If y ~ 0.3 y C 88.4% for 70 nm @ 174 degC 96.2% for 150 nm@30
deg modeC97.7% for 500 nm@7 deg mode
phase jitter observed from fringe scan: about 200 mrad ?? C 98 %
(????) Phase (relative position) jitter13/05/29ECFA LC 2013 *
ECFA LC 2013
** In order to achieve the high luminosity needed atILC
necessary for the anticipated detection and detailed research of
new physics the vertical electron beamsize must be focused down to
5.7 nm.
At ATF2, the FFS test facility prototype for ILC, Goal 1 is to
focus the vertical beam size down to 37 nmFor this purpose, the
Shintake monitor, or called IPBSM, is installed at the virtual IP
of ATF2 and plays an indispensible role in small beam size tuning
there.
Here is the outline of my talk:I will start with general
introduction of IPBSMs measurement methods, expectations, and major
rolesThen report on recent beam time results, and hardware upgrades
, error studies, then a summary and some future
goals----------------------------------------------------------------------------------------------------------------------*
Here is the measurement method of IPBSMOwing to the scheme of
using laser interference fringes as a target for the e beam, IPBSM
is the only existing device capable of measuring beamsizes below
100 nm
The laser transported to IP area is first split into upper and
lower paths by a beam splitter we call the half mirror , than while
controlling their relative phase with a delay line, they are
crossed at IP to form laser interference fringes.
The e beam, also focused to waist at IP, is collided
perpendicularly against these fringes, and the resulting Comp
scattered photons enter a downstream gamma detectorMeanwhile the
beam, having finished its role , is safely disposed into a
dump.
*What is actually measured by the gamma detector is the
modulation depth(in these slides written as M) in the Comp
signal.
Modulation is produced by scanning relative pos between laser
phase and beam position
If the beam is thin , or well focused , compared to the fringe
pitches, M will be large, While M is small for a large beam
size
It can also be explained as that the no. of Comp scattered
photons N is large when e beam interact with fringe
mountain/valley, and small in between. This changes more
significantly for a small beam .
*What determines the measurable beam size is the fringe pitch
(or d here), which is in turn depend on the crossing angle of the
two paths.
By switching between few crossing angle modes, IPBSM is by
design capable of measuring with better than 10% resolution a wide
range of beam sizes from 20 nm to as large as few micron.
This plot shows the relationship between beam size and M. As you
can see here, for larger beam sizes, we use small angle modes such
as 2 - 8 deg, and for smaller beam sizes 30 and 174 deg modes.
In order to always maintain better than 10 % resolution, we must
select the appropriate crossing angle modes in accordance with beam
focusing status. Simulation Condition90 bunches, statistical error
10% y = 0.3 yFringe phase jitter400 mradLaser pulse energy fluc.
6.8%
*This is the laser path of each crossing angle mode.Mode
switching is effectively carried out by individual sets of linear
stages on remotely controlled movers, teamed up with mirror
actuators.
This is 174 deg, then 30 deg mode path,The 28 path is special. A
prism is slid back and forth to enable continuous adjustment
between 2-8 deg , to allow larger freedom in beam tuning for a
larger beam size.*Now, here are some of the major steps in using
IPBSM during beam tuning at AF2.
Highly precise position alignment of laser to e beam in
transverse plane and in longitudinal (or beam) direction.
In the transv direction, we perform what is called a laserwire
scan, in which one laser path at a time is scanned vs the beam in
the transv direction , and laser pos is set to the peak of the
resulting Gaussian Comp signal. The sigma here is supposed to
correspond to laser spot size (in sigma) is about 15 20 micron.
In the long direction, we perform the z scan, for which we
actually conduct fringe scans along the way, and setl ong laser pos
to where it yields the largest M.
After all these preparations, IPBSM is ready to continuously and
stably measure beam sizes using interference scans, and feeds back
the result back to tune the e beam size down, for example in the
multi-kobs tuning process shown here.
