Optimization of electron cooling in the Recycler Page: 2 of 3
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upstream of the cooling section (labeled SPB01 and
SPB02 in Fig. ib). This tuning proved to be sufficient for
demonstrating the electron cooling and successfully
applying it for a dramatic improvement in cooling of
antiprotons in the Recycler [1]. On the other hand, over
the years indications were mounting that the angles of off-
axis electrons are significantly higher than the angles on
axis. The latter, estimated from drag rate measurements
[10] to be -0.1 mrad [11], was fed into BETACOOL [12]
simulations of cooling rates, when the sizes of both beams
are comparable. The simulated rates significantly
exceeded the measured ones, and the discrepancy could
be mitigated only by introducing into the model a large
radial gradient of angles, -0.3 mrad/mm [11]. To check
whether the angles are created by focusing errors, a
dedicated set of beam imaging measurements was
performed with a removable scintillator, YAG crystal [13]
(Fig. 1b), which had been installed into the Recycler
vacuum chamber downstream the 180-degree bend in
2004. When the bend is turned off, a cross section of a
pulsed electron beam coming out of CS can be observed
on the crystal. An unexpected complication was that
radiation from the Main Injector synchrotron, mounted in
the same tunnel, was damaging the camera in a matter of
an hour. As a result, imaging of the electron beam in this
location had to be performed in a dedicated run with the
henm in the Mnin Tnisetnr inhihitpdFigure 2. Images of the beam at the YAG with zero
(1 and 2) and adjusted (3 and 4) quadrupole currents.
Images were recorded at zero current of the lens SPQ01
(1 and 3) and at 10.4 A (2 and 4 ). All other settings are
the same for all images. Beam current was I = 0.1 A, the
pulse duration was 2 s, and the camera was gated for 100
ns at the peak of the pulse.
Recorded images showed a significant deviation of
the beam shape from axially symmetrical (Fig.2). The
main component, an elliptical distortion, was corrected by
adjusting of 6 quadrupoles upstream the CS. For this
correction, a special procedure that used the image
ellipticity as a parameter to minimize, was developed [14]
and applied in several steps. At each step, the changes in
the ellipticity resulting from a small change of the current
in each quadrupole were recorded at two currents of the
lens between CS and YAG (labeled SPQQO1 in Fig. ib),and the quadrupole currents decreasing the ellipticity were
calculated from the resulting matrix. The process
converged, and after several steps the beam did not have a
clear ellipticity at any focusing of SPQ01 (Fig.2).
With the resulting settings, the beam size at YAG
was measured at various currents of SPQ01. From this
data, the beam envelope in CS was reconstructed, found
quite far from cylindrical, and corrected by a proper
adjustment of the lenses SPB01 and SPB02.
The adjustments described above were supposed to
decrease the angles, and, correspondingly, increase
dramatically the cooling force. However, the
measurements followed the procedure showed even a
slightly worse force. We interpret this fact as an
indication that accumulation of secondary ions in the
beam line significantly affects the beam envelope. Note
that plates of all BPMs in the cooler are used as ion
cleaners, but there is no ion cleaning inside the solenoidal
doublets.X
0 YO
60
o 50
g40
i 30
> 20
a 10
0.
00.5 1 1.5 2 2.5 3
Beam offset [mm]Figure 3. Drag rate as a function of the offsets
between antiproton and electron beams. The set labeled
Y0 represents data recorded for vertical offsets before
applying the quadrupole correction. The sets X and Y
show the rates after the correction for offsets in horizontal
and vertical directions, correspondingly. I = 0.1 A.
While somewhat disappointing, the results clearly
indicated that quadrupole perturbations can be the source
of large off-axis angles. With that in mind, several sets of
measurements were performed where the drag rates were
recorded at various settings of the 6 quadrupoles. Starting
with all quadrupoles at zero, their currents were changed
one by one to the point where the drag rate was maximal,
and then the similar scans were made for the two lenses
SPB01 and SPB02 to optimize the axially symmetrical
perturbation. This procedure was repeated for several
times until no further growth of the drag rate was found.
The resulting improvement at various offsets between
centers of the electron and antiproton beams is shown in
Fig.3. Note that part of the increased rates might come
from a lower transverse size of "pencil" antiproton beam,
which is poorly controlled during the measurements.
Cooling rate, the most relevant parameter for
operation, increased considerably as well (Fig.4). At
present, the cooler always operates with the electron beam
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Shemyakin, A.; Burov, A.; Carlson, K.; Prost, L. R.; Sutherland, M. & Warner, A. Optimization of electron cooling in the Recycler, article, April 1, 2009; Batavia, Illinois. (https://digital.library.unt.edu/ark:/67531/metadc933278/m1/2/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.