ACCELERATOR PROJECTS AND
DEVELOPMENT OF THE
ACCELERATOR TECHNOLOGY
Alexander Novokhatski
SLAC National Accelerator Laboratory
Accelerator seminar
KEK, February 2020
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A. Novokhatski, Feb. 21, 2020

Thanks for invitation• I’m so glad to visit KEK, the outstanding High Energy Physics laboratory
and participate in the accelerator study of the fantastic SUPER KEKB
machine.
• Many thanks for invitation. I will be very happy to help.
• I this presentation I will talk about the accelerator projects, I was lucky to be
involved in and about the development of the accelerator technology for
future projects.
• Linear Colliders, B-factories, X-ray FELs and Future Circular Colliders
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Some history• In the middle of 70s the director of BINP Andrei Mikhailovich Budkerorganized a small group of researchers under the leadership of VladimirBalakin to study a new type of an accelerator for colliding beams. He called
this machine a “SuperLinac”, what we now call “Linear Colliders”• The main task of the study was– To demonstrate a possibility to achieve a high gradient of acceleration of 1 MV/cm– And to solve all beam dynamics problems• It is interesting that we managed to solve main problems.
• We developed a technology of fabricating high gradient accelerating structure.
We managed to achieve almost 2 MV/cm in a cavity.
• We understood main beam dynamics problems and found solutions for almost
all of them
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VLEPP Linear Collider

Unfortunately, Andrei Mikhailovich died in1977 just before he became a sixty. A new
director Alexander Nikolayevich Skrinsky
continued the activity on Linear Colliders.
The project of the BINP Linear Collider(VLEPP) was first presented in 1978 at theInternational Symposium devoted A. M.
Budker and All-union particle accelerator
Conference in Dubna.
The results, published in Russian weretranslated to English at SLAC.
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Single bunch instability and BNS damping• One of the main beam dynamics problems was a beam break-up effect, which limited theacceleration of the very high intense beams, needed to achieve high luminosity.
• To understand the structure of force acting in a bunch, we developed a numerical method
to calculate electromagnetic interaction of the beam and accelerating structure.
• With a very good description of these forces we started doing beam dynamics simulationand immediately found a very strong transverse instability of a single bunch.
• Careful analyses of the beam dynamics showed the resonant structure of the instability.
• This important feature was the most impotent point in invention of a new method to damp
the transverse instability, which later got the name “BNS Damping” by the first letters of
the inventors Balakin, Novokhatski and Smirnov.

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Physics of BNS damping. Shortly.
• The bunch head particles do not experience any action of the wake field andfreely oscillate in the focusing lattice at the betatron frequencies.
• However this oscillations produce a periodical force for the particles of the bunch
tail, which experience the action of the wake field.
• As the frequency of the force and the frequency of free oscillations are the samethen the amplitude of oscillations of the tail’s particles grow in time because ofthis resonance.
• An immediate solution for this situation is to destroy the resonance, that means
to give different betatron frequencies to the particle of the bunch head andparticles of the bunch tail.
• It can be done in many different way, but a simple solution is to utilize the factthat a betatron oscillation frequency depends on a particle energy
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Physics of the BNS damping• Since the transverse wake field introduces defocusing the chromatic focusing
can be used for compensation.
• By accelerating a bunch behind the crest of the accelerating field, the tailparticles gain less energy than the head. Therefore, the tail particles are focusedmore by the quadrupoles than the head.
• The longitudinal wake field actually helps to increase the energy spread. The tailparticles loss more energy due to the action of this field.
• With increasing of the particle energy during the acceleration, the energy!!
difference can be reduced. The beam break up effect becomes small ~ ! andthe bunch is now moved ahead of the crest to reduce the energy spread in the
beam.
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BNS damping works very well against a very strong instability

X’ X phase plot for different energy spreads.