==================================================================**Here
is the report on beamtime in 2012By Feb 2012, we have fully
commissioned the 30 deg mode, and at the time we were stably
measuring M of about 0.55, corresponding to about 150 nm.
A major upgrade of the laser optics had been carried out during
the summer shutdown period of 2012, which aimed at suppressing syst
error sources and more reliable laser path construction
During the following winter run, this was shown to have
contributed significantly to performance improvement of IPBSM , and
the remarkably stable IPBSM operation which supported various
aspects of ATF2 beam tuning.
High M was measured at 30 deg mode,And on the last 2 days of Dec
run, we have finally achieved M detection at 174 deg mode, and
measured many times M of 0.15 0.25, corresponding to beam sizes of
70 82 nm. (assuming no M degradation due to IPBSM related
factors)
In this way, the Dec run was a large step forward in achieving
ATF2 goal 1.
These data as well as data taken recently are ongoing undergoing
detailed analysis *During beam time in 2013, IPBSM continued to
contribute stably to beam tuning and studies, which includes
measuring M at 174 deg modes over a few days at a time to enable
application of various linear / nonlinear tuning knobs, aimed at
further focuing down of beam size.
This plot here shows one of the large M measured at 174 deg mode
during this period
Occasionally, after several rounds of tuning knobs, we take
these consistency scans to test the effect of tuning knobs and of
reliability & stability of IPBSM measurements.
The best record for these consecutive scans is about M = 0.3, as
in this example here , where 10 scans have been conducted within
about 40
minutes.----------------------------------------------------------------------------------------In
March operation:We could not apply Y24, Y46 ( normal sext. :
current limit (>0 88A) . of SF5FF PS) We could not apply Y26
(skew sext current limit of SK2FF PS (use QK knobs instead).Plan to
use four 20A PSsfor SK1FFSK4FF, and change SF5FF PS fromHAPS to 10A
PS
*Besides beam focusing, IPBSM was also used for dedicated study
of the beam or errors in IPBSM itself.
I will show some examples here.
For example, for study of wakefield effects, we observe the
dependency of M , or IP beam size on beam intensity. Another
example is this scan of the pos of a ref cavity BPM in a high beta
region is scanned and the response in M is observed.
This plot here shows the dependence of BG levels with beam
intensityHere we can quantatively observe steepness of the response
to intensity by comparing the coefficinet of the linear fits
between different periods of beam tuning. *The optimistic results
reported so far owes largely to the major laser optics upgrade of
2012 summer, which aimed at suppressing syst error sources and more
reliable and precise laser paths
This table lists some main points.For ex, CW alignment laser was
put to use in realigning a large number of optical components on
the vertical table. Then the laser beam itself was made to pass
with high precision along guide of alignment irises and strictly
redefined reference lines.
It is essential to maintain laser path precision when switching
modes.A new IP target was installed which allowed us to check 30
deg and 174 deg mode path within the same screen at IPIn place of
the rotating stages shared between different modes that used to be
used for mode switching, we replaced them with more reliable small
linear stages, an independent set for each mode, teamed up with
actuator attached mirrors.
Also, focal lenses for all modes were enabled with lens position
scanning, to ensure precision in aligning laser focal point to
IP
Furthermore, tuning of the main laser itself is provides careful
regular tuning and maintenance by Spectra Physics company for
stabilizing oscillation, and improving laser profile and spatial
coherence.
**Next I will talk about stability of IPBSM operation based on
fluctuation sources Such as Compton signal jitter, phase stability,
and timing and power stability status*
It can be demonstrated that during 174 deg mode operation this
year, neither the measured M nor other fluctuation factors such as
laser timing or power were significantly drifted when we prolonged
the scan range in this way,
In other words, long term stable performance is maintained under
various scan conditions, whether it is many consecutive scans or
single long scans, or fine step scans.
This table compares comp signal jitter at peak of fringe scans
for a certain set of 2 consecutive long 60 rad scans take on 3/14,
compared to regular range scans immediately before an dafter to
prove this point, as well as 2 otehr long range scans from other
days in 2013.
