![](media/image1.png){width="2.1666666666666665in"
height="10.152777777777779in"}SNS 104050000-TD0010-R01

###### Linac Halo Mitigation {#linac-halo-mitigation .Cover--title}

January 2002

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SNS 104050000-TD0010-R01

# 

# **LINAC HALO MITIGATION**

SNS/AP TECHNICAL NOTE

### D. Jeon, J. Stovall, A. Aleksandrov, J. Wei, ORNL

J. Staples, R. Keller, LBNL

L. Young, H. Takeda, LANL

January 15, 2002

Prepared for the

U.S. Department of Energy

Office of Science

UT-BATTELLE, LLC

managing

Spallation Neutron Source activities at

  ------------------------------------- ---------------------------------
  Argonne National Laboratory           Brookhaven National Laboratory

  Thomas Jefferson National Accelerator Lawrence Berkeley National
  Facility                              Laboratory

  Los Alamos National Laboratory        Oak Ridge National Laboratory
  ------------------------------------- ---------------------------------

under contract DE-AC05-00OR22725

for the

U.S. DEPARTMENT OF ENERGY

CONTENTS

**Page**

LIST OF FIGURES v

1\. SUMMARY 1

2\. INTRODUCTION 1

3\. DTL COLLIMATION 5

4\. PROPOSED PLANS 10

4.1 MEBT SCRAPING 10

4.2 ALTERNATIVE MEBT OPTICS 13

4.3 HYBRID HALO SOLUTION 15

5\. LEBT SCRAPING 16

6\. CONCLUSION 18

ACKNOWLEDGEMENT 18

REFERENCE 18

APPENDIX 1 A-1

APPENDIX 2 A-2

APPENDIX 3 A-3

LIST OF FIGURES

**Figure Page**

2.1 Phase-space projections of the measured and simulated LEBT beam 2

2.2 Transverse (x-x' and y-y') phase-space projections of the beam
distribution\
at the RFQ exit. 2

2.3 Transverse (x-x' and y-y') phase-space projections of the measured
beam\
distribution at the DTL entrance showing horizontal tails which have\
developed in the chopper 3

2.4 Transverse beam envelope profiles in the MEBT 3

2.5 Radial particle distribution of the "measured" beam at 171 MeV
assuming\
no linac errors showing a significant halo extending beyond the CCL
bore. 4

3.1 Schematic diagram of DTL tank 1 showing the location of proposed
collimators 5

3.2 Real space projections of the beam distribution at the MEBT chopper
target and\
at the first 7 of the proposed DT collimators. 6

3.3 Expected energy deposited in 8mm radius DTL apertures 7

3.4 Radial beam distribution at 171MeV with and without DTL collimation
8

3.5 Radial beam distribution at 171MeV with and without DTL collimation
8

3.6 Expected energy deposited in 6 mm radius DTL apertures 9

3.7 Real-space beam distribution at the end of DTL tank 1 for one set of
random\
machine imperfections 9

4.1.1 Schematic layout of the MEBT indicating the location of adjustable
horizontal\
scrapers at the chopper target (red arrow**)** 11

4.1.2 Schematic drawing of adjustable MEBT scrapers and chopper target
11

4.1.3 Beam profile at 171MeV with (red curve) and without MEBT
collimation\
(blue curve) at ±8mm 12

4.1.4 Maximum and minimum power corresponding to beam particles outside
a radius of 8 mm in empty drift tubes after passing through a ±8-mm MEBT
scraper 12

4.2.1 Beam envelope profiles of the modified MEBT optics yielding a more
circular cross section in the anti-chopper 13

4.2.2 Radial beam distribution at 171MeV corresponding to a modified
MEBT optics design (red) showing 87% reduction in beam halo 14

4.2.3 Beam projections at the ring injection foil of a partially chopped
but unrestored\
beam (blue) using alternate MEBT optics compared with the nominal
unchopped\
design (red) 15

4.3.1 Radial beam distribution at 171MeV for the hybrid halo mitigation
solution 16

5.1 Phase-space projections of the measured and simulated LEBT beam in
which\
the LEBT is terminated in a 4 mm aperture 17

