Performance of the Magnetron H Source
on the BNL 200 MeV Linac *
James G. Alessi
Brookhaven National Laboratory
Upton, NY, 11973 USA
Abstract. A magnetron surface plasma H" ion source has been used at Brookhaven since 1982 for injection into the 200
MeV linac. Since 1989, this source has operated with a circular aperture, injecting into an RFQ. The source typically
produces 90-100 mA of H" at 35 keV, in 700 (is pulses at up to 10 Hz. The arc parameters are ~15 A, 150 V, and the
extracted e/H- ratio is 1/2. Ions are extracted from a 2.8 mm diameter aperture (J=1.6 A/cm2). The emittance is
approximately e^ ms = 0.4 n mm mrad. The performance is very reliable, operating continuously for ~6 months, with
essentially no parameter adjustments required once the source is stabilized.
INTRODUCTION
Since 1982, a magnetron surface plasma IT source
has been used on the Brookhaven 200 MeV Linac.
The linac typically runs at a repetition rate of 6.6-7.5
Hz, and a pulse width of ~ 500 |iA, and the current at
the exit of the linac is typically 35-40 mA. This beam
is injected into the Booster synchrotron and then AGS,
for high energy physics. Since the Booster typically
takes only 4-6 pulses every 2.5-3.5 seconds, the rest of
the pulses are switched to a second beamline, where
this ~ 70-100 |iA average current is used for isotope
production.
The magnetron source was first developed in
Novosibirsk (planotron) [1], but the basis of the
version used at BNL is the very clever design
developed by C. Schmidt at FNAL [2]. While changes
have been made over the years to incorporate cathode
focusing, circular aperture, extractor geometry
changes, etc., there have been remarkably few changes
over the years to his basic mechanical design, which is
simple and reliable. From 1982-1989 the source
operated in the dome of a 750 keV Cockcroft-Walton
preinjector [3]. A slit extraction geometry was used at
20 keV, followed by a small 90 degree doublefocusing dipole before the beam entered the high
gradient accelerating column. In 1989, the CockcroftWalton was replaced by an RFQ preinjector. At this
time, the magnetron was converted to circular aperture
extraction at 35 keV, followed by a 2-solenoid
matching into the RFQ [4].
produces a cyclic motion of electrons in the ~ 3 mm
gap between the Mo cathode and anode, producing an
intense hydrogen plasma in this region. Cesium vapor
is fed into the source from an external oven containing
metallic Cs, through a transfer tube kept at ~ 300C.
An all metal valve between the source and the Cs oven
allows the source to be let up to air without having to
reload Cs. Hydrogen is fed into the source through a
pulsed gas valve. The source has a very small
discharge volume (~1 cm3), so good gas pulsing is
effective. The valve is placed close to the source to
reduce the conductance and keep as sharp a gas pulse
as possible.
Cs inlet
FIGURE 1. Schematic of the magnetron source.
MAGNETRON GEOMETRY, LAYOUT
Ions are extracted through 2.8 mm diameter
apertures in both the anode and extractor, across a 4 4.5 mm extractor gap. The extractor tip is an insert
made of tungsten, to reduce erosion coming from
impingement by extracted electrons. We operate with
pulsed extraction voltage. Figure 2 shows the source
without the anode cover on, and Fig. 3 shows the
source with extractor in place.
Figure 1 shows the basic geometry of the
magnetron. A transverse magnetic field of -900 G
The source sits reentrant in the vacuum box. The
only opening between the source vacuum and the
* Work supported under the auspices of the U.S. Department of Energy.
CP642, High Intensity and High Brightness Hadron Beams: 20th ICFA Advanced Beam Dynamics Workshop on
High Intensity and High Brightness Hadron Beams, edited by W. Chou, Y. Mori, D. Neuffer, and J.-F. Ostiguy
© 2002 American Institute of Physics 0-7354-0097-0/02/$ 19.00
279
LEBT line is the 2.8 mm diameter extraction aperture.
