Introduction Both the Main Injector and Recycler share the same abort absorber and 28 meter buried pipe between the MI enclosure and abort enclosure. The Recycler abort line enters the 24 inch buried pipe through a 4 inch flange (red) almost horizontally centered and three inches above the vertical center line, where as the MI line enters a little above the centerline (blue) and about six inches to the left of the Recycler. Both beams emerge from the 24 inch pipe thru a 6 inch flange (green) passing thru a SWIC before hitting the absorber. Figure 1 shows the relative locations of both beams at the entrance and exit of the buried pipe. This SWIC is used to monitor both beams.
Figure 1: Entrance flange (left) and exit flange (right) of the buried pipe between the MI enclosure and the abort absorber enclosure.
The Recycler abort line was designed to be used during the Recycler commissioning. Although, it shares the MI absorber, the line was not designed to transport large emittance / high intensity beam on a regular basis. To this end the aperture and loss level should be re-measured.
The design lattice in the 400 straight section and the abort line are shown in Figure 2. Here, the kicker is located downstream of Q400B with the Lambertson located just downstream of quad Q402B. The right hand plot shows the lattice functions of the abort line. For proton operations the kicker at 400 is used for injecting protons (via LAM328) and to send the beam down the abort channel (via LAM402), while in pbar operations, it is used for pbar extraction to the MI (via LAM328). When the Recycler is converted to a proton accumulator for the MI, only the abort function for the kicker will remain.
Figure 2: Recycler 40 straight section lattice functions (left) and straight section+abort line lattice functions (right). Extracted beam is transported from the Lambertson to the MI enclosure wall (blue dashed line), through a 28 meter buried pipe to the face of the absorber (red box)
The recycler abort line was used during the Recycler commissioning with protons. A search through the Recycler e-log showed that the last time any profiles from the abort SWIC were reported was in October 2000. Figure 3 shows these profiles.
Figure 3: Abort SWIC profiles for Recycler beam.
These profiles were taken with the origional electron cooling high beta straight section in RR30. This straight section was changed to the present configuration in Jan 2001. No other profile was found after this date. It should be noted that the beam passes through an 7 inch air gap and titanium windows located after the Lambertson and just before the first quad in the abort line (close to 403). The data looks to be taken with 1 Booster turn with an intensity of about 1E11. Assuming the emittance was about 10 , expected sigmas on the abort SWIC would be on the order of 4mm horizontally and 5mm vertically. Although the fitted values report values are too large, there does not appear to be any large inconsistency in the profiles. These are 1 mm spacing wires with 48 wires across. If the current profile remains this sharp, the abort line should certainly tolerate 5 mm movement on the SWIC (and dump face).
Recycler modifications for gap clearing kicker installation To keep the kick center as close to the quad as possible, we move the abort kicker (to be a new module) to the space in between the gradient magnet 400A and the quad 400B. We then install the gap clearing kickers just downstream of the quadrupole. The vertical BPM pick up, although it is not used for orbit, could be installed downstream of the gap clearing/bumper magnets. This arrangement is shown in Figure 4. The space between the gradient magnet and quad pole tips is 84.177 inches. Subtracting 5 inches for the magnet end packs, we should have 79.177 inches free to install the existing kicker. This kicker is 66.066 inches flange to flange.
Figure 4: New configuration for Recycler abort kicker and gap cleaning kickers for the Nova project.
Kicker Strengths Since the Nova project will have a new kicker configuration, the required strengths of the kickers need to be determined. The abort kicker will be a full turn abort kicker to be fired once per injection cycle. The existing physical kicker magnet will be removed and a newly constructed module will be installed. On the other hand, the gap clearing kickers will operate at 15 Hz and have a specified rise and fall time of < 60 ns. The bumper kickers are low field kickers which will be used to flatten the gap clearing kicker flattop. These kickers will be timed to fire in the injection gap ½ turn before beam from Booster is injected. This will remove any beam from the injection gap during the slip stacking process prior to the next injection. With an expected MI intensity of around 5E13 per pulse from 12 injections, a few percent loss translates into a few E12 extracted to the abort each MI cycle.
The current abort lattice file was modified to include the gap clearing (i.e. “new”) kickers and new location of the abort kicker configuration shown in figure 4. Since the maximum integrated field of the “new” kickers is 75 G-m, two configurations with either four or five kickers was investigated. The maximum integrated strength of the abort kicker is 370 G-m. The separation of the extracted beam from the circulating beam was measured at each end of the Lambertson. As was the procedure for the injection Lmabertson, an attempt was made to generate enough separation that 10 of a 25 beam would fit in both the circulating and extracted regions of the Lambertson. Figure 5 shows the orbit for circulating beam around the Lambertson septa (left) and beam extraction trajectory thru the Lambertsons by firing the abort kicker (right). Here, positive x is to the inside. This beam separation is 55 mm and is represented by the last column in table 1. Table 1 summerizes the closed orbit and extracted position (and separation) trials and the required kicker strengths. Table 1 is broken into four sections. The first section contains the fitting parameter for the closed orbit and extraction orbit, the next section contains the results for the abort, and the last two sections contain results for 4 and 5 gap clearing kicker modules, respectively. All solutions for the abort kicker integral strength are below the maximum value.
Figure 5: Closed orbit bump around the 402 Lambertson (left) and extraction orbit from the abort kicker thru the Lambertsons (right). The dashed line represents the vertical kick from the Lambertsons.
For the gap clearing kickers, it can be seen that the 4 module solution reaches the maximum integrated strength with only a ~47mm displacement and provides no room for increasing the kicker strength above the maximum. However, for the 5 module case,
we see that even for a 55mm displacement, the kicker is running at only 95% maximum which means we could generate up to a 58mm separation, if required.