A.1 Trace3D output of entire MEBT optics modification A-1

A.2 Beam profile plots at 171MeV for three different MEBT optics without
any collimation A-1

A.3 Schematic drawing of beam scraper for the Drift-Tube-Linac of the
200 MeV Linac at Brookhaven National Laboratory A-2

1\. ABSTRACT

The SNS beam distribution, based on Front End (FE) emittance
measurements and multiparticle simulation studies, develops a
substantial halo that leads to beam loss and radio activation of the SNS
linac. The DOE has recommended that we "develop a plan that would permit
insertion of collimators in DTL tank #1" to remove the halo.

This recommendation has been evaluated and we find that this approach
does not substantially reduce the halo, but would impose infeasible
thermal loads on the drift tubes. Three additional techniques have been
studied, involving LEBT and MEBT collimation and modified MEBT optics.
Based on the results of these studies it appears that we have identified
an alternate scenario that looks highly promising for reducing beam
halo.

2\. Introduction

The Spallation Neutron Source accelerator system is designed to
accelerate intense proton beams to an energy of 1-GeV, delivering more
than 1 MW of beam power to the neutron production target. A primary
concern is potential damage and radio activation of accelerator
components resulting from uncontrolled beam losses. A major source of
loss is beam halo that intercepts the bore of the linac. Presently, beam
halo scrapers and collimators are implemented only in the High-Energy
Beam-Transport (HEBT) line, the accumulator ring, and the Ring-Target
Beam-Transport (RTBT) line. These scrapers do not protect the linac
because they follow the linac exit.

Beam dynamics simulations show that the beam halo develops at low
energy, but some halo particles survive acceleration to higher energies
before being lost on the linac bore. This particle loss at higher
energies results in radio activation of the Coupled-Cavity Linac (CCL).
Whatever halo survives the linac may likewise escape HEBT collimation
and increase the heat load in the ring-injection beam dump.

Several FE components and physical effects may contribute to the
generation of beam halo. These include details of the ion-source design,
plasma dynamics and extraction optics, nonlinear LEBT optics,
phase-space mismatch between the LEBT and the radio frequency quadrupole
(RFQ) linac, the RFQ itself, and the MEBT. The MEBT is the largest
contributor to beam halo. The beam phase width is large enough so that
nonlinear fields in the buncher gaps cause halo growth. More
importantly, because the beam has a large transverse eccentricity over a
relatively long distance, it experiences nonlinear space-charge forces
and develops "tails" in the transverse phase-plane projections. In the
Drift-Tube Linac (DTL), these tails develop into a halo that is
inseparable from the core of the beam.

Figure 2.1 shows the measured beam emittance in the LEBT upon which we
based our beam-dynamics simulations. The upper row of plots shows x-x'
projections and the second row shows y-y'. The two colored plots in the
first column show the raw emittance data. The second column shows the
"analyzed" data after applying a threshold to eliminate noise and
spurious signals. The third column shows the macroparticle distribution
that we derived from the analyzed data and that we used in our numerical
simulations. The derivation algorithm assumed a particular type of
distribution for the unmeasured correlations in the real beam
distribution. The bottom row shows these four projections generated for
the numerical distribution: x-y, x'-y', x-y' and x'-y.

![](media/image2.png){width="6.0in" height="4.8in"}

Fig. 2.1. Phase-space projections of the measured and simulated LEBT
beam.

Figure 2.2 shows the transverse phase-space projections of the measured
beam transported to the RFQ exit. The particles that appear to be
scattered in the angular dimension correspond to low energy particles
(\~0.1MeV) that are destined to be lost longitudinally at low energies
and are of no concern. Otherwise, this beam is only slightly worse than
one originating from an initial water-bag distribution at the RFQ
entrance.

![](media/image3.png){width="5.154861111111111in"
height="1.8590277777777777in"}

> Fig. 2.2. Transverse (x-x' and y-y') phase-space projections of the
> beam distribution at the RFQ exit.

