High-plasma-density helicon source for ion beam application

Serhiy Mordyk,¤ Valentin Miroshnichenko, Anatoliy Nagornyy, Dmitriy
Nagornyy, Dmitriy Shulha, Volodimyr Storizhko, and Vitaly Voznyy
Institute of Applied Physics, National Academy of Sciences of the Ukraine,Sumy, Ukraine
We are testing a high-plasma-density helicon sources for ion beam applications. To operate the
helicon rf source with hydrogen/helium/argon plasma a compact magnetic system with circular
permanent magnets was designed and constructed. The source has been diagnosed by a microwave
interferometer. Measured plasma densities of up to 0.9*1013 cm¡3 (for argon), 2.5*1012 cm¡3 (for
helium), and 8*1011 cm¡3 (for hydrogen) were obtained for working gas pressure < 10 mTorr and
rf power < 350 W (fRF=27.12 MHz).
PACS numbers: 2925Ni,2927Ac,4185Ak,5275-d
I. INTRODUCTION
At present the increase of ion beam brightness in ion
beam applications remains a challenging problem. High
beam brightness can be achieved by extracting the beam
with high current density and low emittance. A neces-
sary condition for the attainment of high current den-
sity in plasma ion sources is a high plasma density in
the source. One way of increasing the plasma density in
ion sources is to create an e±cient inductively coupled
rf discharge with an external magnetic ¯eld by means of
compact permanent magnet systems [1,2]. However, the
increase in the plasma density is a necessary, yet insu±-
cient condition for the beam brightness to be improved.
As the plasma density is increased, the properties of the
plasma boundary are changed, modifying the ion-optic
parameters of the beam. As the rf power and plasma
density is increased, there is a need in higher extracting
voltage and additional beam focusing in the extractor to
reduce the beam losses. The total beam current depends
largely on the transmission losses in the extraction chan-
nel, while emittance on the extractor dimensions, aber-
rations in the optic beam formation system and working
gas pressure in the extraction channel. High-brightness rf
ion sources have to be operated with lowest possible gas
inlet and maximum gas ionization to provide a minimum
increase in the normalized emittance in the ion extractor
owing to ion charge exchange and ion-neutral collisions.
To attain higher beam brightness the Institute of Ap-
plied Physics of the National Academy of Sciences of the
Ukraine is carrying out diverse investigations: design and
construction of plasma generators with high plasma den-
sity; plasma density measurements using the interferome-
try technique which provide a greater accuracy than the
Langmuir probe does; measurements of the beam cur-
rent, phase characteristics, energy spread, average en-
ergy, and mass composition; calculations of the ion source
optics with the conservative matrizant method involving
the experimental data; derivation of information about
¤Electronic address: mordyk@ipflab.sumy.ua
FIG. 1: Experimental setup.
the channel of brightness losses from the experimental
data and theoretical predictions; and simulation of the
extraction system in the high brightness ion source. This
paper touches upon some of these investigations.
II. EXPERIMENTAL SETUP
The schematic of the experimental setup is shown in
Fig.1. and was reported elsewhere [3]. This paper dis-
cusses two types of rf ion source operated in the helicon
frequency range !ci<< ! << !ce (where !c® = eB0/m®c
is the electron/ion cyclotron frequency). The di®erence
between the two sources is in their magnetic systems
and positioning of basic ion source components (antenna,
magnets and extraction system – AME mode; magnets,
antenna and extraction system – MAE mode). The ion
sources have 4 – turn helical antennas (copper wire of
4 mm diameter). In the extraction system the cathode
channel has a 3 mm length and a 0.6 mm diameter. The
tube is made from quartz and has an outer diameter of
30 mm and length of 200 mm. The tube length was
increased to implement a helicon discharge in an exter-
nal magnetic ¯eld. A 27.12 MHz, 40 W oscillator and
800 W rf power ampli¯er (Acom 1000) are connected t tion of this rather strong e®ect. Traditional interpreta-
tions involve the production of complex coupled modes
with participating helicon, Trivelpiece-Gould, and sur-
face waves [7], in combination with the active production
and con¯nement of fast electrons [6]. In the present pa-
per this e®ect is used in the hydrogen/helium ion source
in AME mode. The rf ion source [8] comprises a helicon
plasma generator (in which a longitudinal magnetic ¯eld
is created by a compact magnetic system of permanent
circular magnets), an extraction system (in which a cath-
ode is separated from the plasma by a quartz disk with a
hole) and a movable source outlet aperture. To operate
the helicon rf ion source with hydrogen/helium plasma a
magnetic system with circular permanent magnets (Nd-
FeB) was designed and constructed, permitting a genera-
tion of a longitudinal magnetic ¯eld Bz »100 G along the
length of the RF antenna and of a longitudinal magnetic
¯eld of »1000 G with e®ective ¯eld length of about 10 cm
to con¯ne and transfer the plasma to the extraction sys-
tem. The design of the magnetic system has to meet the
following requirements: 1)to generate a magnetic ¯eld of
strength and structure permitting an e±cient RF power
input into the plasma, 2)to have small size, 3)to avoid
an increase in the normalized ion source emittance, and
4)to provide optimum proton/helium beam focusing in
the ion source extraction- and preliminary acceleration
areas. The magnetic ¯elds were calculated involving a
POISSON-SUPERFISCH numerical code (LANL). Fig.
4 represents a map of magnetic force lines and a pattern
of the longitudinal magnetic ¯eld distribution along the
source axis. The choice of the magnetic ¯eld structure
was made using the optimization calculations of the non-
linear beam dynamics in the ion source extraction- and
preliminary acceleration areas.
The application of a nonuniform magnetic ¯eld with
an antenna placed in the weak magnetic ¯eld in the heli-
con rf ion source permits steady, high-brightness source
operation to be achieved with a relatively large proton
percentage in the beam. The ion current density in the
extracting electrode in the source was 90 mA/cm2 with
high percentage of protons in the beam (»80 % ). In
Fig.5 the hydrogen ion current is represented as a func-
tion of rf power for a helicon ion source (AME mode, p
= 10 mTorr). As can be seen in the ¯gure, measured
ion current is higher than needed for operation mode
of accelerator-based microprobe (0.1-10 ¹A). For a ¯xed
beam current the increase in brightness can be achieved
by increasing the phase density of the beam core with
subsequent beam collimation or reducing emission hole
for ¯xed another parameters of an extraction structure.
The 1.0 mm outlet aperture under conditions of the opti-
mum beam formation (highest current density and small-
est divergence angles) made it possible to obtain a hydro-
gen and helium beam with »100A/(m2rad2eV) bright-
ness. As distinct from the rf source operation in the
H-mode (with an inductive RF discharge) the W-mode
(helicon) operation is characterized by a high peaking
current pro¯le. This circumstance can be ascribed to
di®ering structure of the plasma density pro¯les in the
H-mode and W-mode and to additional focusing of the
ion beam in the extraction structure. The highest ab-
sorption of the rf-power in the W-mode occurs along the
source axis while in the H-mode near the antenna [9].
IV. SUMMARY
Small-size plasma generators with high plasma den-
sity (5×1011{ 1013 cm¡3) have been designed and con-
structed for ion beam applications. The plasma mea-
surements were performed using an interferometer. Beam
brightness of » 100 A/(m2rad2eV) was attained. A fur-
ther increase in brightness could be achieved via inte-
grated experimental and simulative investigations of he-
licon plasma generators and ion beam extraction systems.
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