Relative emittance at the exit of the 100 GeV accelerator
section versus the initial energy spread. With the initialenergy spread of 12% the beam can reach at the section
exit with a minimally achievable spread of 3%
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Comparison BNS and Landau damping

Landau damping works
something like BNS damping
but with an opposite sign ofthe energy spread and cannot
damp instability completely

Landau damping (stochastic energy spread) less effective than BNS damping.
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Application• BNS damping was proposed to use in different Linear Collider project• BNS damping helped to increase luminosity of the SLAC Linear Collider many times.
• BNS damping was effectively used during the operation of the SLAC PEP-II B-factoryfor intense injecting beams.
• Naturally, it can work in the multi-bunch regime as well.

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Injector for electron-positron colliders at BINP
Experience with VLEPP project helped to design an electron-positron injector for circularcolliders at BINP. It started operation several years ago (P. Logachev).

SLAC klystrons are used for electron and positron linacs.
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TESLA Linear Collider• The TESLA Linear Collider project at DESY had also FEL, mainly to support the
Collider. However there was a concern about the possibility of the accelerating of a shortbunch in the superconducting accelerator structure. Some accelerator theories predictedthat electromagnetic field of a short bunch, which contains frequencies above 750 GHz
can destroy the Coopers pares and, in this way, quench the superconductivity• The wake field code developed for VLEPP helped to understand this problem

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Fight against transverse wake fields in the PEP-II B-factory rings
q Transverse wake fields are generated in the asymmetrical parts of the beam pipe.
q Transverse wake fields can penetrate through the small hole in the vacuum chamber or
longitudinal slots of shielded bellows, vacuum valves and RF shields.
q Transverse wake fields may propagate long distances.

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Temperature raise in IR vertex bellows

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Temperature raise in shielded bellows

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HOM leaking from TSP heater connector

The pumping section is 5.5 m long and consists of the beamchamber and an ante-chamber. Electromagnetic fields, excitedin the beam chamber penetrate to the ante-chamber and thenthrough the heater feed-through come out.

The power in the wake fields
was high enough to charbeyond use the feed-throughfor the titanium sublimation
pump (TSP).

antenna

HOM spectrum fromSpectrum analyzer
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Water-Cooled Absorber in the First Arc Chamber
HOM power reached level of 1 kW

Absorber effect

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PEP-II collimators are the main source for transverse wave production
HOM power in bellows

HOM power in absorber

We found a strong correlation with the beam position nearcollimators which are far away from the bellows and arc chamber.

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19

Dipole and quadrupole waves generating in collimators

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Transverse wake fields couple to valve cavities• Shielded fingers of some vacuum valves were destroyed bybreakdowns of intensive HOMs excited in the valve cavity.

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21

Straight bellows – HOM absorber design
Absorbing Tile
2.75” long by .24” wide
HOM Trapping Slots
Modes in the chamberpropagate through the slots &are absorbed by the AlNiSiC.

Inconel Spring
FingerGlidCop
Stub

GlidCop RF
Shield Finger

Bellows Cavity

WeldedBellows

Modes that leak past the RFshield finger and are trapped inthis area still see the absorber
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22

Technology and efficiency of the HOM absorberh=Pbefore - Pafter
Pbefore!100% =

145 - 85
!100% = 41%
145

Almost as it was designed!
HOM power in Arc absorber before and after
installation of the straight HOM absorber
Arc ab sorbe r PR04: 20 10 be fore
Arc ab sorbe r PR04: 20 10 after

aprox :145*LER^2
aprox : 85*L ER^2

HOM power [W]

600
500
400
300
200
100
0

LER [mA] 0

500

1000

1500

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2000

23

SLAC has developed high efficiency HOMs absorbers for different
cross-sections and installed 25 in the rings•

A. Novokhatski and S. Weathersby, “RF Modes in the PEP-II
Shielded Vertex Bellows”, Proceedings of the PAC’2003, Portland,
p.2981.