Figure 2.3 shows the transverse phase-space projections of the beam at
the entrance to the DTL. The now well-developed tails in the horizontal
projection are a result of the MEBT optics, which maintains the beam
eccentricity of \~2:1 throughout the \~1.6-m-long chopper section.

![](media/image4.png){width="5.048611111111111in"
height="1.8131944444444446in"}

> Fig. 2.3. Transverse (x-x' and y-y') phase-space projections of the
> measured beam distribution at the DTL entrance showing horizontal
> tails which have developed in the chopper.

Figure 2.4 shows the horizontal (upper) and vertical (lower) envelope
profiles of the beam in the MEBT. The beam is squeezed vertically to
clear the vertical deflection plates of both the chopper and
anti-chopper.

![](media/image5.png){width="5.323611111111111in"
height="1.4409722222222223in"}

> Fig. 2.4. Transverse beam envelope profiles in the MEBT.

In the DTL, the energy associated with the horizontal tails quickly gets
redistributed, resulting in a halo in both horizontal and vertical
emittance projections. We are, of course, concerned with any increase in
the effective emittance of the beam, but more importantly in any
increase in the real-spatial size of the beam, which increases the risk
of interception with the linac bore. The radial particle distribution in
Fig. 2.5 shows that even without including any errors in the beam
dynamics simulation, at the end of the CCL the halo extends beyond the
1.5-cm physical bore radius. Throughout the CCL the focusing strength of
the transverse lattice gradually weakens to smoothly match the focusing
strength in the Superconducting (SRF) linac. As a result, the beam size
is largest near the end of the CCL, so that most of the beam loss occurs
at energies near 170 MeV. By including random errors in the simulations
we readily see the locations of "hot spots" caused by beam loss.

![](media/image6.jpeg){width="5.0125in" height="3.421527777777778in"}

> Fig. 2.5. Radial particle distribution of the "measured" beam at 171
> MeV assuming no linac errors showing a significant halo extending
> beyond the CCL bore.

Our simulations indicate that, without cleaning, the beam halo results
in excessive beam loss in DTL and CCL. Furthermore, the halo effectively
increases beam emittance and causes more particles to miss the injection
foil. We have investigated the effectiveness and feasibility of four
schemes for reducing the halo, including drift-tube collimation
suggested by the DOE. We propose an alternate, more flexible scheme that
can be progressively implemented, as necessary. The proposed scheme has
little or no impact on the construction schedule. Given the uncertainty
of the beam distribution from the LEBT, our goal is to implement a
robust system that can accommodate varying commissioning and operating
beam conditions.

3\. DTL collimation

The DOE has directed us to develop a plan that would permit the
insertion of scrapers into drift tubes in tank 1 that would trim off the
beam halo described above. The focusing lattice in the DTL is FFODDO,
where O means empty drift tubes. We considered inserting circular
collimators in the first 11 empty drift tubes. By using only empty drift
tubes, we avoid the possibility of overheating and possibly approaching
the Curie point of the permanent-magnet quadrupole lenses (PMQs). In
addition, the beam is nearly round in the empty drift tubes making
collimation by circular apertures more effective.

The first 5 lattice periods, spanning 30 drift-tubes, represents a
depressed phase advance of about 2π radians. Therefore, by placing
collimators in the first 10 or 11 empty drift tubes should trim off the
halo, assuming that the halo remains on the perimeter of the beam. The
aperture size would have to follow the beam size. Figure 3.1 shows
schematically the first half of DTL tank 1. The colored drift tubes
contain PMQs. The vertical red arrows show the proposed locations of
collimators.

![](media/image7.wmf){width="6.0in" height="1.9375in"}

Fig. 3.1. Schematic diagram of DTL tank 1 showing the location of
proposed collimators.

We predict the effectiveness of the collimation scheme by identifying
the halo particles "at risk" as the outermost 1%, which corresponds to
those having a radius greater than 0.9 cm in Fig. 2.5. We then track
these particles through the DTL. Figure 3.2 shows the real-space
projections of the beam at the chopper target and at the first 7
locations of the drift-tube collimators. The "at risk" particles are
plotted in red.

