UNITED STATES PARTMENT OF )MMERCE

IBLICATION

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APR 1 6 1970

1 NBS TECHNICAL NOTE 522

Beam Handling

Techniques for

Electron Linear

Accelerators

U.S.

PARTMENT

OF

COMMERCE

National

Bureau

of

Standards
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NBS TECHNICAL NOTE 522

ISSUED APRIL 1970

Nat. Bur. Stand. (U.S.), Tech. Note 522, 21 pages (Apr. 1970)

CODEN: NBTNA

Beam Handling Techniques

for Electron Linear Accelerators

Samuel Penner

Linac Radiation Division

Center for Radiation Research

National Bureau of Standards

Washington, D.C. 20234

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of transient or limited interest. Technical Notes may be

listed or referred to in the open literature.

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C, 20402.

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FOREWORD

This report was originally prepared for presentation at a Seminar

on the subject "Nuclear Investigations on Linear Electron Accelerators

at Low and Medium Energy", held at the P. N. Lebedev Physical Institute,

Moscow, USSR, December 10-16, 1969. The paper presents a fairly extensive review of beam handling techniques for high current electron linear

accelerators and particularly of the NBS facility which has not been

fully described elsewhere. The paper is therefore reproduced here in

its original form in order to be available to a wider audience.

111
CONTENTS

Page

1. Introduction „ 1

2. Beam Optics 1

3. Beam Diagnostics and Control 11

4

.

Handl ing High Power Beams 14

5. Summary 16

6

.

References 18

IV
BEAM HANDLING TECHNIQUES FOR ELECTRON LINEAR ACCELERATORS

S. Penner

The design of beam transport systems for electron

linear accelerators intended for nuclear physics

research is discussed. The subjects covered include

beam optics, diagnostics and control of electron

beams, and problems of handling high power beams.

Key Words :beam handling; beam optics; beam transport;

electron linear accelerator; instrumentation for

electron beams; linac.

1. Introduction

A suitable beam transport system connecting the accelerator with

the experimental apparatus is an essential part of any experimental program using an electron linac. Major progress in both theoretical understanding and experimental techniques has been made during the last ten

years so that, using presently available methods, electron beams of

excellent quality can be provided to suit the needs of the most exacting

experiments. Without referring to any specific experiment, we can define the main properties demanded of the beam transport system. These

include: good momentum resolution and stability, small and stable beam

focal spots at experimental targets, low backgrounds in experimental

areas, beam purity (absence of x-rays and low energy electrons), matching to the accelerator to obtain good current, and the ability to handle

the beam power of the*accelerator. In this report we will discuss the

methods by which these properties are achieved and the quality of the

beams which have been obtained in operating systems.

2. Beam Optics

Most linacs produce an electron beam with a momentum spread of 1 to

1
5 percent, a diameter of the order of 1 cm, and a divergence of about 1

milliradian or less. Typical experiments demand momentum spreads as

4

small as a few parts in 10 and focal spots of the order of 1mm. With

rare exceptions, existing installations employ achromatic beam transport

systems to achieve these properties. The term achromatic is used here

to mean that the electrons which have passed through a momentum-defining

aperture are reassembled into a final beam in which the orbits are

independent of the electrons' s momentum (or very nearly so). A typical

1/* achromatic system is shown in figure 1~.

The main reasons for using an achromatic system are: (1) it is very

much easier to transport the beam along the final beam path than if it

were appreciably dispersed; (2) a small focal spot may be obtained at

any point along the path by a simple pair of quadrupole magnets, permitting multiple experimental setups along a single beam line; and (3) there

is no dependence of beam direction on momentum, which might cause problems in experiments where the angular distribution of the reaction

products is strongly anisotropic (as in electron scattering) . In addition the deflecting magnet which is located after the momentum-defining

aperture provides a strong discrimination against x-rays and degraded

electrons generated at the aperture, thus sharply reducing the background in the experimental area.

