PICOSECOND-RESOLUTION "SLICE" EMITTANCE MEASUREMENT OF ELECTRON-BUNCHES. Page: 2 of 5
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a 20 degrees dipole magnet and a small slice in energy is
selected by a slit. A quadrupole magnet following the slit is
scanned in current and the vertical beam size is measured
on a Beam Profile Monitor (BPM). The BPM comprises a
phosphorous screen to form a luminous image of the beam
intensity profile and a CCD camera to record it. The
distance from the quadrupole to the BPM is 8.96 m. For
clarity we omit from the figure various beam transport
quadrupole lenses in the high energy beam line.
The second linac section, dipole and slit form a filter
that passes only a short slice of the beam pulse downstream
of the slit. The quadrupole lenses and beam profile monitor
downstream of the filter form an analyzer to measure the
beam matrix of the slice. The filter is tuned to a given slice
by changing the phase of the second linac section. Since the
dipole current is constant, the energy of a selected slice is
constant regardless of its position along the bunch, and the
optics of the selected slices in the filter (and before it) are
identical. However, under regular operation of the machine
the slices enter the second linac section at slightly different
phases around the crest of the accelerating wave-form.
Thus they will experience a slightly different focusing. The
maximum relative defocusing kick along a 10 ps long
bunch positioned at the crest is of the order of half a micro-
radian. This is two orders of magnitude smaller than the
angular spread in the approximately one millimeter
diameter beam as it enters the second linac section and thus
The horizontal spot size on the slit is determined by
the beta function at that point and by the energy spread..
The measured horizontal spot size was equivalent to an
energy spread of 0.5 % FWHM with the known 5.3 mm/%
dispersion at the slit.. The slit opening was adjusted to an
opening of 2.6 mm equivalent to an energy bite of about
0.5%. Next, the second linac section was dephased by 29.8
degrees to produce a nearly linear energy chirp of 0.439 %
per picosecond. Due to this chirp, the slit opening
corresponded to 1.1 ps.
The experimental data was obtained by the BPM. The
image of the beam is captured by a CCD camera ( Pulnix
model 745E) with a frame grabber (Spiricon model LBA-
100A). The CCD camera has pixel size of 11 by 13 microns
and the number of pixels is 512 by 480. A remotely
controlled iris is used to avoid saturation in the 8 bit
dynamic range of the frame grabber. The Gd2O2S:Tb
phosphor is linear in light per charge output under our
beam energy and charge conditions. The spatial resolution
of the light emanating from the phosphor is better than 100
microns, thus our resolution is determined by a
combination of the pixel size and the phosphor resolution.
This resolution is the basis for the error estimate in the
emittance measurements. The digitized image from the
frame grabber was written onto a diskette to be analyzed
off-line. The analysis consists of integrating over the
horizontal direction and obtaining the rms beam size for the
vertical direction. Each vertical rms size corresponds to one
point in a quadrupole scan. The beam size as a function of
the quadrupole strength is used in a best-fit analysis to
extract the beam matrix just upstream of the quadrupole. In
this analysis we make the usual assumption that the
transverse phase space distribution is an ellipse.
1.2 Results of the ATF slice emittance measurement
One application of this powerful technique is a study
of emittance compensation in electron photoinjectors.. In
the emittance compensation technique, suggested by Bruce
Carlsten , the observed emittance growth due to linear
components of space charge forces in the photoinjector is
compensated by passing the electron beam through a
laminar-flow beam-waist. The space charge interaction in
this beam waist results in a differential rotation of the slice
ellipses to bring them into alignment at some point
downstream of this waist. To understand this emittance
growth and compensation, one has to look at the slice
emittance of a number of slices as they evolve. To go
beyond this correction that is characterized by a single
variable (the solenoid current), it is necessary to monitor
the slice by slice emittance of the beam and deduce a non-
linear (multi-parameter) correction.
The beam matrix provides us with the emittance of the
measured slice and the orientation of the phase-space.
ellipse. The measured normalized rms emittances of the
three slices are 3.5 1.1 , 2.8 1.1 and 2.3 1.1 for the
front, center and end of the bunch slices, respectively.
To illustrate the relative orientation between the slice
ellipses, they were rotated with a known transfer matrix so
that the phase space ellipse of the end slice is an erect
ellipse and has a fixed beam size in each of the plots.
Figure 2 shows only the relative rotation of the slice
ellipses and a reversal in this differential rotation at some
solenoid field does not imply an absolute reversal. The best
emittance compensation is achieved when all the slice
ellipses line up. This will reduce the emittance of the whole
electron bunch by producing the smallest projected area,
with summation weighted by the charge in each slice.
As the beam passes through the waist created by the
solenoid, the ellipses of the slices rotate relative to each
other since the space-charge force is not uniform along the
bunch. According to Carlsten's theory, the angle of rotation
is a function of the solenoid field and the charge
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BEN-ZVI,I.; QIU,J.X. & WANG,X. PICOSECOND-RESOLUTION "SLICE" EMITTANCE MEASUREMENT OF ELECTRON-BUNCHES., article, May 12, 1997; [Upton, New York]. (digital.library.unt.edu/ark:/67531/metadc892001/m1/2/: accessed January 16, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.