+----------------------------------+-----------------------------------+
| ![](m                            | ![]                               |
| edia/image8.jpeg){width="2.15in" | (media/image9.jpeg){width="2.2in" |
| height="1.7527777777777778in"}   | height="1.7527777777777778in"}    |
|                                  |                                   |
| chopper target                   | drift tube 1                      |
+----------------------------------+-----------------------------------+
| ![](medi                         | ![](                              |
| a/image10.jpeg){width="2.1625in" | media/image11.jpeg){width="2.2in" |
| height="1.7715277777777778in"}   | height="1.7715277777777778in"}    |
|                                  |                                   |
| drift tube 4                     | drift tube 7                      |
+----------------------------------+-----------------------------------+
| ![](me                           | ![](                              |
| dia/image12.jpeg){width="2.15in" | media/image13.jpeg){width="2.2in" |
| height="1.7527777777777778in"}   | height="1.7527777777777778in"}    |
|                                  |                                   |
| drift tube 10                    | drift tube 13                     |
+----------------------------------+-----------------------------------+
| ![](me                           | ![](                              |
| dia/image14.jpeg){width="2.15in" | media/image15.jpeg){width="2.2in" |
| height="1.8277777777777777in"}   | height="1.8277777777777777in"}    |
|                                  |                                   |
| drift tube 16                    | drift tube 19                     |
+----------------------------------+-----------------------------------+

> Fig. 3.2. Real space projections of the beam distribution at the MEBT
> chopper target and at the first 7 of the proposed DT collimators. Halo
> particles "at risk" are plotted in red.

Halo particles have more transverse energy than do core particles. They
oscillate through the core with large amplitudes, spending only part of
their time on or near the beam perimeter. The projections in Fig. 3.2
clearly show halo particles inside the core of the beam. In fact, the
halo is completely unobserved at some points (e.g. drift tubes 1 and
16). The tails created in the MEBT have become so well integrated with
the core of the beam that drift-tube collimation is not effective.

To evaluate the effectiveness of DTL collimators, we looked at the
radial particle distribution at 171 MeV for several collimator radii. To
evaluate their feasibility, we calculated the amount of energy deposited
in each drift tube from scraped beam. This energy represents an addition
to the rf thermal load on each drift tube. The water-cooling circuit
must remove this heat in order to hold the structure on resonance.

We first considered 8-mm-radius circular collimators and transported the
beam without including errors such as misaligned drift tubes. The
collimators reduce the bore cross section by 36%, but trim just 0.22%
from the beam. The left-hand plot of Fig. 3.3 is the expected power
deposited in the collimating drift tubes. The maximum power of \~5 W
deposited in drift tube 10 adds 10% to the rf heat load. The expected
beam-loss profile compared with the beam sizes in Fig. 3.2 shows that
the beam completely clears the apertures in drift tubes 13, 16 and 19,
so there is no beam power added to these drift tubes.

  ----------------------------------------------------- -----------------------------------------------------
  ![](media/image16.jpeg){width="2.922222222222222in"   ![](media/image17.jpeg){width="2.922222222222222in"
  height="1.9479166666666667in"}                        height="1.9479166666666667in"}

  ----------------------------------------------------- -----------------------------------------------------

> Fig. 3.3. Expected power deposited in 8mm radius DTL apertures. The
> right-hand plot assumes no errors. The left-hand plot summarizes the
> results of 100 linac runs with errors.

The first dipole corrector in DTL tank 1 is in drift tube 49, so there
is no steering spanning the proposed collimators. With this lack of
steering, it is prudent to study the effects of machine imperfections.
The right-hand plot of Fig. 3.3 summarizes the simulated particle
dynamics in 100 linacs that included random alignment errors. The plot
shows the maximum and minimum power lost at each aperture. The maximum
power, again at drift tube 10, was 31W, adding 65% to rf heating.

