RFQ Designed to Accept Beam from a Weak Focusing LEBT*
RFQ Designed to Accept Beam from a Weak Focusing LEBT^
L. M. Young
L. M. Young
Los Alamos National Laboratory,
Los Alamos, New Mexico 87545
Los Alamos National Laboratory, Los Alamos, New Mexico 87545
Abstract.
Abstract.
TheLEDA
LEDARFQ
RFQ isis aa 350-MHz
350-MHz continuous-wave
continuous-wave (CW)
(CW) radio-frequency quadrupole
The
quadrupole linac.
linac. LEDA
LEDA was
was designed
designed asasthe
thefull
full power
power
front-endprototype
prototypefor
forthe
theaccelerator
accelerator production
production of
of tritium
tritium (APT)
(APT) linac. This machine has
front-end
has accelerated
accelerated aa100-mA
100-mACW
CWproton
protonbeam
beam
from75
75keV
keVtoto6.7
6.7MeV.
MeV. The
The 8-m-long
8-m-long RFQ
RFQ accepts a dc, 75–keV,
from
75-keV, ~110-mA
~110-mA HH++ beam
beam from
from the
theLEDA
LEDAinjector,
injector,bunches
bunchesthe
thebeam,
beam,
andaccelerates
acceleratesitittotofull
full energy
energy with
with -94%
~94% transmission.
transmission. Output beam power is
and
is 670
670 kW.
kW. This
This RFQ
RFQconsists
consistsofoffour
four2-meter-long
2-meter-long
RFQsjoined
joinedwith
withresonant
resonantcoupling
couplingto
to form
form an
an 8-meter-long RFQ.
RFQ.
RFQs
•
INTRODUCTION
INTRODUCTION
The RFQ
RFQ [1-5]
[1-5] receives
receives aa continuous
continuous stream of 75The
keV protons
protons from
from the
the IfH+ injector,
injector, [6,7]
[6,7] forms
forms it into
keV
buncheswith
with aahigh
high capture
capture efficiency
efficiency (~
(~ 94%), and then
bunches
acceleratesthese
thesebunches
bunchesto
to an
an energy
energy of
of 6.7 MeV. Figure
accelerates
shows the
the coupled
coupled RFQ
RFQ structure mounted in the tuning
11shows
tuning
laboratory. Figure
Figure 22 shows
shows the
the RFQ
RFQ structure
laboratory.
configuration
configurationincluding
including tapered
tapered RF
RF power
power feeds,
feeds, vacuumportplacement,
placement,and
andsection
section nomenclature.
nomenclature. Figure
Figure 3 shows
port
photograph of
of the
the completed
completed RFQ
RFQ assembly in the
aa photograph
LEDA
LEDAtunnel
tunnel with
with the
the injector
injector pulled
pulled back.
back. The
The array of
vacuum
vacuum manifolds,
manifolds, water-cooling
water-cooling manifolds,
manifolds, and RF
waveguide
waveguide almost
almost completely
completely hides
hides the
the accelerating
accelerating
structure.
structure.
•
•
It has
has aa significantly
significantly larger
larger aperture
apertureand
andgap
gapvoltage
voltage
in the
the accelerating
accelerating section
section than
than previously
previouslydesigned
designed
RFQs at
at this
this frequency.
frequency.
Transverse
Transverse focusing
focusing atat the
the RFQ
RFQ exit
exit isis reduced
reduced toto
match the
the focusing
focusing strength
strength inin the
thenext
nextaccelerating
accelerating
match
structure.
structure.
RF
RF power
power from
from three
three 1-MW
1-MW klystrons
klystrons isiscoupled
coupledtoto
the RFQ
RFQ through
through six
six waveguide
waveguideirises.
irises.The
Thestructure
structure
combines the
the RF
RFpower.
power.
itself combines
Design
Design features
features
With
Withoutput
outputenergy
energy of
of 6.7
6.7 MeV
MeV the
the LEDA
LEDA RFQ
RFQ [1,8]
isisthe
thehighest
highestenergy
energyand
andhighest
highest power
power RFQ
RFQ in the world
[3,
[3, 5,5, 9-11].