LEBT line is the 2.8 mm diameter extraction aperture.
This
results
a rapidmm
reduction
inextraction
pressure aperture.
along the
LEBT
line isin
diameterin
This results
inthea 2.8
rapid reduction
pressure along
the
This results
in a rapid
inlosses.
pressureThe
along
the
beamline,
reducing
IT
stripping
source
−reduction
beamline, reducing H− stripping losses. The source
beamline,
losses.
The source
chamber
pumped
aa 2200
2200 l/s
1/s
turbopump,
while
chamber isisreducing
pumpedHby
by stripping
turbopump,
while
chamber
is
pumped
by
a
2200
l/s
turbopump,
while
only
a
1701/s
turbopump
is
needed
on
the
LEBT
line.
only a 170 l/s turbopump is needed on the LEBT line.
only a 170 l/s turbopump is needed on the LEBT line.
FIGURE2.
2. Source
Source with
with the
anode
cover
removed.
FIGURE
theanode
anodecover
coverremoved.
removed.
FIGUREmagnets
2. Source
the
Permanent
arewith
visible
on the sides
of the source.
Permanent
magnets
are
visible
on
the
sides
of
thesource.
source.
Permanent magnets are visible on the sides of the
what it was initially. This improvement leads us to
what it was initially. This improvement leads us to
believe
thatinitially.
the source could
easily operate at
what
it that
was
ushigher
to
believe
the source This
couldimprovement
easily operateleads
at higher
duty
factors
than
those
now
used.
believe
that
the
source
could
easily
operate
at
higher
duty factors than those now used.
duty factors than those now used.
TABLE
1. Typical
running
parameters
TABLE
1. Typical
running
parameters
Hcurrent
90-100
TABLE
1. Typical running parameters
Hcurrent
90-100
mA mA
2
2
Hcurrent
90-100
mA
J(H-)
1.5
A/cm
1.5 A/cm
J (H-)
2
1.5
A/cm
JExtraction
(H-)
Extraction
voltage
35 kV
voltage
35 kV
Extraction
voltage
35 0.5kV
Electron/HElectron/H0.51.0 1.0
Electron/H1.0
Arc
voltage
140
-160V
Arc
voltage
1400.5– 160
V
Arc
voltage
140
–
160
V note)
Arc
current
8
-18
A (see
Arc current
8 – 18 A (see
note)
Arc
current
8 – 187.5
A (see
note)
7.5 Hz
Rep
rate
Rep
rate
Hz
Rep
rate
7.5
Hz
Pulse
width
Pulse
width
700700
µs us
Pulse
width
700
µs
Duty
factor
Duty
factor
0.5 %
0.5%
Duty
factor
0.5
%
rms
emittance
rms emittance
∼ 0.4
π mm
mradmrad
~ 0.4
71 mm
rms
emittance
∼ <0.4
mm/mg
mrad
Cs
consumption
Cs
consumption
0.5
hr / hr
<π mg
0.5
CsGas
consumption
< ∼0.5
mg / hr
Gas
flow
2~
sccm
flow
2
seem
Gas flow
∼ 2 sccm
(Note:
often
runs
at 15-18
A toAkeep
source
temperature
(Note:
often
runs
at 15-18
to keep
source
temperature
(Note: often runs at 15-18 A to keep source temperature
high)
high)
high)
TABLE
2. 2.
