S. Gammino

Crossref

OPERATIONS OF THE SERSE  
SUPERCONDUCTING ECR ION SOURCE AT 28 GHz 
 
S. Gammino, G. Ciavola, L. Celona, P. Romano, L.Torrisi INFN-LNS, Catania, Italy 
D. Hitz, A. Girard, G. Melin, CEA-DRFMC-SBT, Grenoble, France 
 
Abstract 
The SERSE source has been efficiently working at the 
Laboratori Nazionali del Sud in Catania since 1998, by 
operating at 14 and 18 GHz, with a maximum power of 
about 2 kW. The performance of the source has been 
optimised for the beam injection into the K-800 
Superconducting Cyclotron, i.e. the maximisation of the 
charge states was looked for, provided that the current of 
the beam was about 0.5 to 5 eµA, which is the typical 
analysed current for routinely operations. 
In 2000 a collaboration was established in order to test 
the behavior of SERSE at 28 GHz, so that a new 
operating domain could be studied, featuring a very dense 
plasma (up to 1013 cm-3) and a very high ion current (1 
emA of Pb27+ is the goal of CERN and GSI for future 
heavy ion projects and 0.5 emA was considered to be a 
challenge for our experiment). 
Tests were carried out with a generator based on a 
gyrotron tube which was refurbished and made available 
by CEA; Xenon ions were chosen as a reference being the 
ionisation difficulty for Xe20+ similar to the one of Pb27+. 
The results were excellent and the current of Xe20+ 
exceeded our expectations (more than 0.5 emA in 
afterglow and about 0.3 emA in dc mode). These results, 
obtained with a power limited to 4 kW in dc mode and 6.5 
kW in pulsed mode because of the poor chamber cooling, 
can be considered as a remarkable step, in view of the 
third generation of electron cyclotron resonance ion 
source (ECRIS). 
The details of the experiments will be described, along 
with the design of the next superconducting ECRIS, the 
GyroSERSE source, which is expected to produce beam 
currents above 1 mA for intermediate charge states as 
well as very high charge states, 50+ to 60+ for the heaviest 
ions, with currents above 1 eµA. 
1  INTRODUCTION 
Under the frame of a collaboration between CERN - 
Geneva, GSI – Darmstadt, CEA – Grenoble, ISN – 
Grenoble and INFN – LNS, a research program was 
established in order to produce very intense beams with 
an ECRIS [1]. The idea was to increase the electronic 
density ne of an ECRIS by using a frequency heating 
higher than the typical 6.4, 10, 14 or 18 GHz commonly 
used in an ECRIS. The 28 GHz frequency was chosen, 
which should increase ne by a factor 2.4 as compared to 
the 18 GHz operation. However, the maximum “effective” 
mirror ratio R of SERSE should be, in that case, reduced 
from 2.4 at 18 GHz down to 1.6 at 28 GHz. The 
“effective” mirror ratio R is defined as R = Bmax/Bres, 
where Bmax is the value of the magnetic field at the mirror 
throat and Bres is the magnetic field at the resonance (1 T 
at 28 GHz). By changing R from 2.4 to 1.6, the electron 
confinement was lowered, but the tests of SERSE at 28 
GHz permitted to obtain the highest currents ever 
achieved for any Xenon charge state above 20+ by means 
of the higher plasma density. 
 
 
Figure 1: The analysis section of the SERSE beamline, 
including the focusing solenoid and the analyzing magnet. 
2  THE EXPERIMENTAL SETUP 
The experiment was carried out by using the SERSE 
source and its analyzing magnet (fig. 1). Ion currents were 
at first measured on the Faraday Cup FC3 in order to get 
the most of the extracted beam. Analyzed beam currents 
are measured by the Faraday cup FC4 located at the 
image point of the magnet, immediately after the ± 10 mm 
slits. 
No major changes were made to the SERSE source 
[2,3]. The source injection flange was modified to include 
a window for high power 28 GHz microwave injection. 
The flange also included a window for 18 GHz 
microwave injection in order to allow comparison 
between the two modes of operation. The main difference 
with respect to the standard flange consisted of the biased 
disk which was fixed and this was perceived to be a 
limiting factor. The extraction gap was shortened in order 
to get the maximum current from the 12 mm extraction 
hole, but the extraction optics was not changed. 
CP600, Cyclotrons and Their Applications 2001, Sixteenth International Conference, edited by F. Marti
© 2001 American Institute of Physics 0-7354-0044-X/01/$18.00
223
The extraction voltage was limited to 26 kV and most 
of the tests were carried out at 20 kV. 
 