9-11]. The
The beam
beam power
power is
is 670
670 kW
kW when operated
with
withthe
thedesign-value
design-value 100-mA
100-mA CW
CW proton
proton beam,
beam, making
ititthe
the second-most
second-most powerful
powerful linear
linear accelerator
accelerator (after
(after the
LANSCE
LANSCE 800-MeV
800-MeV linac).
linac). Some
Some of
of its
its unique design
features
featuresare
areas
asfollows:
follows:
•• Transverse
is
Transverse focusing
focusing strength
strength at the RFQ
RFQ entrance is
reduced
reduced for
for easier
easier beam
beam injection.
injection. This
This allows
placement
placement of
of the
the final
final focusing
focusing solenoid
solenoid in the low
energy
energy beam
beam transport
transport (LEBT)
(LEBT) at the optimum
distance
distancefrom
fromthe
theRFQ
RFQ for
for input
input matching.
matching.
•• ItIt employs
[12,13] between the
the
employs resonant
resonant coupling
coupling [12,13]
four
four 2-m-long
2-m-long segments,
segments, providing
providing high
high RF field
field
stability
stabilitythroughout
throughoutthe
the entire
entire structure
structure length.
FIGURE 2:
2: RFQ
RFQlayout,
layout,showing
showingRF
RFfeeds,
feeds,vacuum
vacuumports,
ports,and
and
FIGURE
segmentnomenclature.
nomenclature.
segment
Resonant Coupling
Coupling
Resonant
typical RFQ
RFQ that
that has
has constant
constant focusing
focusingstrength
strength
In aa typical
constant gap
gap voltage,
voltage, as
asvane
vanemodulation
modulationincreases
increasestoto
and constant
the beam,
beam, the
the aperture
apertureshrinks
shrinksand
andbeam
beamcan
canbebe
accelerate the
on the
the vane
vane tips.
tips. As
As the
the energy
energy rises
rises the
the cell
cell length
length
lost on
and for
for aa given
given modulation,
modulation, the
the accelerating
accelerating
increases, and
decreases inversely
inversely with
with cell
cell length.
length. Since
Since the
the
gradient decreases
maximum practical
practical modulation
modulation isisabout
about2,2,the
theRFQ
RFQwould
would
maximum
very long
long ifif the
the gap
gap voltage
voltage remained
remained constant.
constant.
become very
beam loss
loss and
and shorten
shortenthe
theRFQ,
RFQ,we
wemaintain
maintainaa
To reduce beam
Figure
adjustableslug
slugtuners
tunerscan
canbe
beseen
seenininthis
thispicture.
picture.
Figure1.1. Eight-meter-long
Eight-meter-long RFQ
RFQ in the tuning laboratory. The adjustable
_________________________________________
**Work
Worksupported
supportedby
bythe
theUS
USDOE,
DOE, Defense
Defense Programs.
Programs.
t†
[email protected]
[email protected]
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
105
large aperture, and increase the vane voltage to partially
large aperture,
aperture,
and increase
increase
the vane
partially
counter
the decrease
in transverse
focusing
vane
large
and
the
voltageasto the
partially
counter the
theincreases.
decrease in
in transverse
transverse focusing as the
counter
decrease
the vane
vane
modulation
modulation
increases.
modulation
increases.
The increased
gap voltage substantially increases the
The increased
increased
gap
voltage
substantially
increases
the
The
increases
the
accelerating
field, gap
thusvoltage
shortening
the RFQ.
However,
accelerating
field,
thus
RFQ.
However,
accelerating
field,
thus
shortening
the
However,
even with this increased gap voltage, eight meters of
even with
with
this increased
increased
even
this
gap the
voltage,
meters
of
length
is
required
to accelerate
beameight
to 6.7meters
MeV. of
A
length isis required
required
to accelerate
accelerate
6.7
A
length
to
the beam
6.7 MeV.
MeV.
A
conventional
8-m-long,
350-MHz
RFQ to
would
not be
conventional
8-m-long,
would
be
conventional
350-MHz
wouldthenot
notfield
be
stable.
Small 8-m-long,
perturbations
wouldRFQ
distort
stable. Small
Small
perturbations
distort
field
stable.
perturbations
the
field
distribution
intolerably
[12,13]. would
Therefore,
four the
2-m-long
distribution
intolerably
four
2-m-long
distribution
intolerably
[12,13].