The radial particle distributions plotted in Fig. 3.4 show the
effectiveness in reducing halo by 8-mm drift tube collimators. The blue
curve is the radial beam distribution at 171 MeV without collimation,
and the red curve is the distribution after being trimmed. Even though
0.22% of the beam has been trimmed from the beam there is no observable
reduction in the beam halo.

![](media/image18.jpeg){width="5.0125in" height="3.4125in"}

> Fig. 3.4. Radial beam distribution at 171MeV with and without DTL
> collimation. DTL apertures of 8-mm radius do not reduce beam halo.

Next we considered 6-mm-radius collimators, which reduces the bore cross
section by 64%. Indeed, the beam halo at 171 MeV is reduced as shown in
Fig. 3.5. The smaller apertures remove over 1% of the beam and reduce
the current in the halo (r \> 1.1 cm) by 90%. Unfortunately, the energy
deposited in the 6-mm apertures is excessive.

![](media/image19.jpeg){width="4.975in" height="3.4125in"}

> Fig. 3.5. Radial beam distribution at 171 MeV with and without DTL
> collimation. Apertures of 6-mm radius reduce beam halo by 90%, but
> result in excessive drift tube heating.

The left-hand plot of Fig. 3.6 shows that, excluding errors, the beam
power deposited in drift tube 10 would double the design thermal load.
Including errors (right-hand plot of Fig. 3.6), the maximum expected
power deposited in drift tube 22 is 444 W, which is \~6 times the design
cooling capacity of this drift tube.

  ----------------------------------------------------- -----------------------------------------------------
  ![](media/image20.jpeg){width="2.922222222222222in"   ![](media/image21.jpeg){width="2.922222222222222in"
  height="1.9479166666666667in"}                        height="1.9479166666666667in"}

  ----------------------------------------------------- -----------------------------------------------------

> Fig. 3.6. Expected energy deposited in 6 mm radius DTL apertures. The
> left-hand plot assumes no errors. The right-hand plot summarizes the
> results of 100 linac runs with errors.

The random misalignment of the PMQs steers the beam, so the beam
centroid is seldom on axis. Without beam-position monitors (BPMs) in the
low-energy end of tank 1 we have no way to steer the MEBT beam onto the
DTL axis. As a consequence, the beam will scrape the bore asymmetrically
as shown in Fig. 3.7. Although we did not quantify this effect, it is
clear that even small drift-tube apertures would not assure a
significant reduction in the halo, but would guarantee serious heating
problems.

![](media/image22.jpeg){width="2.825in" height="2.3340277777777776in"}

> Fig. 3.7. Real-space beam distribution at the end of DTL tank 1 for
> one set of random machine imperfections. The resulting asymmetric
> scraping leaves a significant halo.

We conclude that placing apertures in drift tubes is neither an
effective nor safe approach to removing halo particles that are expected
to be lost in an uncontrolled way.

-   Too small an aperture (\~6-mm radius) is required to effect any
    useful reduction in the beam halo.

-   Severe thermal loading of the drift tubes will exceed the ability of
    the present drift-tube cooling design.

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-   Apertures of fixed size and shape lack the flexibility required to
    accommodate beam matching and steering.

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-   Asymmetric scraping is expected to result from unpredictable
    construction and steering errors that would nullify any expected
    benefit.

4\. Proposed Plans

We propose an alternate scheme for halo mitigation that reduces the
opportunity for formation of transverse tails, and trims the tails
before they have an opportunity to mix with the beam core. The proposed
scheme utilizes adjustable scrapers at the location of the MEBT chopper
target and/or adjustable scrapers in the anti-chopper box (in the case
that anti-chopper is unnecessary). We also need three more power
supplies to make the MEBT quadrupoles independently adjustable, thus
providing adjustable MEBT optics to control beam halo generation.

