Metallic element beams from ECRIS sources
M. Cavenago1, A. Galatá1 , T. Kulevoy1,2, S. Petrenko1,2, M. Sattin1 , A. Facco1
1 INFN, Laboratori Nazionali di Legnaro, 2 ITEP, Moscow
ECRIS VAPOR CAPTURE
Oven vapor has a low thermal speed (generally believed
to be good for trapping inside ECRIS plasma), vth =
(8Ti /πmi )1/2 where Ti is the ion temperature and mi =
mu A the ion mass; so vth = 540 m/s for tin with Ti =
0.14 eV. Forward sputtered atoms instead have a broad
energy distribution, with maximum probability at Ki ∼
=4
eV, so a typical estimate for vi is 2.5 km/s ( 4.0 and 9.8
km/s respectively for breeder and MEVVA case[3]).
The mean flight time before ionization of ion X i+ is
f
ti = 1/νi+ , with νi+ the (i + 1)-th ionization probability.
Let us recall estimates for the electron density ne , temperature Te and ion confinement times τi , discussed in
Ref. [4]: a first relation is ne / < τi >≡ k2 = 1.06 × 1021
m−3 Hz, inferred from the total extracted current density j = 18 A/m2 . Other equations are approximately
rf oven
Q
Db
S
P
V
OVENS
Recent results of rf oven with copper coil for Sn and
Pr were described elsewhere [5]; copper coil limits the
sample temperature Ts ≤ 1800 K, but it allows a better
efficiency than coils of W or Mo, used in the refractory rf
oven. Here we report the 2006 experiments for Ti beams
using a Mo coil oven, both online (limited by ECRIS
scheduling) and offline in a new test facility (without
ECRIS). An adequate rf amplifier A3 (350 W nominal)
was finally available. The new offline test facility (see
Fig. 1) was prepared with oven axis mounting horizontal
(like in ECRIS), to verify also oven crucible alignment.
Test facility is separated by a gate valve G from the
main MetAlice vacuum chamber, and can be independently vented to air, opened, and pumped by a rotary
pump. In the oven vapor/light path we have now a
slightly off axis deposition balance Db , a shutter S, a
disk of quartz Q where vapor stops, a vacuum viewport
V and a pyrometer P. In previous installation, shutter
was before deposition balance, with the inconvenient of
two measurement protocols. In procedure I, shutter was
left open, so that progressive metal deposit on Q distorted the pyrometer reading, while Db reading was un2100
1800
FIG. 1: Scheme of oven test facility: vacuum chamber and
main parts
60
D(I) [pm/s]
Ib(I) [uA]
1500
1200
50
40
900
Ts (II)/Celsius
10*Zo(II)/ohm
10*Zo(I)/ohm
600
300
0
G
70
dep. rate D, current Ib
We report on sputter probes and on further progresses
of rf induction ovens, applied as metallic element vapor sources for Electron Cyclotron Resonance (ECR) Ion
Sources (ECRIS). As well known[1], ECRIS are the most
versatile injectors for many nuclear physics complexes,
since they can provide highly charged ions (i2 > A where
i is the charge state and A is the mass number), they
work at a low pressure pa (order of 100 µPa), and can be
optimized for any A. In particular, to optimize yield of a
heavy element A2 > 100, it is convenient to use a mixing
gas A1 which provide most of the pressure pa , with the
additional benefit of a low consumption of A2 .
Induction ovens are well established in different areas,
from atomic physics to semiconductor furnaces [2]. Rf
ovens were successfully coupled to ECRIS firstly at LNL,
using the Alice source, a 14.4 GHz compact ECRIS. A resistively heated oven (a common option for ECRIS) was
also installed. Rf oven development, which continues for
refractory elements, physical understanding and preliminary comparisons with sputter probes are discussed in
following sections.
given by well known Golovanivsky diagram [1] and by
the observed extracted current distribution, from which
we infer k1 =√
ne hτi i ≥ 1015 m3 s, and moreover Te ∼
= 200
eV. So ne = k1 k2 ≥ 1.03 × 1018 m−3 and hτi i ≥ 0.9(7)
ms. From the Lotz formula for tin with Te = 200 eV, we
have tf0 = 4.2 µs, tf1 = 11.6 µs, tf2 = 22.2 µs.
A simple condition for trapping is that the first ionization length LI be much smaller than ECRIS length
L1 = 18 cm. This seems very easily satisfied for sputter
(LI = tf0 vi = 1 cm) and ovens (LI = tf0 vth < 3 mm).
temperature Ts (C)
INTRODUCTION
0
100 Power Po [W] 200
30
20
10
0
300
FIG. 2: Refractory rf oven results for Ti with procedures I
and II; note also Ib the bias current with a Vb = −20 V
.