Therefore,
2-m-long
RFQs (labeled A, B, C and D in Figure 2) are resonantly
RFQs (labeled
(labeled
A, B,
B,
C 8-m-long
and D in Figure
are resonantly
RFQs
A,
and
resonantly
coupled
to form
theC
LEDA2) RFQ.
This is
coupled
to
form
the
8-m-long
RFQ.
This
is
coupled
to
form
the
8-m-long
LEDA
This by
is
implemented by separating the four 2-m RFQs
implemented
by
separating
2-m
RFQs
by
implemented
by
separating
the
four
RFQs
by
coupling plates. An axial hole in the coupling plates
coupling
plates.
An axial
hole touch.
in the The
coupling
plates
coupling
An
coupling
plates
allows
theplates.
vane tips
to nearly
capacitance
allows
the
vane
tips
to
nearly
touch.
The
capacitance
allows the
to one
nearly
capacitance
between
the vane
vane tips
tips of
RFQ and the next
provides
between the
the vane
vane tips
tips of
of one RFQ and
the
between
andsegments.
the next
next provides
provides
the
RF coupling
between
the 2-m-long
The
gap
theRF
RFcoupling
coupling between
between the
the 2-m-long segments.
The
gap
the
segments.
between the vane tips at the coupling joint is 0.32The
cm. gap
To
between the
the vane
vane tips
tips at
at the
the coupling joint is 0.32
cm.
To
between
0.32the
cm.gap
To
minimize
the effect
of this
gap on the beam,
minimize the
the effect
effect of
of this gap on the beam, the gap
minimize
position corresponds to a zero crossing of the RF electric
position corresponds
corresponds to
to a zero crossing of
electric
position
of the
the RF
RF
electric
field
when
the
bunch
passes
the gap. The RF field
field is in
field when
when the
the bunch
bunch passes
passes the gap. The RF
field
RF field isis in
in
phase
The “coupling mode”
has aa
phasein
inall
all four
four segments.
phase
in
all
four
segments. The “coupling
"coupling mode”
mode" has
has a
strong
the 0.32-cm gap and has
has one
strongelectric
electricfield
field across
strong
electric
field
across the 0.32-cm gap and
and has one
one
longitudinal
node
in
each
2-m
RFQ.
The
coupling
mode’s
longitudinal node
node in
in each 2-m RFQ. The coupling
longitudinal
coupling mode’s
mode's
longitudinal
RF
longitudinal component
component of
of electric
electric field
field transmits
transmits
longitudinal
component
transmits RF
RF
power,
providing
the
field
stability.
power,providing
providing the
the field
field stability.
power,
description of the input beam is required to accurately
description
the input
input
beam
requiredtotoaccurately
description
ofof the
beam
isisSimulations
required
simulate
beam
losses
in the
RFQ.
ofaccurately
the beam
simulatebeam
beam
losses
theRFQ.
RFQ.
Simulations
the[16,17]
beam
simulate
losses
the
ofofthe
beam
transport
through
theinin
LEBT
[15]Simulations
with
PARMELA
transportathrough
through
theLEBT
LEBT
[15]with
withPARMELA
[16,17]
transport
the
[15]
produce
more realistic
distribution
ofPARMELA
particles [16,17]
for
input
produce
realistic
ofofparticles
produce
amore
more
realistic
distribution
particlesforforinput
input
into
the aRFQ
codes
thandistribution
the
ideal input
distributions.
into
intothe
theRFQ
RFQcodes
codesthan
thanthe
theideal
idealinput
inputdistributions.
distributions.
RFQ Entrance
RFQ
RFQEntrance
Entrance
To implement the reduced focusing strength at the
To
the
reduced
To implement
implement
theand
reduced
focusingstrength
strengthatat
entrance
of the RFQ
havefocusing
adequate
focusing
inthethe
the
entrance
of
the
RFQ
and
have
adequate
focusing
in
the
entrance
of
the
RFQ
and
have
adequate
focusing
in
the
interior of the RFQ, the transverse focusing parameter
interior
the
the
transverse
focusing
interior ofofsmoothly
the RFQ,
RFQ,
the3.088
transverse
focusing
parameter
increases
from
to 6.981
over parameter
the
first 32
increases
3.088
toto6.981
the
increases
smoothly
from
3.088parameter
6.981over
thefirst
first3232
cm
of thesmoothly
RFQ. Thefrom
focusing
isover
proportional
to
2 the
cm
The
focusing
parameter
is
to
cmof
of
theRFQ.
