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THE 1 MV 114 MHZ ELECTRON ACCELERATING SYSTEM FOR THE CERNB
B.J.
Evans,
R. Garoby,
PS Division,
R. Hohbach, G. Nassibian,
CERN, 1211 Geneva 23,
--Summary.
In addition
tc its original
function
as a
accelerator,
the PS will
become
part of the LEP
:njector
(.hain [I] accelerating
electrons
and
positrons
from 600 MeV to 3, 5 GeV. For this
purpose,
two 114 MHz cavities
with a gap voltage
of 500 kV have
been installed,
Thi.s paper describes
the (cavities
together
with their
related
auxiliary
systems
and the
Special
features
include
electronics.
low level
mechanical
short
circuiting
of t,he gaps in order to
avoid perturbing
the high-intensity
proton
beams, fast
tuners
using perpendicularly
biased
ferrites
and
higher
order mode dampers.
proton
Parameters
The RF voltaqe
is obtained
using
two high Q
They are independently
fed via 40 m coaxial
situated
outside
the ring and
cables
by amplifiers
accessible
during
operation.
The amplifiers,
rated at
50 kW CW and 200 kW pulsed,
are housed in a Faraday
cage to avoid interference
with the navigation
system
of the a-i-port
nearby [Z].
The cavity
The shape of the cavity
(Fig.
I) was chosen
to give high values
of Q and R/Q (evaluated
by Superfislh [3]) and small deformations
under atmospheric
pressure
(vcriflrd
using Safeshell
[4]).
As the cavity
has to be non-magnetic
due to
its proximity
to the PS magnets,
it was decided
to use
a stainless
steel
shell
internally
copper--plated
The nosc cones are machined from solid
OFHC
(0.1
mm).
We measured a Q of 90 % of the value calculcopper
any effects
of the many holes for
at cd , Ignoring
A further
10 9, drop is due
tuners i pumps and :;horts.
to the penetration
of the tuning
cylinders
and the
short-ing
elements.
of
the
cavity
Resonant
frequency
Tuning range (slow)
Tuning range (fast)
Gap voltage
Q value
(Superfish)
Q value
(measured)
or
cavities.
S. Talas
Due to the poor thermal
conductivity
of
:stainless
steel,
it was necessary
to enclose
the whole
cavity
surface
in a water
jacket,
which is divided
into a large number of radial
channels
in order to
maintain
a low-temperature
gradient.
The nose cones
are separately
cooled by water flowing
through
holes
drilled
in the massive copper forgings
The cavity
is
pumped by two 400 l/set
ion pumps.
Jntroduction
--.-__The basic operating
modes [l]
for electron
pos:tron
acceleration
in the CF.5 require
a new RF
system with the following
specification:
:
240
Harmonic
number
:
114.511
MHz
Frequency
6.1
mA
Nominal DC beam current:
kV
: 1000
Peak RF voltage
P. Marchand,
Switzerland
R/Q
114.511
MHz
MHz
MHz
kV
0.260
0.020
500
70'000
fully
equipped
56’000
180
Shunt
Power
impedance
for 500 kV
Cavity
10
Q
MQ
12.5
kW
tuninq
The inital
setting
of the resonant
frequency
of the cavities
was done by machining
the tips of the
nose cones with a tolerance
of l/10
mm which
corresponds
to 15 kHz.
A pair of piston
260 kHz within
a minute.
tuners
covers
the
range
of
For displacing
the beam + IO mm radially,
a
fast tuner is being developed
with a range of
The tuner consists
of a coaxial
line,
& 10 kHz.
partly
filled
with rings
of microwave
ferrites
(+ 160 mm) and coupled
to the cavity.
A coil
is wound
around the external
conductor
producing
a bias field
perpendicular
to the RF flux
(Fig.
2).
The magnetic
circuit
is closed
by a laminated
iron sore. The
perpendicularly
biased microwave
ferrite
rings
(G 810),
exhibit
a Q of over 1000 at permeabilities
below 4 [5,6].
With the nominal
gap voltage
and the
desired
tuning
range each of the two tuners
has power
losses
below
1 kW. Ordinary
ferrites
with parallel
biasing
would generate
10 times
these losses.
r--
Fig.
1
The 114 MHz cavity
N = nose cone
D = damper
W = RF window
C = cable
F
S
P
V
1901
=
=:
=
=
to
ferrite
short
piston
vacuum
amp.
tuner
tuner
pump
CH2387-9~R7;0000- lgO1 9 I .OO G: IEEE
PAC 1987
Rshunt [ knl
10000
IRONYOKE
COIL
AIR FLOW
FERRITE
COAX
LINE
200
Fig.