Mechanical 
Compensator 
Watercooled 
RF 
Window 
TE02 
TE01
Gyrotron 
TE02 
Arc 
Detector 
TE02  TE01 
Converter 
30 kV 
DC 
Break
SERSE
Bi-Directional 
Coupler 
Mode 
Filter 90° Corrugated 
Bend (TE01) 
 
Figure 2: The waveguide line. 
2.1  Microwave coupling 
The microwave power from a gyrotron has been 
coupled to SERSE through the waveguide line described 
in fig. 2. The output mode of the 28 GHz - 10 kW 
gyrotron was TE02; a conversion into TE01 was performed 
to minimize ohmic losses, then oversized circular 
waveguides were used. The gyrotron itself was protected 
by an arc detector and a mode filter (the gyrotron does not 
suffer, as reflected power, more than 10% of the incident 
power). Mechanical compensators were used to prevent 
from any thermal expansion. A 90° bend with corrugated 
walls was used and a dc break was placed just before the 
source to insulate the waveguide up to 30 kV; then a 
water cooled sapphire window was used to insulate the 
waveguide from the vacuum. A bidirectional coupler, 
calibrated with a water-load, measured the incident and 
reflected power in the TE02 mode. During the tests the 
maximum observed reverse power was less than 150 W, 
which enabled a safe operation of the gyrotron. The line 
was about 8 m long and, when 10 kW were extracted from 
the gyrotron, the losses in the rf line were about 400-500 
W, most of the losses (300 W) being in the mode filter. 
Being 700 mm of the line under vacuum a resonance 
occurred in the waveguide, inducing a plasma; this 
phenomenon lasted a few hours during the waveguide 
outgassing. In conclusion, the rf launching into Serse was 
successful, and it is an important achievement, 
considering that it is the first time that 28 GHz 
microwaves have been coupled to an ECR ion source. 
3   SCALING EFFECTS AT 28 GHZ 
We have remarked in a previous paper [4] that the 
Geller’s semi-empirical scaling laws [5] are able to 
explain the results of many existing ECR sources, even if 
they do not represent the ECRIS behavior exhaustively; 
two laws are particularly relevant to our tests: 
Prf ∝  V ω1/2 qopt3 (1) 
 
Iq+ ∝ ω 2 Mi−α (2) 
where qopt  is the optimal charge state, Prf is the 
microwave power, ω = 2πf, Iq+ is the intensity of the 
charge state q, Mi is the ions mass and α is an adjustable 
parameter variable between 0.5 and 1. 
It was demonstrated at lower frequency [6,7,8] that the 
scaling laws and in particular eq.(2) are effective only if 
the confining field is high enough to support a higher 
electron density. 
The conclusions of the scaling laws tests at frequencies 
between 2.45 and 18 GHz [4,9] were the following: 
• the ratio between the radial field and the resonance 
field should be higher than two; 
• the axial field at the injection side must be as high 
as possible, in order to close the loss cone; 
• the axial field at the extraction side should be 
optimized around the value of the radial field, so 
that the escape of plasma on the wall is not favored 
with respect to the beam extraction;  
• the frequency should be increased, provided that a 
high magnetic confinement is maintained; 
• the CSD improves with the microwave power, but 
the amount of power that can be coupled to the 
plasma increases with the confining field. 
 
It was shown in ref [9] that the increase of current 
obtained with SERSE by increasing the frequency from 
14.5 to 18 GHz is at least proportional to (f2/f1)2, once 
that the magnetic field is scaled by a factor f2/f1. 
The increase for the highest charge states was even 
more relevant than expected according to the f2 law. In 
fact for the highest charge states of xenon the increase 
was up to a factor 4 [9]. 
Fig. 3 compares the Xe27+ intensities extracted from 
SERSE at three different frequencies and for a different 
radial field. It is clear that the performance of the source 
is improving with the increase of radial field and that a net 
increase of the highly charged ion beam current is 
obtained by increasing the frequency. We arrived at the 
same conclusion when changing the axial field; the 
statements above cited [4,9] are confirmed even at 28 
GHz, i.e. the higher is the field the better the performance. 
A complete report of the study of dependence on the 
magnetic field and frequency may be found in ref [10]. 
4  PRODUCTION OF INTENSE BEAMS 
The study for the production of Xenon ions with charge 
states q≥20 was carried out either in dc mode and in 
pulsed mode. In both cases the reproducibility of the 
results was excellent but the long term stability was not, 
mainly because of technical problems (outgassing, high 
voltage discharges), especially in the case of a poorer 
confinement of the plasma. Because of outgassing the 
power was limited to 3 kW in dc mode (except for a short 
test at 4 kW) and to 6.5 kW in afterglow mode. The 
dependence of Xe20+ intensity on the microwave power 
was about linear up to 3 kW then it saturated and 
decreased above 3.5 kW because of outgassing; on the 
other way in pulsed mode there was not an optimum but a 
continuous beam current increase with the power was 
observed. 
0
20
40
60
80
100
1 1,2 1,4 1,6 1,8 2 2,2 2,4 2,6 2,8 3
Brad / Bres
In
te
ns
ity
 (e
µA
) 28 GHz
18 GHz
14 GHz
Xe27+
 
Figure 3: Xe27+ currents obtained by operating SERSE at 
different frequencies versus increasing radial field (20 kV 
extraction voltage). 
 
0
10
20
30
40
50
640 680 720 760 800
xenon19-09-00
In
te
ns
ity
 (e
µA
)
Analyzing magnet field (gauss)
39+
34+
35+
36+
37+
38+
 
Figure 4: High charge states spectrum (power=4 kW). 
 
