Collimation of Halo from Helically Separated Beams D. A. Fmley and S. M. Pruss

Fermi National Accelerator Laboratory*

Talk presented at the APS Meeting April 18,1990 Washington D.C.

Historical Context

The 1st and 2nd Tevatron Collider runs had one low beta insert at the accelerator straight section called BO with CDF as the single high

priority experiment. During the 2nd run, the backgrounds at CDF could be greatly reduced by making subtle adjustments to the tune

and coupling. However, the backgrounds at the other (lower priority) experiments could become excessive for the first several

hours of each store.

In order to reduce the background rates at CDF further, a single beam scraper was employed [l]. This scraper removed large vertical

amplitude particles and was used at the beginning of each store. Most of these particles were protons since the proton transverse emittance was deliberately increased after injection in order to

reduce the beam beam force [2]. When this force was not controlled, the initial luminosity lifetime was significantly smaller and the useful integrated luminosity dropped [3]. During the scraping

process the backgrounds at CDF were large enough that they could not keep thdr tracking chambers on and they waited until the

scraping was completed to begin taking data.

[1] S. M. Pruss, C. Crawford, D. Finley and M. Harrison, Operational Experience with Using Collimators to Remove Halo in the Tevatron Collider. Proceedings 1989 IEEE Particle Accelerator Conference, Chicago, IL p 439 ff.

[2] L. Evans, The Beam-Beam Interaction, Proceedings of the 1983 CERN Accelerator School, p319 ff.

[3] G. Dugan, Tcyatron Status. 1989 IEEE ibid., p 426 ff.

•Operated by Universities Research Association, Inc. Under contract with the U. S. Department of Energy

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Historical Context continued

The scraping process was done in a short period at the beginning of a store. The intent is to cause most of those particles which are

destined to cause backgrounds to do so in a short time period, rather than over the full duration of the store.

Future Context

The 3rd collider run is scheduled to begin in 1991. A second large detector will be installed in the accelerator straight section called DO and the CDF detector will be reinstalled at BO. The old BO low beta

insert will be removed and replaced with a matched insert. A second matched insert of identical magnetic design will be installed at DO. Each of these inserts is designed to provided a 13 * of 50 cm, with a

possibility of going as low as 25 cm. Both DO and CDF are considered as high priority experiments. It will not be acceptable to reduce the

backgrounds at only one of these experiments.

Electrostatic beam separators will be used in the 3rd run. These will give the protons and antiprotons different closed orbits. They are to be arranged in four sets of 3-bumps so that the beam

bunches collide head on at BO and DO, but pass by one other on separated helical orbits elsewhere [4]. This is being done in order to control the beam beam force which was the main Tevatron limit to the integrated luminosity in the 2nd run. Naively, this separation scheme will reduce the force by a factor of six since there will only

be two collisions per tum instead of twelve with six particle bunches in each beam.

The success of these separators is one of the key elements in the Fermilab collider upgrade. Since it will not be necessary to dilute the

proton brightness, it is expected that the initial luminosity will be larger by about a factor of two from increased proton brightness

alone.

[4] B2 6 Helical Separated Orbits in the Tevatron. G. P. Goderre, this conference.

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TheCDFSVX

For the 3rd collider nm, CDF is going to install a silicon vertex detector (SVX). This detector mounts directly on the 1.5 inch

diameter beam pipe at BO and it is necessary to have its electronic readout as close as possible to the detector. The detector and its

electronics cost about 2M$. One of its functions is to provide for the reconstruction of V's from B-physics decays. The SVX electronics are

expected to degrade by a factor of two just from the radiation produced by an integrated luminosity of about 40 / pb.

The 2nd collider run delivered nearly 10 / pb. The integrated luminosity from the 3rd collider run is expected to exceed 40 / pb. Thus, it is highly desirable to keep other sources of radiation on the

SVX as low as is reasonably possible. One of these sources is the radiation resulting from the act of scraping the beams at the

beginning of each store.

It is not intended to use this particular SVX after the 3rd run. Instead, a radiation hardened version is intended to be used.

Nevertheless, the SVX for the 3rd run will require unusual measures to protect it.

