Study (A): obtain the proper llrf overhead under the certain perturbations. These

tobascothwackUrban and Civil

Nov 15, 2013 (3 years and 7 months ago)

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Proposal of LLRF study at FLASH


1.

Possible
perturbations

and overhead

(LLRF/HLRF/Cavity/Gun)

It is necessary to check the various overhead (such as the micro phonic effect and so
on) experimentally by making use of
FLASH
.

Three issues of particular importan
ce for the
I
LC LLRF are

1)
cavity detuning (
Lorentz force detuning

and

m
icrophonics
) and 2
) b
eam
loading

and
3
)
high
-
voltage (HV)

regulation of klystron.

1)

cavity

detuning
:
Lorentz force
d
etuning
is approximately 600 Hz
for operation at
design gradient in t
he main linac (31.5

MV/m).

it is essential to cancel the Lorentz
force detuning by a fast frequency

tuner (piezoelectric actuators).

The
allotted

power
overhead

for the cavity detuning (Lorentz force and microphonics) is 2%
corresponding to +/
-
40 Hz.

2)

Beam
loading
:
Slow fluctuation in the beam (slower than feedback response such
as 100us)
requires

a
dditional rf power in case of larger beam loading. Allotted
rf
power for additional beam
current

is 1% corresponding to +1% beam current.

3)

HV
flatness
: S
i
nce the r
f output power depends on the modulator power supply, The
output power is quite
sensitive

to applied HV. In principle, the fluctuation of +/
-
0.5%
in HV results in +/
-
1.25%
in power
and 5deg.
in

phase respectively.


In order to satisfy the
tight
llrf stabil
ity requirements
(such as 0.07% in amplitude and
0.24 deg. in phase as in RDR table 3.9
-
1)
and build up a cost
-
effective rf system, llrf team
has worked to
pile up these perturbations in simulations. These simulations imply that more
feedback overhead woul
d be necessary. On the other hand, the experiments under the
ilc
-
like condition should be carried out to support these estimations.


Study (A):
o
btain the proper llrf overhead under the certain perturbations.
These
perturbations should be monitored synch
ronously with llrf measurements. For
instance, the beam current (by beam monitor),
HV applied to the klystron (by HV
monitor) a
nd rf field (by llrf

monitor
) should be monitored in the same rf pulse. The
long time operation (one shift (8 hours) or more) wil
l be proposed to obtain the long
time drift of
the operational
rf power.


Table 1. Perturbations and their measurements.

perturbation
specification
measurements
piezo compensation
microphonics
beam current
+/-1%
beam current
klystron HV
+/-0.5%
HV
klystron saturation
w&w/o linearlization
-5% or -10%
---
+/- 40Hz
rf phase


Study (B):
o
perate near the klystron saturation.
The present operation point of the
klystron is
-
5% from it
s saturation (the worst case
*
). The llrf performance should be
evaluated under the
circumstance

and compare with the case of
-
10% or more.


*

RF power budget

cavity

input 8.02 MW (33 MV/m * 1.038 m * 26 cav. * 9 mA)


a)
reflection from waveguide system 1% (VSWR~1.2 )

b)

non
-
optimal coupling 2% (if over
-
coupling x1.3)

(We should also consider the rf
-
output reduction due to the rf reflection to klystron)



c)
rf loss

8.54% (should be minimized!)



d)
beam fluctuation 1% (should be compensated by fast feedforward)



e)
modulator ripple 2.5% (pulse
-
to
-
pulse +/
-

0.5%HV ripple)



f)
cavity detuning 2% (
40 Hz
peak

of Lorentz force and microphonics)

Remained rf power:

10 MW



㠮〲0jt*(ㄮ〱0* ㄮ〲0⨠ㄮ〱⨠ㄮ〲㔠0 ㄮ〲0/(1
-
〮〸㔴)=〮Q㝍t

䱌i䘠c敥e扡bk 潶敲桥慤



㠮〲⨠(ㄮ〱0⨠ㄮ〲0⨠ㄮ〱0⨠ㄮ〲㔠0 ㄮ〲⨠u )/(1
-
〮0㠵㐩8㄰

u=ㄮ〴㤠0㔥5
(2.5% in amplitude)




2.

Exception handling (LLRF/Cavity)

Since the margin of the
rf power is limited, some exception handling should be studied,

Study (C):
operate with one cavity failure.
Present llrf operation point is not
considering the cavity failure. (If llrf had enough margin, we can continue rf operation
in case of cavity failu
res by detuning the cavity.) This study clarif
ies

the effect on the
field regulation in case of cavity failure.



3.

Proper rf distribution system and its
commissioning

(HLRF/Cavity)

Variations of loaded Q

s induce the field slope during rf operation. And du
e to the
variation of cavity quench limit,
it is desirable
that each cavity is operated near its
quench
-
limit
.
Up to now some
schemes

are proposed to select loaded Q (Q

s) and power
splitter (P

s)

to satisfy flatten the field and operating individual opera
tion field
.

1)

Individual Q

s and P

s. F
lat field in the cavity with a beam
*
. Most cost effective

in field
flatness but lose some rf power due to the cavity mismatch.

(by
Chris Adolphsen
)

2)

Common Q

s and individual P

s. Fl
at without a beam.
S
mall

gradient dis
tributions are
introduced

with beam and small rf power loss due to smaller mismatch.

(by
Julien
Branlard
)

3)

Common Q

s and individual P

s.
Similar approach
scheme 2)
, while grouping of the
cavity gradient is taken into account.
(by Shuichi Noguchi)


These are

presented at SCRF meeting in FNAL on Apr.
24.
(
http://ilcagenda.linearcollider.org/conferenceDisplay.py?confId=2650
)

T
hese t
hree schemes are involving the P’s and /or Ql’s adjustment.

Qi’s and Pk’s issues
require the experimental evaluation.



Study (D)
:

evaluate these three schemes and compare the performance and estimate
the
tenability

and operability.
S
i
nce the mismatch of Q

s introduce the additional rf
power, this is also important to evaluate the rf power margin.