The performance for the highest charge states had a 
similar trend and for ions like Xe30+ and Xe33+ the increase 
of current with the power was steeper between 3 and 4 
kW. During a short test devoted to the optimization of the 
high charge states production in dc mode, a higher power 
and a very low amount of gas were injected, and a few 
charge states were optimized at power level above 4 kW. 
In spite of the relatively high pressure inside the plasma 
chamber (~2÷3×10-7 mbar), we observed about 100 eµA 
of Xe30+ , about 8 eµA of Xe38+ and 0.5 eµA of Xe42+. 
Unfortunately these levels could not be maintained for 
a long time and after a few minutes they decreased by a 
factor two or more, but anyway they confirm that a 
remarkable increase can be obtained by operating ECRIS 
at 28 GHz. In fig. 4 the section of the spectrum featuring 
the highest charge states for Xenon is reported. An 
accident to the plasma chamber which was damaged by 
electron bombardment obliged us to stop these tests and 
in the following days we have operated the gyrotron only 
in pulsed regime.  
4.1  Afterglow mode 
The current of Xe25+ measured on FC4 is shown in 
fig.5. It can be observed that the enhancement factor was 
about 1.5 or 2 with respect to the yet larger current 
obtained in dc mode. It must be remarked that it was not 
possible to extract larger currents because of the limited 
current available from the high voltage supply (the drain 
current was 15 mA, three times bigger than the current 
measured on the Faraday Cup FC3, because of the 
backstreaming electrons from the puller and because of 
the beam losses in the extractor and in the first part of the 
beam line). 
In fig. 6 the performance of the source in dc mode and 
in afterglow mode are compared. It must be noticed that 
the optimum trap for dc mode production is the optimum 
also for the afterglow mode. 
The results concerning the peak stability were less good 
than at lower frequency, when the source operates in High 
B mode, but it is still acceptable for most of applications, 
being about ±10%. 
 
-50
0
50
100
150
200
250
300
350
400
450
500
-70 -50 -30 -10 10 30 50 70 90 110 130 150
Time (ms)
In
te
ns
ity
 (e
µA
)
 
Figure 5:  A Xe25+ beam pulse in afterglow mode. 
0
50
100
150
200
250
300
1,2 1,25 1,3 1,35 1,4 1,45 1,5
Radial field (T)
In
te
ns
ity
 (e
µµ µµA
)
Xe25+
afterglow
dc
Binj = 2.34 T
Bmin = 0.50 T
Bext = 1.33 T
Brad = 1.45 T
 
Figure 6: Comparison of Xe25+ currents in dc and 
afterglow mode versus the radial confining field. 
 
Figure 7: Emittance plot for Xenon beams. 
5  TECHNICAL PROBLEMS 
5.1  High voltage discharge and extraction 
process 
High voltage sparks often occurred because of the high 
gas load which was needed to optimize the production of 
intense beams. A further load derived from the beam 
divergence: we carried out the tests with the same 
extraction system which we optimized for the low current, 
low emittance beams used for the injection into the 
cyclotron and it seemed to be not appropriate. In fig. 7 a 
simulation is shown, which features a very large beam 
emittance (about 500 to 900 πmm.mrad for the typical 
total current of 4 to 15 mA from the extractor [11]) 
whereas the acceptance of the analysis beamline is about a 
factor five smaller. The large emittance led to relevant 
beam loss either between the extraction and FC3 (the 
beam pipe became warm after a few hours of operations 
in dc mode) and between the two Faraday cups (we 
evaluated the loss in this case and we estimated that about 
35% of the beam is lost inside the analyzing magnet, 
whose gap is only 54 mm). 
The beam losses at the extraction are also important, as 
it is evident in fig. 8, where the typical triangular shape of 
the beam on the puller is visible; this losses generated an 
important outgassing which further increased the gas load 
and finally damaged the plasma electrode, which was 
melted (fig. 9). In spite of these troubles, we were able to 
find a suitable set of parameters which permitted us to 
complete the tests with a safe regime for electrodes. 
5.2  X-ray and LHe consumption 
The high flux of X-ray which we measured around the 
source was generated by the electrons lost on the chamber 
wall, because of the insufficient plasma confinement. 
In fig. 10 the X-ray measurements in the radial 
direction at 18 and 28 GHz are compared (the source 
setting was similar in the two cases). The energy 
distribution was not much different at 28 GHz, but a low 
energy population is also present. The X-ray level in the 
radial direction is much higher at 28 GHz, which is a hint 
of the bad confinement (in fact on the axis, the level of 
radiation measured during 28 GHz operations was not 
higher than the one which we measured at 18 GHz). 
A further evidence of this intense X-ray bombardment 
consisted of the higher LHe consumption which was 
associated with the lower radial magnetic field. The LHe 
consumption increased from 4.5 to 10 l/h and because of 
this effect and because of the plasma instability we could 
not decrease the radial field below 1.17 T in our test of 
scaling laws [10]. In fact the energy release in the cryostat 
decreased the safety margin of the magnets and we got 
quenches which were caused by the X-ray bursts, in spite 
of the magnet setting, which was considered safe. 
 
Figure 8: A picture of the puller 
 
Figure 9: A picture of the eroded plasma electrode 
100
1000
104
50 100 150 200 250
C
ou
nt
s (
a.
u.
)
X-ray energy (keV)
28  GHz
18 GHz
 
Figure 10: Comparison of the X-ray emission rate. 
 