*In between small beamsize tuning, we gather data that enables a
grasp of IPBSM operation stability, and how it affects fringe scan
measurements.
These are the 2 consecutive long range scans, during which
conditions are proven to remain stable.
These higher statistics data are used for analyzing signal
jitters or phase drift and jitters.
Phase stability, which can also be interpretted as relative
position stability, is a factor that easily changes condition over
time, and are analyzed using consistency scans, as shown here. The
initial phase of consecutive fringe scans are plotted against times
passed.The largest drift observed for this method is about 70 mrad
/ min. Since the typical scan for beam tuning is about 20 rad ,
taking about 80 sec, this is not a major issue.On the other hand
there are stable periods with almost no drift al all.
*This table shows the main statistical fluctuation sources , and
how much each factor is estimated to contribute to overall signal
jitters
These consist of
The effect from relative position jitter is still uncertain, and
different methods are currently being tried out to investigate
this.
Meanwhile, signal jitter is analyzed separately from individual
fringe scans themselves, for example at the peaks, mid points, or
bottoms of fringe scans
The sig jitter at fringe scan peaks are considerably consistent
at about 20 25%. *A difficult error source to assess is relative
pos jitter., or equivalently phase jitters. It is hard to separate
rel pos jitter from other jitter sources, and it is not accurate to
push all of the seemingly horizontal fluc on fringe scan plot to
phase jitters.
Vertical rel pos jitter (this y here) could potentially be a
dominant M reduction factor, but we are not sure, so this is
currently undergoing investigation.In general, by design, beam
position jitter in ATF2 beamline is below 0.3 x beamsize, but since
we are still awaiting full commissioning of IPBPMs, there hasnt
been a way to confirm this confidently.
On the other hand, horizontal pos jitter affect fringe scans by
causing signal jitters. This has been investigated using high
statistics laserwire scan (such as this one here, where 99 points
are taken at each location) , and the standard deviation errors at
each place is plotted as a function of the scanned positions and
fitted with a function that takes into account the various dominant
fluctuation sources, as well as relative position jitter.
This rel pos jitter is a convolution between jitter of the e
beam and of laser pointing stability. This of course also vary from
diff data time to time, but assuming about 2.5 4.5 micron of H rel
position jitter, it is estimated to contribute about 4-6% to signal
jitter .
Investigation of this issue is taken seriously and plans are to
consider alternative methods, as well as acquire data of higher
statistics to improve precision in analysis.
*In order to resolve signal jitters, and achievemore stable
scans for beam tuning and a more precisely fitted Modulation, we
need to (1) imrpve IPBSM hardware. Another things is to try to
apply offline data selection after a thorough investigation of the
fluctuation sources.
Some studies have been carried out where signal jitter
dependence on sugnal energy is observed, such as in this model
here, where there a statistics dominant terms proportional to sqrt
of E, linear on E, which relates to laser conditions, and const
terms.
This plot shows laser timing gathered into a histogram for one
single fringe scanWe can apply timing cut, such as vetoing data
outside 1 sigma .
Also, recently IPBSM related data have been synchronized with
info from all ATF2 beampline BPMs all within the same fringe scan
data, These can be checked for correlation of orbit fluctuations to
signal jitters in the IPBSM. For example in a vertical IP phase BPM
here, and some in high beta locations. **Next I would like to
explain about a study of the various systematic errors affecting
IPBSM measurements , which are interpretted using what is called M
reduction factors.The c1, c2, so on. Here represent each individual
type of bias factors, and these work to smear the fringe contrast
and consequently lead to under-evaluation of M, or over-evaluation
of beamsize.
I have classified 2 ways to evaluate M reduction
Method 1 I call direct method, where total M reduction is
estimated using data where mode is switched under the same beam
condition within a short time. M is measured at each mode, and
consistency is compared to results from higher crossing angle
modes. For example, a beamsize that is large enough to be able to
measured, but small enough to give a very large M near the
resolution limit at a low crossing angle , and we can observe the
limit as to how high M can be measured.