4.1 MEBT scraping

There are only a few places where scrapers will fit in the MEBT. One
convenient place is at the chopper target. Figure 4.1.1 shows the layout
of the MEBT. A pair of adjustable horizontal scrapers would be installed
in the chopper target box (at the red arrow). The chopper target itself
is located above the mid-plane to intercept beam that is deflected
upward. Scrapers mounted on horizontal actuators will not interfere with
the function of the target. This assembly is shown schematically in Fig.
4.1.2. This scraper implementation has the advantage that it is readily
adjustable to accommodate the actual beam conditions, which are expected
to vary with different operating conditions such as beam current,
ion-source performance, LEBT, RFQ, and MEBT tuning.

![](media/image23.jpeg){width="5.809722222222222in"
height="3.4159722222222224in"}

> Fig. 4.1.1. Schematic layout of the MEBT indicating the location of
> adjustable horizontal scrapers at the chopper target (red arrow). A
> second 4-jaw adjustable aperture could replace the anti-chopper as a
> backup (green arrow).

![](media/image24.jpeg){width="3.9034722222222222in"
height="2.0395833333333333in"}

> Fig. 4.1.2. Schematic drawing of adjustable MEBT scrapers and chopper
> target.

To compare with the DTL aperture study, we fixed the horizontal MEBT
scraper aperture at ±8 mm (see Fig. 3.2). Figure 4.1.3 shows that,
without errors, the halo is reduced by 84%. We then ran 100
beam-dynamics simulations through the DTL including errors. To compare
MEBT scraping with DTL scraping, we counted (and summed the energy of)
halo particles whose radius exceeded 8 mm at empty drift tubes in
tank 1. As indicated in Fig. 4.1.4, only a small fraction of beam
particles experienced radial excursions greater than 8 mm. In the worst
case, the corresponding power amounts to less than 5 W. For a comparable
reduction in halo, DTL scraping causes 90 times this power loss. These
results suggest that MEBT collimation may be sufficient.

![](media/image25.jpeg){width="4.9875in" height="3.4125in"}

> Fig. 4.1.3. Beam profile at 171MeV with (red curve) and without MEBT
> collimation (blue curve) at ±8mm. Halo is reduced by 84%.

![](media/image26.jpeg){width="3.439583333333333in"
height="2.286111111111111in"}

> Fig. 4.1.4. Maximum and minimum power corresponding to beam particles
> outside a radius of 8 mm in empty drift tubes after passing through a
> ±8-mm MEBT scraper. Column "CT" corresponds to beam scraped in the
> MEBT.

4.2 Alternative MEBT Optics

We believe that the primary cause of halo generation is nonlinear
space-charge forces acting on a beam having a large transverse
eccentricity. This condition occurs in both the MEBT chopper and
anti-chopper where the beam is compressed vertically to facilitate
chopping. The MEBT anti-chopper ideally operates at a vertical betatron
phase of 90 degrees (zero-current phase advances) from the chopper
target, and 180 degrees from the MEBT chopper, to restore any unchopped
portion of the beam to its "original" position in phase space. Figure
2.4 shows that this condition produces a beam profile that is symmetric
about the chopper target.

In an alternative design, we preserve the 90° phase advance from the
chopper to the target, but we relax it to 63° from the target to the
anti-chopper. The resulting beam cross section is more circular as shown
in Fig. 4.2.1. Now, the anti-chopper no longer restores a partially
chopped portion of the beam to its original (on-axis) position in phase
space, if indeed that were desirable. Also, the beam now has a larger
vertical extent and approaches the anti-chopper plates as designed.
Alternative modes of anti-chopper operation are under study.

![](media/image27.png){width="5.344444444444444in"
height="1.4916666666666667in"}

> Fig. 4.2.1. Beam envelope profiles of the modified MEBT optics
> yielding a more circular cross section in the anti-chopper.

This simple modification to the optics reduces the formation of
transverse tails and improves the beam quality in the downstream linac.
Figure 4.2.2 shows that beam halo at 171MeV is reduced by 87%. The only
expense of this marked improvement is providing independent adjustment
of the two MEBT quadrupole triplets. The halo reduction is comparable to
the effect of MEBT collimation for the baseline MEBT optics, but
involves no beam loss at all.