Conceptual Desi&n of Scrapers

Tevatron scrapers are L-shaped blocks of steel which can be moved within the beam pipe. They can be between 2 and 6 feet long

depending on the application. They are remotely adjustable and can provide vertical or horizontal aperture limits. When a scraper is

moved into the aperture, it intercepts particles with large betatron amplitudes. If these particles strike the face of the scraper

sufficiently far from the edge of the scraper they will be lost from the beam [5]. If they strike the edge of the scraper and scatter

sufficiently deeply into the scraper they will also be lost. However, if they strike the edge of the scraper and scatter away from it, they are

not immediately lost from the beam.

151 A. Van Ginneken, Elastic scattering in thick targets and edge scattering. Phys. Rev. D 37, 3292 (1988).

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Conceptual Desien of Scrawrs continued •

If these edge scattered particles from the scraper are allowed to cause radiation on the CDP SVX, then this is not considered to be a

good design. Instead, a second scraper can be employed to intercept these edge scattered particles. In principle, one could consider edge scatters from the second scraper, but this is not under consideration

at this time.

Secondar_y Scraper at 90 Deerees Phase Advance

Figure 1 shows the situation for an exaggerated view of what happens when a pair of scrapers are put 90 degrees apart. The

upper left sketch shows the undisturbed phase space. The upper right sketch shows the introduction of a scraper. The amount of

beam intercepted is exaggerated to be visible. The lower left sketch shows a tail of particles scattered by the edge of the scraper. The

lower right sketch shows that this tail can be interecepted by a second scraper placed 90 degrees from the first scraper.

Secondary Scraper at 165 Peerees Phase Advance

However, 90 degrees is not the ideal phase advance for the location of the second scraper. Figure 2 shows the situation for a second

scraper placed at 165 degrees. The number of particles which can be intercepted is larger as can be seen from the lower right sketch.

Scrapers in Circular Accelerators

Figure 3 shows a more appropriate interpretation for a circular accelerator such as the Tevatron. All particles with betatron

amplitudes greater than the distance between the closed orbit and the edge of the scraper will necessarily intercept the scraper on

subsequent turns. Thus, the lower left sketch is more representative of the phase space immediately downstream of the first scraper. As

shown in the lower right sketch, the second scraper can be moved closer to the beam than figure 2 implies.

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Conceptual Design of Scrapers Figure 1

ax+ 13 x'

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Conceptual Design of Scrapers Figure 2

. ax+px'

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• Conceptual Design of Scrapers

Figure 3

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Special Consideration for the Tevatron

The Tevatron is a superconducting accelerator. If sufficient energy is deposited in the magnets, they will quench and end any possibility of

having beams until the magnets are recooled. This makes it imperative to place scrapers in the lattice with care. Although the particles produced in the scraper are no longer part of the beam,

their energy must be carefully absorbed in devices other than the Tevatron magnets. In practice, this means placeing scrapers in straight sections which usually include other room temperature

devices.

In order to protect the CDF SVX from the act of scraping, the relationship among them should be: first scraper, second scraper and

then the SVX. In addition, it would be good not to have the DO detector between the first and second scraper.

Advantace of Helical Orbits

Helical orbits allow for the independent scraping of proton and antiproton beams in either the horizontal or vertical planes. Figure 4

is a sketch in real space (not phase space) and demonstrates the principle for L-shaped collimators as used in the Tevatron. It is to be noted that the proton and antiproton scrapers are necessarily located

at (slightly) different locations along the beam line.

Scraper Role Reversal

The lattice phase advance between the first and second scraper is the same for protons and antiprotons. Thus, if one has established lattice

locations for scraping protons, the same locations will serve for scraping antiprotons if one reverses the roles of first and second

scrapers.

Conclusions

It is intended to install scrapers into the Tevatron for the 3rd collider run based on the principles given in this talk. Whether they will

serve to protect the CDP SVX sufficiently remains to be seen.

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.,

•

Independent Scraping of Protons and Antiprotons on Helical Orbits

Figure 4

antiproton scraper

0 antiprotons

protons C) proton scraper

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