Another is what I call the indirect method, where for each
individual bias factor, the corresponding contrast reduction factor
is evaluated to best of what is known, .Although this is a more
generalized than the 1st part, for some factors we do not yet have
a clear way to assess quantatively, and from some we can only get
the worst limit.So an overall M reduction may be difficult. *There
are two main points to how to assess M reduction factors:The 1st
priority is to suppress error sources through improvements in
hardware and alignment methods.The 2nd priority is to evaluate any
residual errors to the best precision possible, in order to be able
to derive the so called true beam size from the measured value.
So the next question here is how to assess the so called unknown
or uncertain errors:Some of these can be proven to not depend on
crossing angle mode, or not much affected by beam condition, in
other words not much change over time.For uncertain individual
factors: (e.g. Relative position jitter, spatial coherence, ect )Do
we really have large bias ? how much ? how to find out ??
For example, we are not sure yet of the quantative effect that
spatial laser coherence has on fringe contrast.
This goes back to my explanation of the direct method for
evaluating total M reduction factor.For ex we can try to measure a
beamsize that gives a very high theoretic value for M near
resolution limit at a low crossing angle mode, like about 0.98 or
so, Then lets say we get about 0.97 , not one time, but a average
after a few almost stable scans, then that is the very worst
contrast reduction factor overall we can get.In this case no
individual factor can be worse than 0.97.In fact, as there are many
small error sources, probably no big individual errors at all.
But then again some other error sources are not so easy to
generalize since they change over time, and depend on angle mode.
*
This table shows individual systematic errors affecting IPBSM
measurements based on data taken at 174 deg mode in 2013
These are derived using beam time data , or from measurements
conducted during beam off time specifically for the purpose of
error studies
Some factors we particularly put effort into suppressing are
those related to polarization and fringe tilt , which if left
unattended to, would become a dominant M reduction factor. (this I
will explain in detail in a moment)
Others include
Some factors , such as spatial coherence and phase jitter, are
still difficult to evaluate quantatively
Certain factors which have nearly no dependence on crossing
angle mode, or does not vary much , while others need to be
confirmed for individual scans.*The IPBSM laser optics is designed
for linearly S polarized laser.A fine and precise measurement of
the actual polarization of the IPBSM laser was carried out just
before the continuous run of May 2013.
Here is the setup: Just after injection onto the vertical table
at IP, we use a half lambda plate to rotate the polarization state
of the laser, in other words, its angle is scanned while a high
intensity beamsplitter is set up to reflect S laser component
upward into a power meter to be measured, while letting P component
pass.The results show the IPBSM laser to be very close to purely S
linearly polarized state, with P contamination of less than 1.5
%.
There should be very little M reduction from polarization once
we set the half lambda plate to the optimum setting.
However to evaluate any residual error, if there is any, we need
to investigate what happens after the laser pass through numerous
optical components all the way to IP. The reflective properties of
the esp important half mirror has been measured, and resulted to be
well within catalog specifications. Furthermore we need to measure
the polarization separately for U and L paths,, just immediately in
front of the lenses, the setup for this has been prepared, and will
be conducted in June. *In this way, we have determined the setting
that yield this pure S polarized state
We confirmed these particular angles, referred to as S peaks
here, to be also the one that maximizes M, as confirmed using the
beam during beamtime, where M is measured while scanning half
lambda plate angle.
If laser power is imbalanced between U and L paths, there will
also be some M reduction . By measuring power as a function of half
lambda plate angle immediately in front of the lenses near IP, it
was found that the setting for S state also were the ones that
realizes the best power balance. *Fringe tilt correspond to the
mismatch in axis of laser interference fringes and beam axis.It
takes the form of fringe pitch (in longitudinal direction) , or
roll (in transv plane)
This can be observed from the relative offset wrt lens center
for U and L paths. However, the observation precision is limited to
about 0.5 mm, and the fact that the e beam itself may be rotated in
the transv plane, in which case it is difficult to say to what
reference we are defining fringe tilt.Some other issues are that
laser drift may well have occurred from time of pre-beamtime path
making and when we actually conduct beamsize measurements with
IPBSM.