Figure 11: A sketch of the GyroSERSE source. 
6  THE GYROSERSE PROJECT 
The successful completion of the study of SERSE at 28 
GHz reinforced our intention to build the GyroSERSE 
source [2,12]. This source is designed to operate in high-
B mode at a frequency equal or higher than 28 GHz, 
getting a hot dense plasma (1013 cm-3). 
The main features of this design compared to the ones 
of SERSE are given in tab. 1. Fig. 11 shows a sketch of 
the GyroSERSE source, with the cryostat, the solenoids 
and the hexapole surrounded by an iron yoke. 
The mechanical constraints have obliged to choose a 
well larger inner bore than for SERSE, because of the 
boundary conditions for the hexapole (the stored energy is 
above 300 kJ). The warm bore thickness corresponds to 
an internal diameter of the plasma chamber of 180 mm, 
50 mm larger than the one of SERSE. The maximum field 
in the conductor is about 7.5 T. The length of the plasma 
chamber will be about 700 mm and the volume will be 
larger than 14 liters, well higher than any other existing 
source. The large diameter will help to get a pressure in 
the order of 10-8 mbar inside the chamber and will make 
easier the design of the injection flange that will host the 
ports for 14, 18 and 28 GHz injection, two gas inputs and 
a 35 mm flange for the oven, the biased disk, the 
sputtering system or the target for laser ablation.  
On the outer side a 5 mm double wall watercooled 
stainless steel chamber will be able to dissipate a 
maximum power of 10 kW. On the outer a 3 mm thick 
PEEK tube will insulate the cryostat (at ground) from the 
chamber (at 40 or 50 kV). The extraction system will be 
further studied [11]. In conclusion, this source seems to 
be feasible and not more expensive than SERSE, as many 
components will be inspired by the design of SERSE. An 
estimate of the time involved with the completion of such 
a source has been roughly carried out, and we believe that 
3.5 years from the date of funding (expected for the 
summer 2001) are needed.  
ACKNOWLEDGMENTS 
This work has been performed under the frame of a  
collaboration of CEA/DRFMC/SBT with GSI/Darmstadt 
(N.Angert, P. Spädtke, K. Tinschert), CERN (C. Hill, H. 
Haseroth), ISN/ Grenoble (P. Sortais) under contract # K 
598/PS and in the frame of a Memorandum of 
Understanding between CEA and INFN/LNS. We 
acknowledge the support of the European Community – 
Access to Research Infrastructure action of the Improving 
Human Potential Programme. 
The precious help of F. Chines, S. Marletta, E. 
Messina, J. Debernardi, J.M. Mathonnet during the 
experiment has been appreciated. 
We gratefully thank P. Spadtke for the simulations of 
the extraction process. 
 
Table 1: Comparison between the design features of 
SERSE and GyroSERSE. 
 SERSE GyroSERSE 
f 14-18 GHz 28-35 GHz 
Bradial 1.55 T 2.7 T 
B1 2.7 T 4.5 T 
B2 1.6 T 3.5 T 
φ chamber 130 mm 180 mm 
L chamber 550 mm 700 mm 
φ cryostat 1000 mm ~1000 mm 
L cryostat 1310 mm 1540 mm 
Vextr 20-25 kV 40-50 kV 
Lhe consumption ~4l/h (100 l/day) 0 
O8+ ~7 pµA ~50÷100 pµA 
Ar18+ >20 pnA ~1 pµA 
Au45 ~1 pnA ~0.1 pµA 
Xe42+ ---- ~.02 pµA 
Pb52+ ---- ~.01 pµA 
REFERENCES 
[1] N. Angert et al., Proc. of the 14th Int. Workshop on 
ECR ion sources, Geneve, (1999) 220 
[2]  S. Gammino and G. Ciavola, Rev. Sci. Instr. 71(2), 
(2000) 631 
[3]  G. Ciavola and S. Gammino, Rev. Sci. Instr. 63(4), 
(1992) 2881 
[4] S. Gammino,  G. Ciavola, Proc. of 14th Int. Conf. on 
Cyclotrons & Applications, Capetown, Sudafrica 
(1995) 377 
[5] R. Geller et al, Proc. of the 8th Int. Workshop on 
ECR ion sources, East Lansing, (1987) 1 
[6]  G. Ciavola and S. Gammino, Plasma Sources 
Science and Technology, 5 (1996) 19 
[7] T. Antaya and S. Gammino, Rev. Sci. Instr. 65(5), 
(1994) 1723 
[8] S. Gammino et al., Rev. Sci. Instr. 67(12), (1996) 
4109 and references therein 
[9]  S. Gammino et al., Rev. Sci. Instr. 70(9), (1999) 
3577 
[10] S. Gammino et al, submitted to Rev. Sci. Instr., 2001 
[11] P. Spadtke, private communication (2001)  
[12] S. Gammino, G. Ciavola, L. Celona, The 
GyroSERSE project, Proc. Workshop “Production 
of Intense Beams of Highly charged ions”, Catania 
(2000), to be published by Società Italiana di Fisica, 
Bologna, Vol. 72 (2001) 