RFQ.
The
focusing
parameter
isproportional
proportional
to
V/r
where
V
is
the
voltage
between
adjacent
vane
tips
0
V/r
V/r0022rwhere
where VVisisthe
thevoltage
voltagebetween
betweenadjacent
adjacentvane
vanetips
tips
and
0 is the average aperture. The voltage is held constant
and
r
is
the
average
aperture.
The
voltage
is
held
constant
and
r
is
the
average
aperture.
The
voltage
is
held
constant
0
0
in this region and the aperture is reduced to increase the
in
and
isisreduced
totoincrease
in this
this region
region
andthe
theaperture
aperture
reduced
increase
focusing
parameter.
On
entry,
the beam
is notthethe
yet
focusing
parameter.
On
entry,
the
beam
isisnot
focusing
parameter.
On
entry,
the
beam
notyetyet
bunched, allowing the use of weak transverse focusing.
bunched,
bunched, allowing
allowingthe
theuse
useofofweak
weaktransverse
transversefocusing.
focusing.
By the time the beam starts to bunch, the focusing is
By
By the
the time
time the
the beam
beam starts
startstotobunch,
bunch,the
thefocusing
focusingis is
strong enough to confine the bunched beam.
strong
strongenough
enoughtotoconfine
confinethe
thebunched
bunchedbeam.
beam.
The low focusing strength at the RFQ entrance means
The
The low
lowfocusing
focusingstrength
strengthatatthe
theRFQ
RFQentrance
entrancemeans
means
that the
the matched beam
beam sizeisisrelatively
relatively large,allowing
allowing
that
that the matched
matched beamsize
size is relativelylarge,
large, allowing
some space,
space, as shown ininfigure
figure 4, betweenthe
the second
some
some space, asasshown
shown in figure4,4,between
between thesecond
second
LEBT
solenoid
and
the
RFQ
entrance.
Without
this
LEBT
LEBT solenoid
solenoid and
and the
the RFQ
RFQ entrance.
entrance.Without
Withoutthis
this
feature,
the
solenoid
would
be
right
at
the
RFQ
entrance.
feature,
feature,the
thesolenoid
solenoidwould
wouldbeberight
rightatatthe
theRFQ
RFQentrance.
entrance.
DC1
DC1
VD1
VD1
DC2 VD2
DC2 VD2
AC toroid 3
AC toroid 3
Inside
RFQ
Inside
RFQ
end end
wallwall
is “match
point”
is “match
point”
Electron
Trap
Electron
Trap
–1 kV
–1 kV
Collimator
Collimator
(water
cooled)
(water
cooled)
FIGURE
sketch
the
LEDALEBT.
LEBT.
FIGURE
FIGURE4.4.
4.AA
Asketch
sketchofof
ofthe
theLEDA
TRANSMISSION
TRANSMISSION
THROUGH
THERFQ
RFQ
TRANSMISSIONTHROUGH
THROUGHTHE
FIGURE33.
3. . A
Aphotograph
photographof
of the
the
entrance
end
of
FIGURE
A
photograph
of
the entrance
entrance end
end of
of the
the RFQ
RFQ
FIGURE
the
RFQ
mountedin
inthe
thesupport/alignment
support/alignment frame.
frame.
The
accelerating
mounted
in
the
support/alignment
frame. The
The accelerating
accelerating
mounted
structureisis
isburied
buriedwithin
withinthe
thearray
array
of
water-cooling
structure
buried
within
the
arrayof
ofwater-cooling
water-coolingmanifolds,
manifolds,
structure
manifolds,
vacuum
manifolds,
and
RF
waveguide.
vacuum
manifolds,
and
RF
waveguide.
vacuum manifolds, and RF waveguide.
RFQELECTROMAGNETIC
ELECTROMAGNETIC DESIGN
DESIGN
RFQ
ELECTROMAGNETIC
DESIGN
RFQ
TheRFQ
RFQ was
was designed
designed with
with
the
code
PARMTEQM
The
RFQ
was
designed
with the
the code
code PARMTEQM
PARMTEQM
The
(Phase
and
Radial
Motion
in
Transverse
(Phase
and
Radial
Motion
in
Transverse
Electric
(Phase and Radial Motion in Transverse Electric
Electric
Quadrupole; Multipoles)
Multipoles) [14].