So, we put into practice what we call the tilt scan .Here,
fringe is intentionally tilted by adjusting these mirrors in the
174 deg mode path and responding change in M is observed. Mirrors
are set to where M is maximized.From this we can estimate tilt
(either pitch or roll) with regard to e beam itself.
*Shintake monitor (or IPBSM), a beamsize monitor using laser
interference as target for the beam, is the only existing device
capable of measuring y < 100 nm And plays an Indispensible role
in achieving ATF2 goals and realizing ILC
It has been contributing with remarkably stable operation to
ATF2 continuous beam size tuning.Its performance have been improved
by laser optics reforms and laser tuning. Consistent measurement of
M > 0.3 174 mode) have been achievedcorresponding to y ~ 65 nm
(assuming no M reduction)Other new records during recent beam run
include application of various linear / non-linear multi-
knobs.
Dedicated studies have been done relating to e beam and IPBSM
errorsOffline detailed analysis is still ongoing. As ATF2 is moving
still closer to achieving the 37 nm goal, It becomes all the more
imporatnt to maintain and improve performance in terms of stability
and precision.Furthermore residual syst errors must be assessed to
be able to derive the true beam size. **A difficult fluc source to
assess is relative pos jitter., or equivalently phase jitters. It
is hard to separate rel pos jitter from other jitter sources, and
it is not accurate to push all of the seemingly horizontal fluc on
fringe scan plot to phase jitters.
We are not sure whether this is a dominant contribution to
sugnal jitters, as well as a M reduction factor.
It is generally said that by design, beam position jitter in
ATF2 beamline is about 0.3 x beamsize, but since we are still
awaiting full commissioning of IPBPMs, teher hasnt been a way to
confirm this confidently.
Recently a method is being tried out that compares the
difference in sig jitter between peaks, mid points, and bottoms of
a fringe scan.
Investigation of this issue is taken seriously and plans are to
consider alternative methods, as well as acquire data of higher
statistics to improve precision in analysis.
*What determines the measurable beam size is the fringe pitch
(or d here), which is in turn depend on the crossing angle of the
two paths.By switching between few sets of angle modes, IPBSM is by
design capable of measuring with better than 10% resolution a wide
range of beam sizes from 25 nm to as large as few micron.
This plot shows the relationship between beam size and M. As you
can see here, for larger beam sizes, we use small angle modes such
as 2 - 8 deg, and for smaller beam sizes 30 and 174 deg modes.
This right hand plot sjows beam size vs measurement resolution.
In order to always maintain better than 10 % resolution, we must
select the appropriate crossing angle modes in accordance with beam
focusing status. Simulation Condition90 bunches, statistical error
10% y = 0.3 yFringe phase jitter400 mradLaser pulse energy fluc.
6.8%
*
Optical delay line
8mm1buncht*
Convolution
Cos exp
Y_yconstrast M
*CsI(Tl)PMT,5cm 10 cm33 cm17.7 radiation length
*Optical Delay line10cm
*
()
Id like to show you the difference between the conventional beam
size monitor and shintake monitor.wire scanner or laser wire,
normally used in accelerator study, scans the wire like this,and
gamma peak and the size can be detected.Shintake monitor scans the
laser interferrence as the ``slit'' of the strength of magnetic
field.,This scheme enables to measure smaller beam than the
wavelength of laser.
Using laser interferrence as the ``slit'' of photons enables it
to measure such a small beam size. The conventional beam size
monitors, like laser wire or wire scanner cannot measure such small
beam size.
*misalignmentrlower
misalignemnt
laser **misalignmentrlower
misalignemnt
laser *300 nm
Mard IPBPMcommissioning
-------------------------------------------------------------------LorentzP
S (e- rest frame
*
