Fermilab main injector: High intensity operation and beam loss control

From 2005 through 2012, the Fermilab Main Injector provided intense beams of 120 GeV protons to produce neutrino beams and antiprotons. Hardware improvements in conjunction with improved diagnostics allowed the system to reach sustained operation at 400 kW beam power. Transmission was very high except for beam lost at or near the 8 GeV injection energy where 95% beam transmission results in about 1.5 kW of beam loss. By minimizing and localizing loss, residual radiation levels fell while beam power was doubled. Lost beam was directed to either the collimation system or to the beam abort. Critical apertures were increased while improved instrumentation allowed optimal use of available apertures. We will summarize the improvements required to achieve high intensity, the impact of various loss control tools and the status and trends in residual radiation in the Main Injector.

Figure 1

Sampled intensity per cycle from January 1999 through April 2012 with full scale of $5\times {10}^{13}$ protons per pulse (50 Tp per pulse). Vertical grid lines are at May 2, 2002, August 31, 2005, and December 30, 2008.

Figure 2

Beam loss monitor display from a Main Injector cycle in January 2009 at an intensity of $4.4\times {10}^{13}$ protons. The three decade logarithmic vertical scale in $\mathrm{rads}/\mathrm{pulse}$ ranges from 1 millirad to 1 rad. Values shown are integrated loss at each BLM in the ring at the end of the cycle in green, overlaid in yellow by loss integral after 1.5% acceleration, then overlaid in blue by the loss integral at the end of injection. In typical operation, most of the green loss results from the extraction process at the very end of the cycle. Functional areas of the ring are marked by colored braces that are identified below the display.

Figure 3

Beam loss monitor display from Main Injector cycle in September 2011 at an intensity of $4\times {10}^{13}$ protons prior to the end of pbar production. Loss monitors are shown sequentially around the Main Injector. Compare to Fig. 2 above. Some remaining BLM signals are due to pedestal offset, not beam loss.

Figure 4

Left panel shows logic of slip stacking (see Sec. 4a). Five (red) Booster batches are injected into buckets of the first rf system. The frequency is reduced to decelerate them and the next five (blue) batches are injected. When the bunches in the red and blue batches are aligned, the 1 MV rf system captures them. Then the final blue batch is injected. Right panel shows wall current monitor signal during injection for slip stacking operation using 11 Booster batches. Horizontal axis shows time for (nearly) one Main Injector revolution. Main Injector revolution time is 11134 ns at injection. Vertical axis shows bunch intensity with later turns offset vertically. Four double batches followed by one single batch for neutrino production are phased for acceleration at the top of the figure. The first injected batch for pbar production begins at the lower left, slips to the right and is joined by the second pbar injection, arriving at acceleration phase before the top of the figure where it is at the far right. The final neutrino batch arrives following the recapture to the left of the gap for pbar extraction. This panel is from Ref. [1]. With this mode, Main Injector intensities of up to $4.6\times {10}^{13}$ protons per cycle are achieved.

Figure 5

Tomograph showing momentum offset against time for a recaptured bucket of slip stacked beam as reconstructed from resistive wall monitor signal after creation of 1 MV rf capture bucket. This figure is from Ref. [37].

Figure 6

Wall current monitor plot for slip stacked beam during development of ($5+2$) slip stacking. For the upper panel, the horizontal axis shows time for (nearly) one Main Injector revolution. Vertical axis shows bunch intensity after recapture with selected turns offset vertically. The lower panel shows portions of one turn, just before extraction, expanded to show the beam captured in the gaps for the rising (left) and falling (right) edges of pbar extraction kicker pulse. This figure is from Ref. [1].

Figure 7

One of four secondary collimators that employ a thick stainless steel vacuum chamber surrounded by a steel absorber to contain the shower particles. Radiation shielding for personnel is provided by 12 cm of marble placed on the top, ends, and aisle side of the collimator. At the upstream end a polyethylene block reduces neutron flux to upstream magnet coils. At the downstream end, masks are placed to absorb small angle outscattered particles.

Figure 8

Typical beam operation for pbar plus NuMI beam production shows the beam intensity vs time in the acceleration cycle. Acceleration begins at 0.755 sec. Intensities shown are blue (sum of injected beam), green (circulating beam in Main Injector), red (loss from injection process), and magenta (total loss in ring).

Figure 9

Aperture improvement using WQB large aperture quadrupoles are shown in this end view of the vacuum pipes (old and new) and the Lambertson that is downstream.

Figure 10

Residual radiation on contact with top of beam pipe at locations between an upstream dipole and defocusing quadrupole after cool down of a few hours. Data taken on June 11, 2004.

Figure 11

Geometric distortions of beam pipes permitted beam loss at locations upstream of vertically focusing quadrupoles. The resulting residual radiation distribution on top of beam pipe near Q113 (upper panel); restriction due to beam pipe flexing under vacuum load (center panel) for various locations; and restriction near Q113 from misalignment due to asymmetric stress relieving of insertion-induced beam pipe stress. All locations experienced flexing; many experienced offset.

Figure 12

Elliptical formed bellows used throughout Main Injector to connect beam pipes. Fingers shield beam to provide smooth transition for rf image currents. Failed bellows removed on March 7, 2011. Misinstalled bellows allowed fingers to escape so vertical aperture was reduced by $\sim 5\mathrm{mm}$.

Figure 13

Main Injector residual radiation history from injection to abort region (upper) and from abort region to injection (lower) measured on contact at barcoded locations. Ratio of measured radiation (November 2011/August 2008) is shown below the measurement sets. Ratio is somewhat overstated due to less cool down time for 2008 data. Major loss points: injection (50 m), collimation (1000 m), abort (1675 m), proton extraction to Tevatron (and other transfers) (2500 m), NuMI Extraction (2780 m), antiproton extraction to Tevatron (2950 m).

Figure 14

Residual Radiation History at barcoded locations fitted to BLM loss weighted by set of isotope half lives. LAM522 fit to LI522A loss (top), Top of Q104 Downstream End fitted to LI104 loss (center) and Top of Sextupole S408 fitted to LI408 loss (bottom). Reduced loss at Q104DS followed commissioning of Gap Clearing Kickers.

Figure 15

Weekly Summary of Protons Accelerated in Main Injector for delivery to the pbar and NuMI targets. This stacked bar chart shows pbar weekly beam in black bars with colored bars indicating the NuMI weekly beam (each year a new color) so the top of the bar indicates the total weekly delivered 120 GeV beam.