![](media/image28.jpeg){width="4.9in" height="3.327777777777778in"}

> Fig. 4.2.2. Radial beam distribution at 171MeV corresponding to a
> modified MEBT optics design (red) showing 87% reduction in beam halo.
> The blue line corresponds to the baseline.

To realize this alternative optics scenario, three additional power
supplies are needed to provide independent tuning of the quadrupole
triplets. While the anti-chopper will not completely restore a partially
chopped beam, it may not be required. Figure 4.2.3 shows that even
without the anti-chopper, a partially chopped beam remains inside the
envelope of the nominal beam at the foil. In this simulation, the
chopper deflected the beam so that 50% of it was removed at the chopper
target. This result is consistent with an earlier study by B. Blind
(LANL), and strongly suggests that we adopt the proposed alternative
MEBT optics with or without the anti-chopper.

![](media/image29.jpeg){width="5.0625in" height="3.61875in"}

> Fig. 4.2.3. Beam projections at the ring injection foil of a partially
> chopped, but unrestored, beam (blue) using alternate MEBT optics
> compared with the nominal unchopped design (red). The partially
> chopped and deflected beam remains within the envelope of the nominal
> beam.

4.3 Hybrid halo solution

We also investigated the effectiveness of adding MEBT collimation in
combination with the alternative optics design. In this scenario, we
added MEBT collimation at the two locations indicated by arrows in Fig.
4.1.1. We adjusted horizontal scrapers to ±8 mm at the chopper target
and introduced a square aperture of ±6 mm in the anti-chopper box.
Figure 4.3.1 shows the radial beam distribution at 171 MeV resulting
from this hybrid solution. The halo is reduced to only 3% of the
baseline case.

![](media/image30.jpeg){width="4.9875in" height="3.421527777777778in"}

> Fig. 4.3.1. Radial beam distribution at 171MeV for the hybrid halo
> mitigation solution. The red curve shows a 97% reduction in the halo
> with the combination of two collimators and modified MEBT optics. The
> blue line corresponds to the baseline performance.

Modifying the MEBT to create a more circular beam profile clearly
eliminates much of the halo seen in the baseline design. In addition,
with the introduction of variable collimators, we expect to reduce
uncontrolled beam loss associated with halo to a manageable level.

-   The alternative MEBT optics reduces beam halo formation.

-   Additional beam scraping in the MEBT removes potential halo created
    in the chopper before it becomes integrated into the core.

-   Implementation of the hybrid solution costs little and has no impact
    on schedule.

-   Because the quadrupole lenses and all the scrapers are adjustable,
    this scheme works in any operational scenario.

-   By tailoring the beam in the MEBT, rather than in the linac, its
    final quality will be insensitive to construction errors.

-   MEBT scrapers intercepting ≤ 1% of the beam can be cooled.

5\. LEBT Scraping

The beam produced by the ion source and subsequently accelerated through
the LEBT exhibits a distorted projection in phase space (see Fig. 2.1).
A poor shape of the H-minus emitting meniscus is primarily responsible
for the distortion. The present design of the outlet aperture and
extraction electrode produces a meniscus that emits a more divergent
beam than desired. The beam further encounters increased spherical
aberration in the LEBT elements, as it does not match the beam size
assumed for the LEBT design.

At the exit of the LEBT, in the second of two einzel lenses, the LEBT
chopper deflects the beam into a grounded target at the end of the LEBT,
which also serves as a diagnostic element. This aperture currently has a
diameter of 7.6 mm. Reducing the aperture diameter of this diagnostic
element would remove some of the peripheral beam, which will reduce
slightly the halo present in the RFQ beam. This change increases the
on-off ratio of the LEBT chopper, and it reduces slightly the spherical
aberration of the last lens. LEBT transmission will be slightly reduced.