The particular system of figure 1 has the additional property that

an initially parallel beam is transformed to a parallel final beam.

Although this is not a general property of achromatic systems it is useful because the beam obtained from the linac is very nearly parallel,

*Literature references at the end of this paper.

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and particularly desireable when several experimental setups are located

along a single beam path. At the NBS linac, a system of this design

provides beam to any of five experiments which occupy a beam path length

of nearly 25 meters.

In principle, the momentum-defining aperture for the system of

figure 1 could be located at the symmetry plane of the system. Aside

from the mechanical problems involved in locating an adjustable aperture

inside a quadrupole magnet, this would be a poor location for the aperture because changes in beam steering along the linac would change the

momentum bin passed by the aperture. In the NBS system this effect

-3

would be 1.4X10 " fractional momentum change per milliradian change in

beam direction. Therefore, an entrance aperture (CA in figure 1) is

installed between the linac and the beam transport system, and the momentum-defining aperture (E) is located at the point where the first

deflecting magnet (Dl) and central quadrupole (Q0) form an image of CA

(in the horizontal plane optics). These apertures guarantee that the

momentum of the beam selected by aperture E is independent of linac

steering.

Careful design of the slit edges in the momentum defining aperture

is necessary in order to minimize low-energy tails in the transmitted

beam. Electrons which just graze the edge of the slit should be made

to suffer as much energy loss and multiple scattering as possible so

that they can be effectively absorbed in clean-up collimators before

entering the experimental area. The way to accomplish this aim is to

have a large radius of curvature slit -edge made of high Z material.

Figure 2 shows the NBS energy slit design. The radius of curvature of

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the edge is about 0.5m. The slit edges are made of water-cooled copper

(because of heat removal requirements), on which a .05mm gold surface

layer is evaporated.

It is important to note that the distance from CA to the first

deflecting magnet should be as large as possible (about 5m in the NBS

system where the bending radius is 0.7m). The reason is that if CA were

moved closer to the deflecting magnet, its image would move farther

downstream from Q0. The magnification of the image would increase but

the dispersion would decrease rapidly. Thus to obtain a given resolution, the horizontal opening of CA would have to be decreased, resulting

in reduced beam current. In the NBS system the dispersion at E is

.0927o/mm and the magnification of CA at E is -.26. Our highest resolution electron scattering work is done with the CA opening set at 2.5mm,

resulting in a momentum spread of 0.06 percent (FWHM)

.

Although a beam transport system demands a reduction of beam size

for high resolution operation it may be capable of accepting a more

divergent beam than that produced by the linac. The phase space emittance of the beam can be more closely matched to the acceptance of the

beam transport system by means of quadrupole lenses (QA, QB in figure 1)

located in front of the entrance aperture CA. These quadrupoles can

minimize the reduction of beam current described above. At NBS the

angular acceptance of 4 mrad is limited by a 20mm diameter aperture

(CB) just in front of the first deflecting magnet. This aperture

restricts the beam to the uniform-field region of the deflecting

magnets in order to insure good resolution, and prevents impingement

of the beam on the vacuum chamber which could cause thermal failure
(discussed in section 4, below) or large background in the experimental

area if even a small fraction of the beam impinges on the vacuum

pipe along the final beam line. In principle, the quadrupole pair

QA, QB, in the NBS system provides a good match of the beam transport

system phase acceptance (1 mrad-cm in the horizontal plane for .O67o

resolution, and 4 mrad-cm in the vertical plane) to the linac emittance (1 to 2 mrad cm)*. In practice, we have not been highly successful in increasing the transmitted current. This seems to be due to a

misalignment of the quadrupoles plus a lack of sufficient beam position

information and steering control. These problems will be discussed in

detail in section 3.