English

Operations of the SERSE superconducting ECR ion source at 28 GHz

OPERATIONS OF THE SERSE  
SUPERCONDUCTING ECR ION SOURCE AT 28 GHz 
 
S. Gammino, G. Ciavola, L. Celona, P. Romano, L.Torrisi INFN-LNS, Catania, Italy 
D. Hitz, A. Girard, G. Melin, CEA-DRFMC-SBT, Grenoble, France 
 
Abstract 
The SERSE source has been efficiently working at the 
Laboratori Nazionali del Sud in Catania since 1998, by 
operating at 14 and 18 GHz, with a maximum power of 
about 2 kW. The performance of the source has been 
optimised for the beam injection into the K-800 
Superconducting Cyclotron, i.e. the maximisation of the 
charge states was looked for, provided that the current of 
the beam was about 0.5 to 5 eµA, which is the typical 
analysed current for routinely operations. 
In 2000 a collaboration was established in order to test 
the behavior of SERSE at 28 GHz, so that a new 
operating domain could be studied, featuring a very dense 
plasma (up to 1013 cm-3) and a very high ion current (1 
emA of Pb27+ is the goal of CERN and GSI for future 
heavy ion projects and 0.5 emA was considered to be a 
challenge for our experiment). 
Tests were carried out with a generator based on a 
gyrotron tube which was refurbished and made available 
by CEA; Xenon ions were chosen as a reference being the 
ionisation difficulty for Xe20+ similar to the one of Pb27+. 
The results were excellent and the current of Xe20+ 
exceeded our expectations (more than 0.5 emA in 
afterglow and about 0.3 emA in dc mode). These results, 
obtained with a power limited to 4 kW in dc mode and 6.5 
kW in pulsed mode because of the poor chamber cooling, 
can be considered as a remarkable step, in view of the 
third generation of electron cyclotron resonance ion 
source (ECRIS). 
The details of the experiments will be described, along 
with the design of the next superconducting ECRIS, the 
GyroSERSE source, which is expected to produce beam 
currents above 1 mA for intermediate charge states as 
well as very high charge states, 50+ to 60+ for the heaviest 
ions, with currents above 1 eµA. 
1  INTRODUCTION 
Under the frame of a collaboration between CERN - 
Geneva, GSI – Darmstadt, CEA – Grenoble, ISN – 
Grenoble and INFN – LNS, a research program was 
established in order to produce very intense beams with 
an ECRIS [1]. The idea was to increase the electronic 
density ne of an ECRIS by using a frequency heating 
higher than the typical 6.4, 10, 14 or 18 GHz commonly 
used in an ECRIS. The 28 GHz frequency was chosen, 
which should increase ne by a factor 2.4 as compared to 
the 18 GHz operation. However, the maximum “effective” 
mirror ratio R of SERSE should be, in that case, reduced 
from 2.4 at 18 GHz down to 1.6 at 28 GHz. The 
“effective” mirror ratio R is defined as R = Bmax/Bres, 
where Bmax is the value of the magnetic field at the mirror 
throat and Bres is the magnetic field at the resonance (1 T 
at 28 GHz). By changing R from 2.4 to 1.6, the electron 
confinement was lowered, but the tests of SERSE at 28 
GHz permitted to obtain the highest currents ever 
achieved for any Xenon charge state above 20+ by means 
of the higher plasma density. 
 
 
Figure 1: The analysis section of the SERSE beamline, 
including the focusing solenoid and the analyzing magnet. 
2  THE EXPERIMENTAL SETUP 
The experiment was carried out by using the SERSE 
source and its analyzing magnet (fig. 1). Ion currents were 
at first measured on the Faraday Cup FC3 in order to get 
the most of the extracted beam. Analyzed beam currents 
are measured by the Faraday cup FC4 located at the 
image point of the magnet, immediately after the ± 10 mm 
slits. 
No major changes were made to the SERSE source 
[2,3]. The source injection flange was modified to include 
a window for high power 28 GHz microwave injection. 
The flange also included a window for 18 GHz 
microwave injection in order to allow comparison 
between the two modes of operation. The main difference 
with respect to the standard flange consisted of the biased 
disk which was fixed and this was perceived to be a 
limiting factor. The extraction gap was shortened in order 
to get the maximum current from the 12 mm extraction 
hole, but the extraction optics was not changed. 
CP600, Cyclotrons and Their Applications 2001, Sixteenth International Conference, edited by F. Marti
© 2001 American Institute of Physics 0-7354-0044-X/01/$18.00
223
The extraction voltage was limited to 26 kV and most 
of the tests were carried out at 20 kV. 
 
Mechanical 
Compensator 
Watercooled 
RF 
Window 
TE02 
TE01
Gyrotron 
TE02 
Arc 
Detector 
TE02  TE01 
Converter 
30 kV 
DC 
Break
SERSE
Bi-Directional 
Coupler 
Mode 
Filter 90° Corrugated 
Bend (TE01) 
 