We performed a beam-dynamics simulation with the exit aperture reduced
from 7.6 mm to 4.0 mm. The input beam is modeled on experimental data
taken 22 October 2001 with the potential of both einzel lenses set at
-40 kV, the settings that result in maximum simulated beam through the
RFQ. Figure 2.1 shows the generation of an input ensemble with the\
7.6-mm aperture diameter at the LEBT exit. Figure 5.1 shows the
generation of an input ensemble with a 4.0-mm aperture diameter at the
LEBT exit. Reducing the aperture reduced transmission to the exit of the
LEBT in the simulation by 3.8%.

![](media/image31.png){width="6.0in" height="4.8in"}

Fig. 5.1. Phase-space projections of the measured and simulated LEBT
beam in which the LEBT is terminated in a 4 mm aperture.

We used the code Toutatis to transport both particle ensembles through
the RFQ with a 38-mA current. The beam from the RFQ was then transported
though the MEBT, using the latest tune derived by LANL and then though
the room-temperature linac to an energy of 187 MeV using the Parmila
code. Differences observed in the halo from these two distributions are
statistically insignificant.

6\. Conclusion

After investigating the potential for mitigating halo by introducing
apertures in the DTL, we conclude that this scheme is neither effective
nor practical. Too small an aperture is required to reduce halo
significantly, and results in severe thermal loading of the drift tubes.
Asymmetric scraping, resulting from errors would nullify any potential
benefit. Fixed apertures would limit our ability accommodate any variety
of beam conditions.

Modifying the MEBT optics and introducing adjustable scrapers as needed
is a preferred alternative for mitigating halo by preempting its
formation. The hybrid solution does not involve any redesign that would
impact the construction schedule. Because the lenses and scrapers are
all adjustable, this scheme is adaptable to any operational scenario.

Acknowledgement

We would like to thank G. Murdoch, D. Raparia, et al for having provided
information on the beam scraping at ISIS and BNL.

References

\[1\] Jim Stovall *et al*, "Final design & expected beam performance of
SNS linac", ASAC Review, September 2001.

Appendices

##### Appendix 1: Halo enhancement in MEBT

Beam halo can be significantly reduced with revised MEBT optics. In the
unlikely case that MEBT chopper can be avoided, one can further relax
the beta function at the MEBT chopper using the independent MEBT triplet
quadrupoles. Entire MEBT optics is modified, unlike the MEBT optics
modification mentioned in the Proposed Plan section. The Trace3D output
is shown in Fig. A.1. Figure A.2 compares the amount of beam halo at
171MeV for the same input beam into the MEBT.

![](media/image32.png){width="5.452083333333333in" height="1.525in"}

> Fig. A.1. Trace3D output of entire MEBT optics modification.

![](media/image33.jpeg){width="5.0125in" height="3.46875in"}

> Fig. A.2. Beam profile plots at 171MeV for three different MEBT optics
> without any collimation. Blue is for the baseline optics, Red for the
> MEBT with 2^nd^ half of optics modified, and green for the MEBT with
> entire MEBT optics modified. Significant reduction of halo is
> observed.

#### Appendix 2: Beam Scraper at BNL Drift Tube Linac 

![](media/image34.wmf)

> Fig. A.3. Schematic drawing of beam scraper for the Drift-Tube-Linac
> of the 200 MeV Linac at Brookhaven National Laboratory.

The Brookhaven National Laboratory linac deliverers 200 MeV proton or
H^-^ beams. The first tube of the Drift Tube Linac is fit with an
elliptical scraper made of tungsten with only contact cooling with the
Cu tube as shown in Fig. A.1. The beam energy at the scraper is 750 keV.
In simulation, about 5% of the 100 μA beam is collected by the scraper.
The elliptical scraper is rotated by 90 degrees when the beam is
switched from proton to H^-^ beam.

##### Appendix 3: Beam Scraper at ISIS Drift Tube Linac

The ISIS linac at the Rutherford-Appleton Laboratory delivers 70 MeV
H^-^ beams. The first six tubes of the DTL tank 2 are fit with fixed
scrapers made of reactor-grade high-density graphite. The scrapers are
split longitudinally and sprung into place with only contact cooling to
the Cu tube. The inner diameters of the scrapers are progressively
reduced along the beam direction.