History
of improvements
in BNL
magnetron
TABLE
History
of improvements
in BNL
magnetron
TABLE
2. History
of
improvements
power
efficiency in BNL magnetron
power
efficiency
power
Aperture/
H- efficiency
Arc I Arc V Pwr effic
Aperture/
Arc I ArcV
Aperture/
H-H- Arc
PwrPwr
efficeffic
cathode
focusing (mA)
(A) I Arc
(V) V (mA/kW)
cathode
focusing (mA)
(mA) 150
(mA/kW)
cathode
focusing
(A)(A) 150
(V) (V) (mA/kW)
Slit/flat
50
2.2
Slit/flat
Slit/flat
50 50 150
150150
2.2 2.2
Slit/grooved
50 150 150
6.7
Slit/grooved
Slit/grooved
50 50 10
50 50 150
150150
6.7 6.7
Circular/dimpled
100
67
Circular/dimpled 100100 10 10 150150
Circular/dimpled
67 67
OPERATING EXPERIENCE
OPERATING
EXPERIENCE
OPERATING
EXPERIENCE
in
Typical
sourceshould
operating
parameters
are extracted
given in
Table
1.
One
note
the
very
low
Typical source operating parameters are given in
Table
1. One should
note
the very
low extracted
electron
We often
operate
higher
Table
1. current.
One should
note
the the
verysource
low atextracted
electron
current.
operate
the source
higher
arc current
than We
is often
required
to achieve
theatdesired
electron
current.
We
often operate
the source atdesired
higher
arc
current
thanThis
is required
beam
current.
is partly to
justachieve
to keepthe
the source
arc
current
than
is
required
to
achieve
the
desired
beam
current.highThis
is partly
justgood
to keep
the source
temperature
enough
to allow
Cs distribution
beam
This
is by
partly
justgood
tothe
keep
the in
source
temperature
high
enough
tooperating
allow
Cssource
distribution
in thecurrent.
source,
but
also
the
temperature
high
enough
to
allow
good
Cs
distribution
in
the source,
also mode,
by operating
the current
source inpulse
the
space
charge but
limited
the beam
inspace
the source,
but also
by operating
thecurrent
source pulse
in the
charge
limited
mode,
the beam
becomes
somewhat
flatter.
space
charge
limited
mode,
the
beam
current
pulse
becomes somewhat flatter.
The good
powerflatter.
efficiency of the magnetron has
becomes
somewhat
The good
power The
efficiency
the magnetron
has
helped
reliability.
powerofefficiency
improved
The good
power
efficiency
the
magnetron
has
helped
reliability.
power
efficiency
improved
dramatically
as
we The
went
from of
the
conventional
flat
helped
The power
efficiency
cathodereliability.
to as
geometrical
focusing
of
ions improved
fromflata
dramatically
we went
from
the
conventional
dramatically
as we went
from
the ofconventional
flat
grooved tocathode
into focusing
the slit
extractor,
cathode
geometrical
ions fromand
a
cathode
tocathode
geometrical
focusing
ions
from
subsequently
spherical
the
cathode
into
grooved
intofocusing
the from
slit of
extractor,
and a
a circular cathode
aperture.
Table
2 shows
how
the power
grooved
into
the
slit the
extractor,
and
subsequently
spherical
focusing
from
cathode
into
hasspherical
now improved
a factor
of
from
aefficiency
circular aperture.
Table
2 by
shows
how
the30power
subsequently
focusing
from
the cathode
into
now improved
a factor
30 power
from
aefficiency
circular has
aperture.
Table 2 by
shows
howofthe
With continuous 24-hour/day operation, times
With
continuous
24-hour/day
Withsource
continuous
24-hour/day
operation,
times
between
maintenance
are ∼ 6operation,
months.
(Ittimes
can
between
maintenance
∼ 6~ months.
(It
can
between
source
maintenance
are
6 months.
(It can
vary
fromsource
3-9
months;
almostare
always
shutting
down
vary
3-93-9months;
always
shutting
down
varyfrom
from
months;
almost
always
shutting
down
when
the
program
ends,almost
rather
than
due
to
failure).
when
thetheprogram
ends,
rather
than
due
to made
failure).
when
program
ends,
rather
than
due
to failure).
The
source
can
be shut
down,
a minor
repair
or
The
source
can
shut
down,
a minor
made
or or
the
spare
swapped
in,shut
restarted,
and
berepair
running
well
The
source
canbebe
down,
a minor
repair
made
the
in,in,
restarted,
be be
running
again
in 8 swapped
hours.