2
The ferrite
short
500
tuner
;ig.
me
300
400
FREPUENCY[MHz]
4
circuits
When accelerating
electrons,
a high cavity
shunt impedance
saves RF power.
However,
when
accelerating
a high intensity
proton
beam (using
another
RF system)
the impedance of the idle
114 MHz
cavity
must be reduced
to a few kR to avoid
instdbilitles.
R shunt with
(for electrons
and without
damper
and positrons)
R shunt [ kR1
10000-
SHORTS
CLOSED
I
I
I
1000,.
having
shown that the desired
result
could not be obtained
using coupling
I oops and loads,
we decided
to use two shorting
bars bridging
the gap.
The lowest
mode is then at 180 MHz and both Q and R/Q
-ire very much reduced.
Furthermore,
all the
resonances
of the short-crrcuited
cavity
fall
wrtbin
the range ,of the dampers.
Q measurements
indicate
that
in the open position
the shorting
devices
account
for
3 9, of the total
cavity
drssipation.
-Nfl
nAMDFD
Tests
servo-motor
100.
10.
1,
ii0
The shorts
are actuated
by dn electrical
with controlled
acceleration.
Fig
5
R shunt
(for
Problems
have arisen
and under CW operation
::hr,rt-I:ircliitlnrJ
bars overheat.
version
is bernq prepared.
bellows,
with t,he lifetime
at full
power
A water-cooled
of
the
The
in the cavity
are
of a 50 R load
tuned to the funda-
Low-level
---._.-
The hisher
damper
RF power
amplrt
1er
RF beam__--_----.---------control
dnd
cavity
servos
The CPS 1s supposed to [I] :
receive
leptons
from EPA using a bunch-intll--bucket
where the CPS RF system is locked
transfer
scheme,
to that of the EPA;
accelerate
them erther
wrth the usual ferrite
cavities,
driven
on h = 16, or with the new 114 MHz
cavities
on h .: 240;
accurately
position
the bunches at high energy
for 3
correct
bunch-.into-bucket
transfer
to the SPS;
shape the bunches so that they are stable
in the SPS
making use of any one of the 3 techniques
envrsaged
(“bunch
compression”
t “bunch expansion”
described
in [a] or “long bunch expansion”
as jn [!+]I
Wi.th 4 dampers,
we attenuate
the shunt
impdance
of the parasitic5
by 30-40 dB (Figs.
4,s).
The
by perturbatron.
At 300 MHz the
R/Q was obtained
short
is rather
inefficient,
but the spectral
density
above 200 MHz.
of the proton
beam decays :strongly
3
and without
The two power ampI if ier:, are commer~:1 al
types,
manufactured
by Herfurth
GmbH. , Hamburg.
Each
consists
of a transistor
broad--band
preamplifier
(300 WI followed
by a selective
driver
stdge with a
The final
grounded
grid
t etrode
(10 kW drssrpation)
:Stdge
USf?:j
a tPtrOde
of 70 kW maximum ~~lSSipdi:lOr~.
The
a half-wave
resonator,
is raparitrvely
anode circuit,
coupled
to the 50 R feed line
(Flexwell
4 I/8”)
A
variable
length
lrne covering
half .a wavelength
is inserted
in series
with the 40 m of cable to the cavity.
The damper has a tridxial
structure.
The
inner coaxial
lrne connects
the loop to the external
The outer
conductor
of this
line and the sur-load.
rounding
enclosure
form a resonator
tuned to 114 MBz
The coupling
loop is connected
between
the innermost
conductor
dnd the cavrty
wall
(Fig.
3) [7].
Fig.
with
protons)
the
The dampers
Higher
mode resonances
attenuated
by dampers consisting
icoupled through
a notch filter
mf:ntal
frequency
of the cavity.
500
400
300
FRECWENCY[MHzl
200
mode damper
1902
PAC 1987
This means the low-level
RF system must be
able to:
drive
two groups of cavities
with the correct
pha:iiny,
both the 10 ferri.te
cavitie:;
on h = 16 or
h :- 8 and the two 114 MHz cavities
on h :: 240;
- synchronize
the RF on EPA at injection
and position
the bunches for SPS at ejection;
damp the coherent
phase oscillations
of the beam.
El.ock
dlasram
description
[IO]
The system (Fig.
6) is based on a 114 MHz RF
synthesizer
which drives
the 114 MHz cavities
through
two phase-locked
loops used as phase shifters.
Condition&
_-~
Conditioning
the cavity
required
some time
and care.