N+
Ti4+,C+
Ti8+,C2+
Ti5+
Ti7+
N2+
400
Ti11+
Ti10+
200
Ti2+
current (nA)
600
mA and Vb drops to nearly zero, probably due to a conduction in the overheated Mo coil insulating cover. Note
that after oven cools, or with oven power off, a voltage Vb
up to a few kV can be applied again. When oven is not in
use, oven crucible acts generally as a bias voltage probe
(that is, a moderately negative Vb improves extracted ion
currents). In conclusion, all issue reaching in Ts ∼
= 2300
K rf oven were solved, so also production of vanadium
beams seems well possible.
0
0.5
B (kG)
1
1.5
FIG. 3: Spectra of extracted ion current with Po = 230W and
Pk = 91 W vs spectrometer field B (solid line, labels show 48 Ti
peaks); fit with theorical distribution (circles and dot-dashed
line)
perturbed. In procedure II, shutter was open only when
P readings were taken, so to minimize Q covering; anyway, the heat radiation load on Db was fluctuating, perturbing the deposition speed reading. Bias current Ib due
a bias voltage Vb applied to crucible is also measured.
Results for Ts and Ti deposition D are given in fig. 2;
note that compatibility of tests I and II is good, since
oven impedance versus oven power is the same in both
cases. By continuing tests II to Po = 288 W, we also
reach Ts = 2350 K, when apparently no Ti was left. In
2005 ECRIS experiment, with oven power limited to Po =
140 W by amplifier A2 , stable current of the 48 Ti10+ ion
48
was only I10
= 29 nA. Since deposition rate D is about
40 times larger at Po = 250 W than it is at Po = 140 W,
a large improvement was expected for ECRIS currents.
With oven power Po = 230 W and ECRIS microwave
48
power Pk = 91 W, spectra of ions extracted shows I10
=
315 nA, see Fig. 3, with comparable currents at i =
7, 11 (charge states in between are covered by oxygen
mixing gas or mixed with carbon). Current increase with
48 ∼
oven power, eg I10
= 400 nA at Po = 250 W, and with
microwave power. Sample duration, after a conditioning
phase, was greater than two days, but due to ECRIS
schedule could not be throughly investigated.
At sample temperature Ts > 2000 K, in ECRIS experiments, the oven bias current saturates as Ib ∼
= −0.6
ECRIS
yoke
anode
iron2,3
cathode
-180
-150
SPUTTER PROBES
In the design of the Penning [6] probe, we noted that
at the assumed cathode position (163 mm from ECRIS
center) the fringe magnetic field of ECRIS Bf is about
1.0 kG and is diverging. We add an iron ring (2) and
an iron pole (3) behind the Penning cathode, see Fig. 4,
so that Bf becomes approximately uniform in the whole
Penning cell: Bf = 2.1 ± 0.1 kG.
The same bias voltage Vb is applied to the cathode and
the anticathode, while anode is held to a voltage Ua respect to cathode. Cathode was made of tin, anticathode
and anode of stainless steel. Most of ECRIS mixing gas
is feeded through the Penning cell.
Preliminary results of the Penning sputter probe for
tin, still in the conditioning phase, show an extracted
120 ∼
116 ∼
current I18
= 160 nA, with nitrogen
= 90 nA and I13
as mixer gas, Ua = 4.2 kV and Vb = −80 V. In this condition, the anodic current Ia is modest Ia = 80 µA, while
cathode current is large Ib = −300 µA. By decreasing
|Vb |, more ECRIS plasma arrives to Penning, so that Ia
increases, while I18 decreases. From known current Ia estimates [4, 7], Penning cell pressure was about 400 µPa.
At Pk ∼
= 98 W, tin melted and dropped onto the anode.
Optimization of the system is in progress. In particular, we build and plan to test an anticathode made
of copper covered by tin (to cool tin and to launch tin
atom nearer to ECRIS plasma). Other ideas include exchanging anode and cathode position. The simplicity of
the sputter hardware and the good trapping efficiency of
ECRIS show that sputter concept deserves further investigations. We thank D. Giora for the construction of the
Penning sputter probe.
coil 1
iron ring1
AC
-120
z
FIG. 4: Magnetic field lines in the Penning sputter cell; dimension in mm; isolators not shown; AC=anticathode
[1] R. Geller, Electron Cyclotron Resonance Ion Sources and
ECR Plasmas, Institute of Physics Publ.,Bristol,1996.
[2] B. D. Sosnowchik, L. Lin, Appl. Phys. Lett., 89, (2006)
193112
[3] A. V. Vodopyanov et al. , Rev. Sci. Instrum., 75 (2004)
1888
[4] M. Cavenago et al., Proceedings EPAC 2006, 1744
[5] M. Cavenago et al. , Rev. Sci. Instrum., 77 (2006), 03A339
[6] A. T. Forrester, Large Ion Beams, John Wiley, NY, 1996
[7] J. Hartwig, J. S. Kouptsidis, J. Vac. Sci. Technol., 11,
(1974) 1154