Figure 2: The waveguide line. 
2.1  Microwave coupling 
The microwave power from a gyrotron has been 
coupled to SERSE through the waveguide line described 
in fig. 2. The output mode of the 28 GHz - 10 kW 
gyrotron was TE02; a conversion into TE01 was performed 
to minimize ohmic losses, then oversized circular 
waveguides were used. The gyrotron itself was protected 
by an arc detector and a mode filter (the gyrotron does not 
suffer, as reflected power, more than 10% of the incident 
power). Mechanical compensators were used to prevent 
from any thermal expansion. A 90° bend with corrugated 
walls was used and a dc break was placed just before the 
source to insulate the waveguide up to 30 kV; then a 
water cooled sapphire window was used to insulate the 
waveguide from the vacuum. A bidirectional coupler, 
calibrated with a water-load, measured the incident and 
reflected power in the TE02 mode. During the tests the 
maximum observed reverse power was less than 150 W, 
which enabled a safe operation of the gyrotron. The line 
was about 8 m long and, when 10 kW were extracted from 
the gyrotron, the losses in the rf line were about 400-500 
W, most of the losses (300 W) being in the mode filter. 
Being 700 mm of the line under vacuum a resonance 
occurred in the waveguide, inducing a plasma; this 
phenomenon lasted a few hours during the waveguide 
outgassing. In conclusion, the rf launching into Serse was 
successful, and it is an important achievement, 
considering that it is the first time that 28 GHz 
microwaves have been coupled to an ECR ion source. 
3   SCALING EFFECTS AT 28 GHZ 
We have remarked in a previous paper [4] that the 
Geller’s semi-empirical scaling laws [5] are able to 
explain the results of many existing ECR sources, even if 
they do not represent the ECRIS behavior exhaustively; 
two laws are particularly relevant to our tests: 
Prf ∝ V ω1/2 qopt3 (1) 
 
Iq+ ∝ ω2 Mi−α (2) 
where qopt  is the optimal charge state, Prf is the 
microwave power, ω = 2πf, Iq+ is the intensity of the 
charge state q, Mi is the ions mass and α is an adjustable 
parameter variable between 0.5 and 1. 
It was demonstrated at lower frequency [6,7,8] that the 
scaling laws and in particular eq.(2) are effective only if 
the confining field is high enough to support a higher 
electron density. 
The conclusions of the scaling laws tests at frequencies 
between 2.45 and 18 GHz [4,9] were the following: 
• the ratio between the radial field and the resonance 
field should be higher than two; 
• the axial field at the injection side must be as high 
as possible, in order to close the loss cone; 
• the axial field at the extraction side should be 
optimized around the value of the radial field, so 
that the escape of plasma on the wall is not favored 
with respect to the beam extraction;  
• the frequency should be increased, provided that a 
high magnetic confinement is maintained; 
• the CSD improves with the microwave power, but 
the amount of power that can be coupled to the 
plasma increases with the confining field. 
 
It was shown in ref [9] that the increase of current 
obtained with SERSE by increasing the frequency from 
14.5 to 18 GHz is at least proportional to (f2/f1)2, once 
that the magnetic field is scaled by a factor f2/f1. 
The increase for the highest charge states was even 
more relevant than expected according to the f2 law. In 
fact for the highest charge states of xenon the increase 
was up to a factor 4 [9]. 
Fig. 3 compares the Xe27+ intensities extracted from 
SERSE at three different frequencies and for a different 
radial field. It is clear that the performance of the source 
is improving with the increase of radial field and that a net 
increase of the highly charged ion beam current is 
obtained by increasing the frequency. We arrived at the 
same conclusion when changing the axial field; the 
statements above cited [4,9] are confirmed even at 28 
GHz, i.e. the higher is the field the better the performance. 
A complete report of the study of dependence on the 
magnetic field and frequency may be found in ref [10]. 
4  PRODUCTION OF INTENSE BEAMS 
The study for the production of Xenon ions with charge 
states q≥20 was carried out either in dc mode and in 
pulsed mode. In both cases the reproducibility of the 
results was excellent but the long term stability was not, 
mainly because of technical problems (outgassing, high 
voltage discharges), especially in the case of a poorer 
confinement of the plasma. Because of outgassing the 
power was limited to 3 kW in dc mode (except for a short 
test at 4 kW) and to 6.5 kW in afterglow mode. The 
dependence of Xe20+ intensity on the microwave power 
was about linear up to 3 kW then it saturated and 
decreased above 3.5 kW because of outgassing; on the 
other way in pulsed mode there was not an optimum but a 
continuous beam current increase with the power was 
observed. 
0
20
40
60
80
100
1 1,2 1,4 1,6 1,8 2 2,2 2,4 2,6 2,8 3
Brad / Bres
In
te
ns
ity
 (e
µA
) 28 GHz
18 GHz
14 GHz
Xe27+
 
Figure 3: Xe27+ currents obtained by operating SERSE at 
different frequencies versus increasing radial field (20 kV 
extraction voltage). 
 
0
10
20
30
40
50
640 680 720 760 800
xenon19-09-00
In
te
ns
ity
 (e
µA
)
Analyzing magnet field (gauss)
39+
34+
35+
36+
37+
38+
 
Figure 4: High charge states spectrum (power=4 kW). 
 