An auxiliary pair of quadrupoles (Ql, Q2 in figure 1) is located

just after the last deflecting magnet of the achromatic system. These

quadrupoles serve to reduce the divergence of the final beam so that it

can be carried for a large distance in reasonable size beam pipe without

further focussing. This is particularly important when entrance quadrupoles (QA, QB) are used. One must be very cautious in using the auxiliary quadrupoles because if they are adjusted for too short a focal

length they can cause some of the degraded electrons which have been

scattered from the momentum-defining aperture to be focussed through

the clean-up collimator (CC)

The clean-up collimator and the shielding wall which follows it are

the final steps taken to insure high beam purity and low background in

the experimental area. The aperture of this collimator should be quite

*We define phase area here as full width of the beam spot times total

angular acceptance. The actual 4-dimensional phase volume acceptance of

the beam transport system is (jt/4) times the product of horizontal and

vertical acceptance.
large because if it is too small it can be a source of more background

than it removes. (In the NBS system, CC has an aperture of 5 cm diameter compared to a typical beam size of 1-1.5 cm in this region and a

beam pipe diameter of 10 cm).

A final beam focus at the experimental target is obtained with a

third pair of quadrupoles (not shown in figure 1) which must be located

rather close to the target if small focal spots are desired. The entire

system tends to be highly astigmatic, which is usually desireable, for

example in electron scattering experiments where the vertical beam spot

dimensions must be small in order to obtain good resolution but the

horizontal beam size is not critical (we assume that the spectrometer

bends in a vertical plane) . However, with two pairs of quadrupoles

along the final beam line, a wide range of beam sizes and shapes can be

obtained at a given target location.

The design of beam transport systems of the type we have been

describing has become a highly developed art. The initial design follows from the principles of first-order optics, and calculations are

easily performed by the transport matrix technique- . For high-quality

4

systems, where resolutions of the order of a part in 10 are demanded,

with phase space acceptances of the order of a few milliradian-centimeters, first order considerations are not sufficient and both geometric

2/ and chromatic aberrations must be considered-. This is most convenient3/ ly done with the aid of digital computer programs such as TRANSPORT^ , which has been developed at SLAC and is in quite general use in many

laboratories.

Careful alignment of all components in the beam transport system

8
3/

is required for proper operation. The TRANSPORT program- allows one to

study the alignment tolerances of the components. Alignment accuracy of

0.1 to 0.2 mm is typically needed for good performance of the system. At

NBS, we were probably not sufficiently careful about the alignment when

we installed the system. As a result some of the quadrupoles in the

system tend to deflect the beam, thus increasing the difficulty of steering the beam properly.

Figure 3 is an overall view of the NBS beam transport system. In

addition to the two-magnet -and-quadrupole achromatic system described

above, another achromatic system of the three magnet type- is also

shown. The latter type of system tends to have smaller aberrations than

the former, but due to the smaller dispersion at the momentum-defining

aperture and poorer rejection of degraded electrons generated at the

slits in the three magnet system, the two-magnet-and-quadrupole arrangement seems preferable when good resolution is of primary concern.

Figure 3 also reveals what we believe to. be a mistake in the general

beam layout in our laboratory: The two deflection systems shown (and a

third system now being installed at the end of the straight beam line)

are in parallel, in the sense that each system operates directly on the

straight -ahead beam from the linac. It would have been better to have

a single achromatic analysis system, followed by as many deflecting

systems as desired. In this scheme only one small region of the transport system must be capable of handling the high beam power continuously

(assuming adequate interlocking against accidental beam impingement

thereafter), resulting in considerable cost savings and simplified operation. Since the laboratory was already built before we understood this

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point, it is impossible for us to change the layout of the transport

system.