Looking at the difference of the position and angle at the up and downstream end of the Lambertson for the abort and gap clearing kickers, we see only a 75 microradian difference in angle and 0.3 mm difference in trajectory at the downstream end of the Lambertson. Figure 6 shows the trajectory through the Lambertsons due to a 71 G-m integrated field in the gap clearing kickers (i.e. the same 55 mm displacement at the Lambertson entrance.
Figure 6: Orbit through the Lambertson due to a integrated field of 71 G-m in the gap clearing kickers.
Kicker error The displacement of the beam centroid at the Lambertson due to an error in the kicker strength is simply given by
If we assume a 5% error in kicker strength, or flattop regulation, then the expected position movement at the Lambertson entrance is
Simulating this error in MAD produced an offset of 2.75 mm and an angle error of about 120 microradian. Figure 7 shows how this error is transported through the abort line to the dump.
Figure 7: Trajectory through the abort line due to a 5% error in the abort kicker strength.
From figure 7 it can be seen that the phase advance between the Lambertson and SWIC is approximately 180 degrees as a positive 2.75 mm turns into a negative 2.7 mm offset at the dump SWIC. Based upon the profile in figure 3, this 5% error in kicker strength would move the centroid of the distribution < 1 sigma on the SWIC, well within the aperture. The displacement in the beam pipe at the end of the MI enclosure (flange of the 24inch pipe) is under 1.2 mm.
Abort line aperture The Recycler abort line is made up of 5 quadrupoles and 3 Recycler arc gradient magnets (RGD) and 1 permanent magnet dipole. The first dipole after the Lambertson is a horizontal bend and the last three dipoles bend vertically. Each of the dipoles are rolled to for matching geometry to the MI abort line. The maximum roll is under 8 degrees. The beam pipe is standard Recycler beam pipe with the approprate orientation in the dipoles (i.e. oriented vertically through the vertical dipoles). Figure 8 shows the 10 sigma envelope for a 25 beam from the Lambertson to the dump. Figure 9 shows the 6 sigma beam envelope. Its clear that there are two locations where beam out to 10 sigma would be problematic. On the other hand beam out to 6 sigma, is not a problem.
Figure 8: Abort line beam envelope (10 of 25) and beam pipe apertures. Horizontal(solid red) and vertical(solid blue) apertures of the standard Recycler beam pipe and the dashed apertures are either 4” round or 24” round buried pipe, as indicated.
Figure 9: Abort line aperture (6 of 25) with the same apertures shown in figure 8.
Abort Lambertson Aperture The Recycler uses a permanent magnet Lambertson located just downstream of Q402B. The field free aperture is that of a 3 inch beam pipe and the field region has a 1.75 inch inner dimension square beam pipe. The square beam pipe wall thickness is 0.135 inches and the round beam pipe thickness is 0.060 inches. The septum is 0.060 inches thick.
Figure 10 shows the lambertson aperture superimposed with the Recycler beam pipe on both up and downstream ends. For Recycler commissioning with protons, only a few Booster turns was used which typically produces 10 beam. Figure 10 also shows beam cross section for a 6 and 10 for a 10 beam. It is clear that the six sigma beam fit nicely through both the Lambertson and beam pipe aperture. Even the 10 fit as well.
Figure 10: Magnet and beam cross section for 10 beam
The orientation of the Lambertson in the figures is based upon the current installation. Figure 11 superimposes 6 and 10 sigma beam ellipse for 25 . The orientation of the Lambertson remains unchanged. From figure 11 its clear that if beam exists out at the 10 sigma amplitude it will get lost on the upstream Recycler beam pipe and/or upstream end of the Recycler Lambertson as it is kicked into the abort channel. Circulating beam is tight at 10 sigma. It is clear, thought, that the 6 sigma beam fits through both channels. Figure 12 shows the 6 and 10 sigma beam in the same aperture but with 20 beam.
Figure 13 compares the apertures of the permanent magnet Lambertson and the modified injection Lambertson (MLAW). Replacement of the existing permanent magnet Lambertson with a powered MLAW Lambertson and replacement of the Recycler beam pipe between 401 and 402 with a MI style beam pipe would open up this aperture to accommodate the 10 sigma of a 25 beam. This last modification is not in the current plan.
Figure 11: Lambertson and Recycler beam pipe aperture with the 6 and 10 sigma beam cross sections of a 25 beam.
Figure 12: Lambertson and Recycler beam pipe aperture with the 6 and 10 sigma beam cross sections of a 20 beam.
Figure 13: Comparison between the permanent magnet Lambertsom and the modified 8 GeV injection Lambertson (MLA)
Summary The RR400 region has been modified to accommodate the new “gap clearing” kickers. The Lambertson and abort line remain unchanged. A new abort kicker module (with the same flange-flange dimensions) was moved upstream between the gradient magnet (G401A) and quad (Q400B). The new gap clearing kickers are installed just downstream of the quad Q400B. A kicker strength for both the abort and gap clearing kickers was determined. The nominal abort kicker angle is 1.086 mr which corresponds to an integrated gradient of 322 G-m. The nominal gap clearing kicker angle is 1.198 mr which corresponds to an integrated gradient of 355 G-m. or 71 G-m per kicker. The differnece in angle at the upstream of the Lambertson due to the two kickers is ~ 100 ur which produces a 0.3 mm shift at the downstream end of the Lambertson. A error in kicker strength of 5% produces a 2.7 mm displacement at the entrance to the Lambertson and at the face of the dump. The aperture through the Lambertson and beamline were investigated. A 10 sigma envelope of a 25 beam will encounter a couple of tight spots through the Lambertson and beamline, however most apertures are quite ample. A 6 sigma envelope is quite comfortable through the entire line.