Once
therestarted,
sourceand
isand
started,
we trywell
towell
thespare
spare
swapped
running
again
8 hours.
thethe
source
we we
try
to
keep
itinin
running.
That
is,
even
forisastarted,
week
linac
again
8 hours.Once
Once
source
is1-2started,
try to
keep
That
is, is,
even
for for
a 1-2
week
linac
shutdown,
we would
typically
choose
leave
thelinac
keepit itrunning.
running.
That
even
ato1-2
week
shutdown,
wewewould
typically
to mostly
leave
the the
source
pulsing,
with
extractor
on.choose
This
is
for
shutdown,
would
typically
choose
to
leave
source
pulsing,
with
extractor
on.
This
is
mostly
for
convenience,
since
the
source
almost
always
returns
to for
source pulsing, with extractor on. This is mostly
convenience,
since
the
source
almost
always
returns
to to
normal
operation
following
a
shutdown
or
power
convenience, since the source almost always returns
normal (On
operation
following
a shutdown
or power
outage.
the
test
bench,
a
source
may
be
turned
on
normal operation following a shutdown or power
outage.
(On the
test bench, a source
be turned
on
and
off daily
Themay
attempt
made
outage.
(Onwithout
the testdifficulty).
bench, a source
may beis turned
on
andkeep
off daily
without difficulty).
The attempt
made
to
the extraction
voltage on whenever
theissource
and
off
daily
without
difficulty).
The
attempt
is
made
to running.
keep the extraction
voltage
on extractor
whenevertipthetends
source
is
Beam heating
of the
to
keep theBeam
extraction voltage
whenevertends
the source
istorunning.
of the on
extractor
keep
it clean, and heating
if the extractor
is turnedtipoff
for to
a
is running.
Beam
of theisextractor
tiptofor
tends
to
keep
it ofclean,
if heating
the
extractor
turned off
couple
hours,and
it may
have
to be reconditioned
geta
keep
it
clean,
and
if
the
extractor
is
turned
off
for
couple
it may have to be reconditioned to get a
back
toof
35hours,
kV operation.
couple
of
hours,
it may have to be reconditioned to get
back to 35 kV operation.
We to
typically
operate the source slightly gas- and
back
35 kV operation.
We typically
operate the
source
gas- and
Cs-starved,
i.e. increasing
either
one slightly
would lower
the
We typically
the
slightly
gasCs-starved,
i.e. increasing
one
would
the and
arc
impedance,
butoperate
it haseither
beensource
found
to lower
be more
Cs-starved,
i.e.
increasing
either
one
would
lower
arc impedance,
it has
been found
to be more
reliable
to keepbutthese
parameters
reduced.
The the
arc impedance,
but
it parameters
hasthebeen
found
to beam
beThe
more
disadvantage
is thatthese
it makes
discharge
(and
reliable
to keep
reduced.
reliable“noisy”.
to iskeep
parameters
reduced.
current)
Once
stabilized
after turnon,
it’sbeam
not The
disadvantage
that
itthese
makes
the discharge
(and
disadvantage
that ittostabilized
makes
discharge
(and
beam
unusual
for
the is
source
run forthe
aafter
month
at constant
current)
“noisy”.
Once
turnon,
it’s not
current
a single
adjustment.
fairly
current)without
Once
stabilized
after Itturnon,
it's not
unusual
for"noisy".
the source
to run
for a month
at isconstant
common
that
slow
buildup
of CsOH
willIt gradually
current
asource
single
isat fairly
unusualwithout
for athe
toadjustment.
run
for a month
constant
common
a slowa buildup
CsOH will gradually
current that
without
single of
adjustment.
It is fairly
efficiency has now improved by a factor of 30 from
common that a slow buildup of CsOH will gradually
FIGURE 3. Source with anode cover and extractor.