The performance for the highest charge states had a 
similar trend and for ions like Xe30+ and Xe33+ the increase 
of current with the power was steeper between 3 and 4 
kW. During a short test devoted to the optimization of the 
high charge states production in dc mode, a higher power 
and a very low amount of gas were injected, and a few 
charge states were optimized at power level above 4 kW. 
In spite of the relatively high pressure inside the plasma 
chamber (~2÷3×10-7 mbar), we observed about 100 eµA 
of Xe30+ , about 8 eµA of Xe38+ and 0.5 eµA of Xe42+. 
Unfortunately these levels could not be maintained for 
a long time and after a few minutes they decreased by a 
factor two or more, but anyway they confirm that a 
remarkable increase can be obtained by operating ECRIS 
at 28 GHz. In fig. 4 the section of the spectrum featuring 
the highest charge states for Xenon is reported. An 
accident to the plasma chamber which was damaged by 
electron bombardment obliged us to stop these tests and 
in the following days we have operated the gyrotron only 
in pulsed regime.  
4.1  Afterglow mode 
The current of Xe25+ measured on FC4 is shown in 
fig.5. It can be observed that the enhancement factor was 
about 1.5 or 2 with respect to the yet larger current 
obtained in dc mode. It must be remarked that it was not 
possible to extract larger currents because of the limited 
current available from the high voltage supply (the drain 
current was 15 mA, three times bigger than the current 
measured on the Faraday Cup FC3, because of the 
backstreaming electrons from the puller and because of 
the beam losses in the extractor and in the first part of the 
beam line). 
In fig. 6 the performance of the source in dc mode and 
in afterglow mode are compared. It must be noticed that 
the optimum trap for dc mode production is the optimum 
also for the afterglow mode. 
The results concerning the peak stability were less good 
than at lower frequency, when the source operates in High 
B mode, but it is still acceptable for most of applications, 
being about ±10%. 
 
-50
0
50
100
150
200
250
300
350
400
450
500
-70 -50 -30 -10 10 30 50 70 90 110 130 150
Time (ms)
In
te
ns
ity
 (e
µA
)
 
Figure 5:  A Xe25+ beam pulse in afterglow mode. 
0
50
100
150
200
250
300
1,2 1,25 1,3 1,35 1,4 1,45 1,5
Radial field (T)
In
te
ns
ity
 (e
µµ µµA
)
Xe25+
afterglow
dc
Binj = 2.34 T
Bmin = 0.50 T
Bext = 1.33 T
Brad = 1.45 T
 
Figure 6: Comparison of Xe25+ currents in dc and 
afterglow mode versus the radial confining field. 
 Figure 7: Emittance plot for Xenon beams. 
5  TECHNICAL PROBLEMS 
5.1  High voltage discharge and extraction 
process 
High voltage sparks often occurred because of the high 
gas load which was needed to optimize the production of 
intense beams. A further load derived from the beam 
divergence: we carried out the tests with the same 
extraction system which we optimized for the low current, 
low emittance beams used for the injection into the 
cyclotron and it seemed to be not appropriate. In fig. 7 a 
simulation is shown, which features a very large beam 
emittance (about 500 to 900 πmm.mrad for the typical 
total current of 4 to 15 mA from the extractor [11]) 
whereas the acceptance of the analysis beamline is about a 
factor five smaller. The large emittance led to relevant 
beam loss either between the extraction and FC3 (the 
beam pipe became warm after a few hours of operations 
in dc mode) and between the two Faraday cups (we 
evaluated the loss in this case and we estimated that about 
35% of the beam is lost inside the analyzing magnet, 
whose gap is only 54 mm). 
The beam losses at the extraction are also important, as 
it is evident in fig. 8, where the typical triangular shape of 
the beam on the puller is visible; this losses generated an 
important outgassing which further increased the gas load 
and finally damaged the plasma electrode, which was 
melted (fig. 9). In spite of these troubles, we were able to 
find a suitable set of parameters which permitted us to 
complete the tests with a safe regime for electrodes. 
5.2  X-ray and LHe consumption 
The high flux of X-ray which we measured around the 
source was generated by the electrons lost on the chamber 
wall, because of the insufficient plasma confinement. 
In fig. 10 the X-ray measurements in the radial 
direction at 18 and 28 GHz are compared (the source 
setting was similar in the two cases). The energy 
distribution was not much different at 28 GHz, but a low 
energy population is also present. The X-ray level in the 
radial direction is much higher at 28 GHz, which is a hint 
of the bad confinement (in fact on the axis, the level of 
radiation measured during 28 GHz operations was not 
higher than the one which we measured at 18 GHz). 
A further evidence of this intense X-ray bombardment 
consisted of the higher LHe consumption which was 
associated with the lower radial magnetic field. The LHe 
consumption increased from 4.5 to 10 l/h and because of 
this effect and because of the plasma instability we could 
not decrease the radial field below 1.17 T in our test of 
scaling laws [10]. In fact the energy release in the cryostat 
decreased the safety margin of the magnets and we got 
quenches which were caused by the X-ray bursts, in spite 
of the magnet setting, which was considered safe. 
 
Figure 8: A picture of the puller 
 
Figure 9: A picture of the eroded plasma electrode 
100
1000
104
50 100 150 200 250
C
ou
nt
s (
a.
u.
)
X-ray energy (keV)
28  GHz
18 GHz
 
Figure 10: Comparison of the X-ray emission rate. 
 