3. Beam Diagnostics and Control

We cannot overemphasize the importance of including adequate beam

monitoring instrumentation in the beam transport system, as well as

along the linac itself. To demonstrate this point, we note that in the

NBS system the steering and focussing provided between the linac and an

experimental target has at least 14 adjustable parameters along each

path. There is some simplification due to the fact that the steering

and focussing power supplies derive their reference voltage from the

same source as the deflecting magnet power supply and thus tend to

"track" when the momentum setting is changed. This tracking is not

perfect due to the differing magnetic saturation properties of various

components and presence of stray magnetic fields. Even more important

are steering and focussing changes needed to account for changes in beam

direction, size, and divergence when the current, energy, or other

operating parameters of the linac are changed. It is obviously a very

difficult job to optimize a system with so many adjustable parameters

without good beam monitoring information.

At NBS, the instrumentation provided when the facility was built was

not adequate. With each improvement in our beam monitoring or controls

we have obtained large benefits in ease of operation, quality of beams

achieved, and the speed with which the operating staff can provide conditions requested by the experimenters. In principle we would like to

have a position monitor associated with (that is, a few meters downstream

from) each steering element and a beam-spot viewer for each pair of

11
quadrupoles, as well as current monitors regularly spaced along all beam

paths

.

As a typical example, we will describe the monitoring and controls

provided between the output of the linac and the first deflecting magnet

of the beam transport system in the NBS system. There are two sets of

steering coils between the linac and the entrance aperture (CA in figure

1) . Each set provides both vertical and horizontal steering. With

these two sets of steerers we can make the beam co-linear with the transport system axis (defined by the centers of CA and CB), independently of

the direction and position of the beam leaving the linac. We can observe the beam visually near both CA and CB to determine when the colinear condition is obtained. It is also necessary to optimize the

transmission of the beam through the apertures. This is accomplished

with the solenoid focussing coils along the linac as well as the quadrupole pair QA, QB. Adjustment of these elements is monitored by observing the beam spot at the viewscreen near CB. In addition, beam current

monitors located at the end of the linac and after CA measure the fraction of the beam current transmitted through CA. This fraction varies

between 75 and 100 percent (depending on the beam current and energy)

for a 10mm aperture.

The viewers mentioned above are simply fluorescent screens viewed

through windows in the vacuum system by a closed-circuit television

system. Beryllium oxide wafers serve as convenient viewscreens. This

2 material is useful in the current -density range of about .01 to 1/iA/mm }

and has the advantage of being able to withstand considerable beam power.

Zinc sulfide (painted on aluminum foil) is more sensitive than beryllium

12
oxide, but rapidly loses its flourescent properties with beam impingement, presumably due to radiation damage. At SLAC this difficulty is

overcome by providing a large number of interchangeable screens at each

4/ station-. Fluorescent screens are very useful monitors and we use many

of them due to their simplicity and relatively low cost. Their main

disadvantage is that they intercept the beam and thus must be retracted

when not in use.

Non-intercepting beam-position monitors of many types have been

developed ' ' . Simultaneous display of the beamposition at several

stations simplifies considerably the alignment of the beam in cases where

the number of possible steering adjustments is large (such as along the

linac itself). One could also use the signal from a non-intercepting

monitor to stabilize the beam alignment by feeding back to the steering

coil power supplies. In our experience, this has not been necessary

due to the good stability of the linac beam direction, provided that

well regulated power supplies are used for the steering coils.

Reliable and reasonably accurate beam current monitoring is obtained with ferrite-core pulse transformers. For pulsed beam currents of

about 10mA or greater, the pulse transformer can directly drive a coax

cable for oscilloscope display. For smaller pulses, low input impedance

current amplifiers are used and careful attention must be given to minimizing electrical pickup-

, which is always a serious problem around

pulsed accelerators. In addition to their use in beam control, the current monitors are very useful in protecting the linac and beam transport

against accidental beam impingement. The signals from successive monitors along the beam path are fed to difference amplifiers followed by

13
discriminators. A current difference greater than a preselected value

results in an inhibit signal which gates the linac injector pulser off

for ~2 seconds. In effect then, beam loss between monitor stations

causes the beam pulse repetition rate to drop to a very low value until

the condition is corrected. The ferrite-core pulse transformers can

also be used effectively as average current monitors by means of suitable integrating circuits- . Although requiring calibration against an

absolute current measurement, the ferrite monitors have proven to be

very useful for average currents above about 1/2iuA. The precision of

current measurements is typically about 0.2%.