FIGURE
3. Source
Source with
FIGURE 3.
with anode
anodecover
coverand
andextractor.
extractor.
SOURCE PERFORMANCE
SOURCE PERFORMANCE
SOURCE
PERFORMANCE
Typical
source operating
parameters are given
280
begin to block the hydrogen inlet, which then requires
one to increase the gas to the source (voltage to the
pulsed gas valve) every 1-2 weeks. This may start
after a few months of operation.
When opening after a long run, the source is
generally very clean inside (polished). Cs is not seen
downstream of the extractor electrode. There is
erosion of the extraction tip (from ions, electrons),
anode aperture (electrons), cathode dimple
(backstreaming ions), and cathode opposite the Cs
feed (discharge). In spite of these very significant
changes in source dimensions for the extraction
geometry and cathode focusing, the performance (and
output from linac) remains very constant. We often
clean the source parts and reinstall them, even with the
heavy erosion.
FIGURE 5. Source output, RFQ input, and RFQ output; 20
mA/div, 100 |is/div.
SCALING TO HIGHER DUTY FACTOR
In 1983, the magnetron source was operating at
150 V x 150 A x 5 Hz x 600 jis = 68 W average
power. With the present power efficiency, simple
scaling would imply that the source could now
produce 100 mA at 4.5% duty factor. The source is
presently uncooled, so one could gain some margin by
adding even simple cooling. At reduced beam
currents, one could either reduce the arc power,
allowing
even
higher
duty
factors,
or
reduce the extraction aperture, resulting in a smaller
emittance and reduced gas flow. In reality, the
situation will be more complicated. Issues to be
explored for higher duty factor would have to include
the effects of the resultant higher gas flow out of the
source, the increased erosion of source parts, and the
increased heating of the extractor electrode.
MATCHING TO THE RFQ
The 35 keV beam is matched from the source to the
RFQ with 2 solenoids, over a distance of -1.3 m. It is
important for the beam to be space charge neutralized
in this LEBT, and this neutralization typically occurs
within 50 |is at a pressure in the line < 10~5 Torr. The
layout of the line is shown in Fig. 4. The pulsed dipole
allows switching between polarized and unpolarized
beam on a pulse-to-pulse basis.
r
CURRENT
TRANSFORMER
ACKNOWLEDGMENTS
The very good performance of the source over the
years is in large part due to the many excellent
engineers and technicians in the BNL linac group who
have operated and made improvements to this source.
REFERENCES
^POLARIZED H-
1. Belchenko, Yu.L, Dimov, G.I., and Dudnikov, V.G.,
Proc. 2nd Symp. On Ion Sources and Formation of Ion
Beams, Berkeley, CA, VHI-1, LBL-3399 (1974).
FIGURE 4. Layout of the matching line between source
and RFQ. The distance from the source to RFQ is ~ 1.3 m.
The 200 MHz RFQ, built by LBL [5], accelerates
the 35 keV input beam to 750 keV. Operation of the
RFQ has been extremely reliable. By operating with
space-charge limited extraction, and "flooding" the
transport line, the transmission numbers suffer, but the
current out of linac is high and stable. RFQ
transmission is typically 80-90% (see Fig. 5), but at
lower currents (40 mA) we have achieved essentially
100% transmission. Under normal conditions, the
emittance out of the RFQ is e(n, rms)~0.4 n mm mrad.
2. Schmidt, C.W. and Curtis, C.D., Proc. Symp. Prod, and
Neut. of Negative Ions and Beams, Brookhaven, BNL50727 (1979) 123.
3. Barton, D.S. and Witkover, R.L., IEEE Trans. Nucl. Sci.
NS-28 (1981) 2681.
4. J.G. Alessi et al., Proc. 1989 IEEE Part. Accel. Conf.,
Chicago, IEEE-89CH2669-0 (1989) 999.
5. R.A. Gough et al., Proc. 1986 Linear Accel. Conf.,
SLAC, SLAC Report No. 303 (1986) 260.
281