The
code
includes
Quadrupole;
[14].
The
code
includes
the
Quadrupole; Multipoles) [14]. The code includes the
the
effect of
of higher-order
higher-order multipoles
multipoles in
the
RFQ
fields,
which
effect
in
the
RFQ
fields,
which
effect of higher-order multipoles in the RFQ fields, which
are important
important in
in accurately
accurately predicting
beam
are
predicting beam
beam loss.
loss. The
The
are
important in
accurately predicting
loss.
The
earlier
code
version,
PARMTEQ,
used
only
the
first
two
earlier
code
version,
PARMTEQ,
used
only
the
first
two
earlier code version, PARMTEQ, used only the first two
terms in
in the
the expansion
expansion of
of the
RFQ
fields.
PARMTEQM
terms
the RFQ
RFQ fields.
fields. PARMTEQM
PARMTEQM
terms
in
the
expansion
of
the
uses the
the first
first eight
eight terms.
terms. In
addition,
uses
In addition,
addition, aaa realistic
realistic
uses
the first
eight terms.
In
realistic
The
The
code
PARMELA
simulates
theLEBT
LEBTbeam
beamwith
with
Thecode
codePARMELA
PARMELAsimulates
simulatesthe
95%
95% space-charge
space-chargeneutralization
neutralization[18].
[18].These
simulations
95%
space-charge
neutralization
[18].
Thesesimulations
simulations
showed
showedthat
thatproper
propermatching
matchingwould
wouldbebepossible
showed
that
proper
matching
would
possiblewith
withanan
electron
electron trap
trapatat
atthe
theRFQ
electron
trap
the
RFQentrance,
entrance,and
andsolenoid-to-RFQ
solenoid-to-RFQ
distance
ring
atat
thethe
distanceof
of15
15cm.
cm.The
Theelectron
electrontrap
trapisisisa ametal
a metal
metal
ring
at
the
distance
of
15
cm.
The
electron
trap
ring
entrance
−1−1kV
entranceof
ofthe
theRFQ.
RFQ.AA
Aring
ringvoltage
voltageofof
of-1
kVblocks
entrance
of
the
RFQ.
ring
voltage
kV
blockslowlowenergy
energyplasma
plasmaelectrons,
electrons,but
butdoes
doesnot
energy
plasma
electrons,
but
does
notaffect
affectthe
the75-keV
75-keV
protons.
The
electron
trap
performs
two
essential
protons.
The
electron
trap
performs
two
essential
protons. The electron trap
functions.
functions. One,
One, ititit improves
improves the
the space
space charge
charge
functions.
One,
improves
the
space
charge
neutralization
in
the
LEBT.
Two,
it
prevents
electrons
neutralization
in
the
LEBT.
Two,
it
prevents
electrons
neutralization in the LEBT. Two, it prevents electrons
from
from streaming
streaming into
intothe
the RFQ
RFQthrough
throughthe
thetorrid
torridand
and
from
streaming
into
the
RFQ
through
the
torrid
and
corrupting
corruptingthe
themeasurement
measurementofof
ofthe
thebeam
beamcurrent.
current.
corrupting
the
measurement
the
beam
current.
Using
Using the
the simulated
simulatedbeam,
beam,two
twoRFQ
RFQcodes
codespredict
predict
Using
the
simulated
beam,
two
RFQ
codes
predict
93%
transmission
with
the
RFQ
operating
at atdesign
93%
transmission
with
the
RFQ
operating
designfield
field
93% transmission with the RFQ operating at design
field
levels.
The
codes
are
PARMTEQM
and
TOUTITIS
[19]
levels.
The
codes
are
PARMTEQM
and
TOUTITIS
[19]
levels. The codes are PARMTEQM and TOUTITIS [19]
that
use
respectively
2D
and
3D
space
charge
effects.
The
that
use
respectively
2D
and
3D
space
charge
effects.
The
that use respectively 2D and 3D space charge effects. The
measured
transmission
has
been
asashigh
asas94%
at at100
measured
transmission
has
been
high
94%
100
measured transmission has been as high as 94% at 100
mA
when
the
RFQ
fields
are
10%
above
thethedesign
field
mA
when
the
RFQ
fields
are
10%
above
design
field
mA when the RFQ fields are 10% above the design field
strength.
strength.
strength.