4. Handling High Power Beams

The design of a beam transport system for a high current electron linac

must explicitly take account of the potential damage which can be done

by the high power beam. The problems encountered include: removal of

large heat flux from components struck by the beam, fatigue failure of

components due to the pulsed nature of the beam, radiation damage to

materials, corrosion, residual radioactivity, and chemical effects such

as radiolysis of water and production of noxious gases. Since we have

8/ recently published a review of these problems and their solutions- , we

will confine our remarks here to a few general comments.

The criteria on which we based the design of components subject to

direct beam bombardment seem to be valid, as evidenced by the fact that

in four years of regular operation we have never had a component fail

due to beam impingement.

Our entire beam transport system is "radiation hard", in the sense

that we have not used organic materials in the high radiation areas for

14
any purpose. However, along the linac there are some organicallyinsulated cables. This insulation has become extremely brittle due to

radiation damage and must be replaced soon, even though the radiation

levels along the linac are normally much lower than in the beam transport system. Other than these cables, the only significant radiation

damage problem in our laboratory is the darkening of the viewing windows

and television camera lenses used with our beam viewing screens. These

components must be replaced occasionally, although they recover partial-'

ly after prolonged exposure to ultraviolet light.

A slow build-up of long-lived residual activities in our beam

transport system has occurred, as expected. In the immediate vicinity

of the entrance aperture and momentum-selecting slits (CA and E in

figure 1) the radiation levels have steady-state values near 1 rad/hr.

Fortunately the high radiation areas are quite localized since normally

beam impingement occurs only at a few collimating apertures. Each of

these is followed by a large lead radiation shield which absorbs most of

the energy in the shower resulting from the impingement, in a geometry

which provides a high degree of self-shielding. As a result of this

localization, a worker will seldom receive a whole-body dose greater

than 20mrad during a full working day in the beam transport area. Since

the number of times our technicians must work in this area is quite

small, the yearly dose received is more than an order of magnitude below the recommended tolerance.

I would like to add one remark on the effective utilization of

high-powered accelerators. Obviously one wants the accelerator to

operate as nearly continuously as staffing levels and maintenance needs

15
permit. This can conflict with the needs of experimenters to work on

their apparatus. When multiple beam paths are provided the conflict is

resolved if adequate shielding between rooms is provided. In our

laboratory we have demonstrated both the desireability and practicability of this approach. Radiation levels in rooms not actually traversed

by the beam are extremely low, permitting full time occupancy of all

such areas.

5 . Summary

In conclusion I would like to summarize the state-of-the-art values

of electron beam properties. The best operational energy resolution

-4

which I know of is the 6x10 which we use quite routinely for electron

scattering at NBS. The new Saclay linac has a design resolution value

of 2.4x10 , but I believe they have not yet reported an experimental

value. The design for the MIT linac now under construction calls for

achromatic beam transport with resolution capability of 3.4x10 . In

addition, the MIT design provides for a dispersive beam with 5.5x10

resolution, for use with their "energy-loss spectrometer" system—

.

There are obviously other important beam properties besides resolution. Unfortunately, many of these cannot easily be assigned quantitative values. Beam energy stability of a few parts in 10 or better

12/ (±2keV at 60MeV— ) seems to be readily available, depending primarily

on the quality of the power supplies for the deflecting magnets. Beam

spot sizes of 1x3mm or less at experimental targets are routinely obtained. The background and beam purity properties of a transport system are

also a great importance to the experiments being performed. Achromatic

systems of the type described here, when provided with adequate monitor16
ing and control features, are quite capable of satisfying the needs

in this regard, of most experiments.