Figure 11: A sketch of the GyroSERSE source. 
6  THE GYROSERSE PROJECT 
The successful completion of the study of SERSE at 28 
GHz reinforced our intention to build the GyroSERSE 
source [2,12]. This source is designed to operate in high-
B mode at a frequency equal or higher than 28 GHz, 
getting a hot dense plasma (1013 cm-3). 
The main features of this design compared to the ones 
of SERSE are given in tab. 1. Fig. 11 shows a sketch of 
the GyroSERSE source, with the cryostat, the solenoids 
and the hexapole surrounded by an iron yoke. 
The mechanical constraints have obliged to choose a 
well larger inner bore than for SERSE, because of the 
boundary conditions for the hexapole (the stored energy is 
above 300 kJ). The warm bore thickness corresponds to 
an internal diameter of the plasma chamber of 180 mm, 
50 mm larger than the one of SERSE. The maximum field 
in the conductor is about 7.5 T. The length of the plasma 
chamber will be about 700 mm and the volume will be 
larger than 14 liters, well higher than any other existing 
source. The large diameter will help to get a pressure in 
the order of 10-8 mbar inside the chamber and will make 
easier the design of the injection flange that will host the 
ports for 14, 18 and 28 GHz injection, two gas inputs and 
a 35 mm flange for the oven, the biased disk, the 
sputtering system or the target for laser ablation.  
On the outer side a 5 mm double wall watercooled 
stainless steel chamber will be able to dissipate a 
maximum power of 10 kW. On the outer a 3 mm thick 
PEEK tube will insulate the cryostat (at ground) from the 
chamber (at 40 or 50 kV). The extraction system will be 
further studied [11]. In conclusion, this source seems to 
be feasible and not more expensive than SERSE, as many 
components will be inspired by the design of SERSE. An 
estimate of the time involved with the completion of such 
a source has been roughly carried out, and we believe that 
3.5 years from the date of funding (expected for the 
summer 2001) are needed.  
ACKNOWLEDGMENTS 
This work has been performed under the frame of a  
collaboration of CEA/DRFMC/SBT with GSI/Darmstadt 
(N.Angert, P. Spädtke, K. Tinschert), CERN (C. Hill, H. 
Haseroth), ISN/ Grenoble (P. Sortais) under contract # K 
598/PS and in the frame of a Memorandum of 
Understanding between CEA and INFN/LNS. We 
acknowledge the support of the European Community – 
Access to Research Infrastructure action of the Improving 
Human Potential Programme. 
The precious help of F. Chines, S. Marletta, E. 
Messina, J. Debernardi, J.M. Mathonnet during the 
experiment has been appreciated. 
We gratefully thank P. Spadtke for the simulations of 
the extraction process. 
 
Table 1: Comparison between the design features of 
SERSE and GyroSERSE. 
 SERSE GyroSERSE 
f 14-18 GHz 28-35 GHz 
Bradial 1.55 T 2.7 T 
B1 2.7 T 4.5 T 
B2 1.6 T 3.5 T 
φ chamber 130 mm 180 mm 
L chamber 550 mm 700 mm 
φ cryostat 1000 mm ~1000 mm 
L cryostat 1310 mm 1540 mm 
Vextr 20-25 kV 40-50 kV 
Lhe consumption ~4l/h (100 l/day) 0 
O8+ ~7 pµA ~50÷100 pµA 
Ar18+ >20 pnA ~1 pµA 
Au45 ~1 pnA ~0.1 pµA 
Xe42+ ---- ~.02 pµA 
Pb52+ ---- ~.01 pµA 
REFERENCES 
[1] N. Angert et al., Proc. of the 14th Int. Workshop on 
ECR ion sources, Geneve, (1999) 220 
[2]  S. Gammino and G. Ciavola, Rev. Sci. Instr. 71(2), 
(2000) 631 
[3]  G. Ciavola and S. Gammino, Rev. Sci. Instr. 63(4), 
(1992) 2881 
[4] S. Gammino,  G. Ciavola, Proc. of 14th Int. Conf. on 
Cyclotrons & Applications, Capetown, Sudafrica 
(1995) 377 
[5] R. Geller et al, Proc. of the 8th Int. Workshop on 
ECR ion sources, East Lansing, (1987) 1 
[6]  G. Ciavola and S. Gammino, Plasma Sources 
Science and Technology, 5 (1996) 19 
[7] T. Antaya and S. Gammino, Rev. Sci. Instr. 65(5), 
(1994) 1723 
[8] S. Gammino et al., Rev. Sci. Instr. 67(12), (1996) 
4109 and references therein 
[9]  S. Gammino et al., Rev. Sci. Instr. 70(9), (1999) 
3577 
[10] S. Gammino et al, submitted to Rev. Sci. Instr., 2001 
[11] P. Spadtke, private communication (2001)  
[12] S. Gammino, G. Ciavola, L. Celona, The 
GyroSERSE project, Proc. Workshop “Production 
of Intense Beams of Highly charged ions”, Catania 
(2000), to be published by Società Italiana di Fisica, 
Bologna, Vol. 72 (2001) 


MUCC (Crossref)

http://accelconf.web.cern.ch/accelconf/c01/cyc2001/paper/G-2.pdf

Operations of the SERSE superconducting ECR ion source at 28 GHz

Abstract

Similar works

Full text

Available Versions

Crossref

Crossref

Crossref

Crossref

MUCC (Crossref)

Crossref

Crossref