106
POSSIBLE
TRAPPING IN
INRFQ
RFQ
POSSIBLE ION TRAPPING
Figure
time dependence
dependence ofof RFQ
RFQ
Figure 55 shows the time
transmission
beam pulse
pulse with
with the
theRFQ
RFQ
transmission in a 300-jis-long
300-µs-long beam
fields
value. At
At about
about 150
150µs
jisinto
into
fields at
at the
the nominal
nominal design value.
the
suddenly drops
drops by
byabout
about10%.
10%.
thepulse,
pulse, the
the transmission
transmission suddenly
As
above the
the design
design value,
value,
As the
the RFQ
RFQ field
field is increased above
transmission
for increasingly
increasinglylonger
longerperiods.
periods.
transmission remains
remains high for
Current (mA)
100
100
80
60
40
20
0
00
100
100
200
200
300
300
400
400
Time
µ s)
Time ((|is)
FIGURE 5.
5. RFQ
RFQ output beam
FIGURE
beam current
current vs.
vs.time
timefor
foraa300-ms300-mslong pulse
pulse at
at -97%
~97% of
long
of the
the design
designRF
RFfield
fieldlevel.
level.
With fields
fields > 105%
105% of
of design,
With
design, the
the transmission
transmissiondrop
dropisisno
no
longer
observed,
even
for
long
pulses
and
CW
operation.
longer observed, even for long pulses and CW operation.
Along with
with the
the transmission
transmission drop,
Along
drop, higher-than-expected
higher-than-expected
activation is
is measured
measured at
activation
at the
the high-energy
high-energy end
end of
ofthe
theRFQ,
RFQ,
indicating significant
significant beam
beam loss
indicating
loss at
at that
that location.
location.Operating
Operating
the RFQ
RFQ with
with fields
fields about
about 10%
the
10% above
above the
the design
design value
value
greatly
reduces
the
magnitude
of
this
beam
greatly reduces the magnitude of this beam loss.
loss. More
More
work isis needed
needed to
to determine
determine unambiguously
work
unambiguouslythe
the cause
causeofof
the time-dependent
time-dependent transmission
transmission anomaly.
the
anomaly. At
At present,
present,
considerable evidence
evidence points
to
the
possibility
that
itit isis
considerable
points
to
the
possibility
that
+
caused
by
low-energy
H
ions
trapped
near
the
axis
by
caused by low-energy FT ions trapped near the axis bythe
the
RFQ fields
fields [20].
[20]. The
The extra
RFQ
extra positive
positive charge
charge from
from the
the
trapped ions
ions causes
causes the
the beam
beam size
trapped
size to
to increase,
increase, reducing
reducing
the RFQ
RFQ transmission,
transmission, and
the
and also
also increasing
increasing the
the beam
beam size
size
theend
end of
of the
the FLEET.
HEBT. This
atatthe
This hypothesis
hypothesis isis consistent
consistentwith
with
the observation
observation that
that the
the collimator
the
collimator ring
ring in
in front
front ofof the
the
beam
stop
glows,
presumably
from
being
beam stop glows, presumably from being struck
struck by
by
incident beam,
beam, when
when the
the RFQ
pressure exceeds 1-2 x10-7 7
incident
+ RFQ pressure exceeds 1-2 xlO"
Torr. The
The low-energy
low-energy H
Torr.
FT ions
ions can
can be
be produced
produced by
bybeam
beam
collisions
with
background
gas
(H
)
near
the
RFQ
collisions with background gas (H22) near the RFQaxis,
axis,oror
by beam
beam collision
collision with
with the
by
the vane
vane tip
tip surfaces.
surfaces. At
At fields
fields <
the
design
value,
the
beam
may
be
sufficiently
the design value, the beam may be sufficiently+large
largethat
that
its fringes
fringes strike
strike the
the RFQ
RFQ vane
its
vane tips,
tips, creating
creating HFTions
ionsthat
that
get trapped
trapped temporarily
temporarily in
get
in the
the beam
beam channel.
channel. As
As the
the
trapped charge accumulates, the beam becomes larger
trapped
charge accumulates, the beam becomes larger
still, until the transmission drops suddenly.
still,
until the transmission drops suddenly.
1
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107