17
6. References

1. S. Penner, Rev. Sci. Instr. 32, 150 (1961).

2. Karl L. Brown, Proc. Int. Symposium on Magnet Technology, page

141, Stanford Linear Accelerator Center Report CONF-650922,

Stanford, California (1965).

3. S. K. Howry, C. H. Moore, Jr. and H. S. Butler, "TRANSPORT,

A Computer Program for Designing Beam Transport Systems",

SLAC Internal Report, Stanford Linear Accelerator Center, Stanford,

Calkfornia (1963).

4. R. W. Coombes and D. Neet, IEEE Trans. Nucl. Sci., NS-14, 3, 1111

(1967).

5. J. L. Menke, IEEE Trans. Nucl. Sci., NS-16, 3, 921 (1969).

6. L. Johnston, J. Faust, W. Pierce and M. Stangenes, IEEE Trans. Nucl.

Sci., NS-14, 3, 1106 (1967).

7. J. L. Menke, Nucl. Instr. and Methods 64, 1, 61 (1968).

8. S. Penner, IEEE Trans. Nucl. Sci., NS-14, 3, 908 (1967).

9. J. B. Bellicard, P. LeConte and J. Mougey, Medium Energy Nuclear

Physics with Electron Linear Accelerators , p. 35, LNS-MIT Report

TID-24667 (1967).

10. Medium Energy Nuclear Physics with Electron Linear Accelerators . p. 17,

LNS-MIT Report TID-24667 (1967).

11. S. Kowalski, W. Bertozzi and C. P. Sargent, Medium Energy Nuclear

Physics with Electron Linear Accelerators , p. 39, LNS-MIT Report

TID-24667 (1967).

12. J. E. Leiss, S. Penner, J. E. Rose and J. B. Broberg, IEEE Trans.

Nucl. Sci., NS-16, 3, 1027 (1969).

18
THE NATIONAL ECONOMIC GOAL

Sustained maximum growth in a free market economy, without inflation, under conditions of full employment

and equal opportunity

THE DEPARTMENT OF COMMERCE

The historic mission of the Department

is "to foster, promote and develop the

foreign and domestic commerce" of the United States. This has evolved, as a

result of legislative and administrative

additions, to encompass broadly the re- sponsibility to foster, serve and promote

the nation's economic development and

technological advancement. The Department seeks to fulfill this mission through

these activities:

<*?***

MISSION AND

FUNCTIONS

OF THE

DEPARTMENT OF

COMMERCE

"to foster, serve and

promote the nation's

economic development

and technological

advancement"

litis©

>#P"

Participating with other government

agencies in the

creation of national

policy, through the

President's Cabinet and its subdivisions.

• Cabinet Committee

on Economic Policy

•Urban Affairs Council

•Environmental

Quality Council

Promoting progressive business policies and

growth.

• Business and Defense Services

Administration

• Office of Field Services

Assisting states, communities and

individuals toward economic progress.

• Economic Development

Administration

• Regional Planning

Commissions

• Office of Minority Business Enterprise

NOTE: This schematic is neither an organization chart nor a program outline for budget purposes. It is a general statement

of the Department's mission in relation to the national goal

of economic development.

JULY 1969

Strengthening

the international economic position

of the United

States.

• Bureau of International Commerce

• Office of Foreign Commercial

Services

• Office of Foreign

Direct Investments

• United States

Travel Service

• Maritime

Administration

Assuring effective use and growth of the

nation's scientific and technical resources.

• Environmental Science Services

Administration

• Patent Office

• National Bureau of Standards

• Office of Telecommunications

• Office of State

Technical Services

Acquiring, analyzing and disseminating

information concerning the nation and

the economy to help achieve increased

social and economic

benefit.

• Bureau of the Census

• Office of Business Economics

6 U. S. GOVERNMENT PRINTING OFFICE : 1970 392-25\/l60

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