In particular,
it was found essential
to
limit
the pressure
to well below low6 torr
while
progressively
raising
the voltage.
At some voltage
levels
extending
from 50 kV
to 400 kV an increase
of even 0.1 dB of RF input
caused an excessive
pressure
rise.
The time rerjuired
to break through
these levels
varied
from hours to
days.
The stray
fields
from the magnets in the ring
also contributed
to these effects.
Beam experiments
MHz (h = 16) or 3.8 MHz (h = 8) are
Cab1 e
derived
by division
from the same source.
delays
are used to obtain
the correct
phases to drive
the 10 ferrite
cavities.
7.6
Signals
from
filtered
around
114.5
in a limiting
amplifier
as beam phase reference
the beam phase loop.
PU’s are bandpass
MHz (Q = 1000) and leveled
off
(60 dB dynamic range)
to serve
for the phase discriminator
of
Successful
results
were obtained
with this
system in
1986;
they are described
in detail
in another
paper
In these proceedings
[ll].
Acknowledqements
electrostatic
RF synchronization
on EPA, before
injection,
by momentarily
locking
the beam phase loop
I :i obtained
on an ad hoc signal
deri.ved
from EPA RF and revolution
frequency
(swLt.ch Sl on “EPA RF*6” In Fi<& 6)
The high power part of the project
was
defined
and started
by W. Pirkl.
The associated
clectronics
were developed
by J. Broere,
M. Croizat
and
G. Serras.
G. Roux was an essential
contributor
for
the low-level
system and for the beam experiments.
P. BarthclPmy
was responsible
for the AVC system.
Fief ere.cnc
“LEP design
report
- Vol.
chain, ’ CERN-LEP/TH/83-29.
Bunch synchronization
on the SPS, before
e j e c 1:ion , 1s effected
by changing
the RF frequency
until
the phase difference
between the bunch position
signal
and an ad hoc signal
derived
from SPS reference
1s at zero [switch
S2 I’on” i.n Fig. 6).
[?I
<:Pyltv
“Superfish
[31 W. Pirkl,
communication.
servo
[21 M. Croizat,
Interferences
system
A medium speed servo system (several
100 Hz
bandwidth)
insures
that the peak crap vol ‘cage follows
the c-omputer- generated
program.
Its dynamic range is
16 dB (10 kV to 500 kV). Slow drift:
of the ~:av~ty
?r:ne, as well as deliberate
detuning
are controlled
by
,% sampled tuning
loop (several
0. 1 Hz bandwidth)
S:lmpling
is done onc‘c per cycle,
before
bcdm
i tijection.
[41 S. Talas,
pub1 ished)
[‘jl
___
-2-z-_kEL~-e. vI AM.5
‘“I-----7
‘b
1
b=l6ial
5rS’FM
h=.i?L@
,' '\~
/
A
“Possible
RF
R. Hohbach,
._,'
PS/RF/Note
83-16.
“Cavity
computation,
for
P. Marchand,
R
R Hohtach,
“Ferrite
tuner, ” PS/RF/Note
[71 R. Hohbach,
ing in the
[El Y. Baronnier
the
\
/L!h\
--
P. Marchand,
MHz cavity,”
114
CERN PS, ”
[Ill
f _,EFa, 6i p+~~~~~~~~
-
Fig.
(to
Poirier,
85-3.
private
be
G. Serras,
“Higher
modes and damp(to be published).
“Electron
et al.,
1983 Part.
Act.
“Organisation
niveau
pour l’ac&lgration
PS/RF/Note
dans le PS,”
[lOI R. Garoby,
‘I
MHz,”
“A possible
et al.,
[91 D. Boussard
lepton
transfer
scheme, PS-SPS,”
LF
lir
114
”
T.G. Arcphy,
R.D. Carllni,
Lhl W.R. Smithe,
C.C. Frledrichs,
D.L Grisham,
G. Spalek and
‘RF cavities
with transversely
L.C. Wilkerson,
Part.
Act. Conf.,
biased
ferrite
tuning,”
Vancouver,
May 1985.
Zr tii Mill
CAW’IES
Cr FERR/TE
CA“I TES
The LEP injector
1.
Y. Baconnier
accelerator
results,“,
beam dynamics
Cllnf.,
IEEE
variant
for t.he
LEP Note 569.
de l’equlpement
RF bas
d’&lectrons/positrons
83-2/Rev.l,
1985.
“The CERN PS as et/eet al.,
in the LEP injector
chain,
first
these proceedings.
5:
6
Beam
control
in
NS.
system
1903
PAC 1987