magnetic field lines. Large enough losses as compared to linear mirror traps are
caused by an additional effect of electron scattering into loss cone in velocity space in the central part of the cusp in
the region of small magnetic field, where the condition of adiabatic motion of electrons along Larmor
circumferences is not met. The latest research in this area was made in [9]. In [9] a modified cusp trap for 14.4 GHZ
ECRIS was designed, but was not tested experimentally.
We propose to apply a higher frequency MW for plasma creation in ECR ion source based on a cusp magnetic
trap as the next step.
RESULTS OF EXPERIMENTS WITH GAS-DYNAMIC ION SOURCE
Experimental setup
The experimental research presented in this work was carried out on the SMIS 37 shown schematically in fig. 1

FIGURE 1. Experimental Setup. 1 – gyrotron, 2 – microwave beam, 3 – vacuum chamber, 4 – coils of magnetic trap (some
magnetic field lines are shown on the figure), 5 – extractor, 6 – Faraday cup or it is possible to put ion lens instead, 7 – ion
analyzer.
Plasma was confined in a cusp trap and created by 37.5 GHz gyrotron radiation with a maximum power of 100
kW injected in pulses of 1.5 ms duration. The gas was nitrogen. It has been shown [10] that, under these conditions,
a quasi-gas-dynamic confinement regime is realized with plasma density of about 1013 cm-3, electron temperature
Te=50-100 eV, and ion life time of m about 10 μs. Having been created in the trap, the plasma spreads along the
magnetic field lines, along the system axis and beyond the trap plug into the expansion chamber (as is shown in fig.
1.), and density of the plasma flow falls down from its maximum value in the trap plug to zero in the region of low
magnetic field. This makes it possible to vary the ion current by placing the extracting aperture in plasmas with
different densities, thus providing additional opportunities to optimize the system for ion beam formation. The
dependence of the ion current density at the system axis on the distance to the trap plug was reported in Ref.11. The
current density of interest in our studies ranged from j = 100 to 1000 mA/cm2.
Beams were formed by use of quasi-Pierce geometry single-aperture and multi-aperture electrode systems.
Extracted beam currents were measured with a Faraday cup (FC), situated behind the extractor.
Measurements of total current of extracted ion beam
Measurements were made for different extraction systems (single 2-electrode and 3-electrode accel/decel
systems, multi-aperture systems) of different geometries. Charge state distribution was the same during all this
experiments (about distributions see below). It was achieved ion current of 4.2 mA with 2-electrode system of
7
5
2
1
3 4
6
following geometry: plasma electrode aperture – 1 mm, puller (accelerating electrode) aperture – 3 mm, distance
between electrodes – 5 mm. This current corresponded to the current density of 630 mA/cm2. Amplification of
plasma electrode aperture led to current increasing and so to beam potential increasing too. In that case it was
necessary to use accel/decel system just to improve the compensation of beam space-charge. And with 1.5 mm
plasma electrode aperture using 3-electrode system current of 6.5 mA was achieved. Normalized emittance of the
beam in that case didn’t exceed the value of 0.15π mm mrad. Such combination of the ion beam current and
emittance id est brightness is the record for ECR ion sources.
Using of multi-aperture system demands plasma radial distribution homogeneity. It was measured that the region
of ion beam homogeneity was 1.5 cm in diameter in the transverse plane (relative to the axis) at a distance of 14 cm
from the trap plug, and the region at the distance of 26 cm was 2.5 cm (with a precision of 10%). Therefore, this
degree of plasma homogeneity is sufficient to use a multi-aperture extraction system. The maximum measured
current in the FC reached 12.7 mA, with a distance between the plug and the plasma electrode being 10 cm and the
current through one hole of 4.2 mA. The ratio of current to the FC and total current to the FC and puller was not less
than 60%. The authors suppose that current may increase still further with a growing number of holes, but this
demands an increase in the degree of compensation of beam space-charge. Thus, even the first experiments with
multi-aperture extraction systems under quasi-gas-dynamic ECR ion source conditions demonstrated that total ion
beam current can be higher than with a single-aperture system. This trend of research may be promising for
advanced technologies.
Investigation of ion charge state distribution
The ion spectrum at voltage of 20 kV and magnetic field in the plugs along the longitudinal axis of 1.7 Tesla,
given that gas input into the trap was optimized for the maximum value of N3+ ion current, is presented in fig. 2.

FIGURE 2. Ion charge state distribution in the beam (rectangles correspond to results of calculations).
The presence of carbon and oxygen impurities is evidently connected with the gas flux from the walls of the
vacuum chamber under the action of plasma bombardment.
COMPARISON BETWEEN THE THEORY AND EXPERIMENTS
A zero-dimensional model was constructed [2] that describes gas breakdown, formation of charge state
distribution in a plasma, and plasma flux through the plugs of the trap. The results of experiments presented above
demonstrate that, under the conditions optimal for formation of N3+ions, the ratios of the majority ion currents are
IN
2+ / IN
+ ≈1.8, IN
2+ / IN
3+ ≈ 3.5, which corresponds, to an accuracy of about 10%, to results of the modeling presented
in fig. 2 for the electron density Ne = 1013 cm-3, absorption rate of microwave power 50%, and electron temperature
Те = 50 eV. Also worthy of notice is a good agreement between the calculated density of ion currents from the trap
and the currents at Faraday cylinder and puller measured in experiment at different distances between the extractor
and the trap plug[11].
40 50 60 70 80 90 100
M А
0
100
200
300
400
Analyser signal, a. u.
C3+
N3+
C2+
N2+
O2+ C+
N+
O+
CONCLUSION
The data obtained were used to design a pulse quasi-gas-dynamic ECR ion source with pumping at the frequency
of 100 GHz, effective trap size 1 m, average ion charge in plasma comparable with that in the best classical MCI
ECR sources (see fig. 3) but with an order of magnitude higher flux density (dozens of A/cm2) and absolute
magnitude of plasma flux through trap plugs. Creation of intense plasma fluxes allows one to extract high-current
MCI beams of high brightness. Transverse homogeneity of a plasma flux makes it possible to use multi-aperture
extraction system for formation on broad intense MCI beams.
FIGURE 3. Ion charge state distribution (for nitrogen) in the beam calculated for 100 GHz, 400 kW, density=9⋅1013 cm-3,
temperature=100eV and effective trap size 1 m.
ACKNOWLEDGMENTS
The authors are grateful to M. Kazakov (IAP RAS) for his help with the experiments and technical assistance.
This work was supported by ISTC grant # 2753, RFBR grant # 05-02-17187 and Russian Science Support
Foundation.
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