•

A. Novokhatski, J.Seeman, M.Sullivan, “RF Heating and Temperature
Oscillations due to a small Gap in a PEP-II vacuum Chamber, PAC
2003
A. Novokhatski, S. DeBarger, F.-J. Decker, A. Kulikov, J. Langton,
M. Petree, J. Seeman, M. Sullivan, "Damping the higher order modesin the pumping chamber of the PEP-II low energy ring", EPAC 2004,
Lucerne, Switzerland, 2004; SLAC-PUB-10531, 2004.
S. Weathersby, M. Kosovsky, N. Kurita, A. Novokhatski, J. Seeman,“A Proposal for a New HOM Absorber in a Straight Section of the
PEP-II Low Energy Ring”, PAC’05, p.2173.
A. Novokhatski, “HOM Effects in Vacuum Chamber with shortbunches”, PAC 2005.
A. Novokhatski, J. Seeman and S. Weathersby, “High efficiency
absorber for damping transverse fields” Phys. Rev. ST Accel. Beams
10, 042003 (2007).
A. Novokhatski, J. Seeman and M. Sullivan, Modeling of the Sparksin Q2-bellows of the PEP-II SLAC B-Factory, PAC 2007.
A. Novokhatski, S. DeBarger, S. Ecklund, N. Kurita, J. Seeman, M.
Sullivan, S. Weathersby, U. Wienands, “A New Q2-Bellows Absorber
for the PEP-II SLAC B-Factory”, PAC 2007
A. Novokhatski, J. Seeman, M. Sullivan, U. Wienands, “Analysis of
the Wake Field Effects in the PEP-II SLAC B-factory”, PAC 2009

•

•

•
•

•
•

•

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LCLS. SLAC LINAC collimator and THz radiation may come out

Gap 2 x 1.6 mm

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Vertical beam position (kick) and relative bunch charge vs a collimatorbottom jaw position (measurement)

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FEL pulse energy, beam emittance and relative bunch charge vs thecollimator jaw position (measurement)

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LCLS horizontal and vertical “dechirpers”

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Originally, we plan to measure the beam profile, but …

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Attosecond measurement in LTU
Thanks to Rick Iverson

Kick the beam horizontally or verticallyby the wake fields exciting by the same
beam moving near corrugated plates

Watch a transverse image on
the screen

Reconstruct bunch
temporal profile

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Application for users. Very short X-ray pulses.
X-corrector is immediately after thehorizontal dechirper

Bunch “tail”

Slide of July 2016

An opposite kick by a corrector tocompensate wake kick acting on
the “tail” of the bunch

X-raysThis will give a large kick to the “head”

X-ray production vs X-corrector

A tail of a bunch will get a perfect,
almost linear kick like from XTCAVOnly “tail” will produce short X-ray pulse
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First experimental result
X-ray spectrum changes with a jaw position

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Dx=0.3

Dx=0.25

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A. Novokhatski, Feb. 21, 2020

The choice of the IR beam pipe for FCC

In the design we tried to achieve
the minimum of theelectromagnetic interaction of
the colliding beams with metal
walls of the IR beam pipe.

A. Novokhatski 2/10/20

We developed a special smoothtransition from two beam pipes
to a common central pipe.

32

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The concept of a low impedance IR beam pipe
Smooth transitions at the pipe connections

Smooth transition fromthe “ellipse” to a roundpipe

IP central
round pipe
A. Novokhatski 2/10/20

Smoothtransition to ahalf of “ellipse”

Full“ellipse”

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Round pipe
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An unavoidable trapped mode in the FCC IR

65 mm

A trapped mode is localized in the region where two beam pipes emerge into one. This mode has
a transverse electrical component near the pipe connection. If we can make longitudinal cuts inthe walls at this tregion then the trapped mode field will radiate out of the beam pipe.
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The concept of the HOM absorber

box

E

Cop
per
pla
te w
ith
colu
mn
s

slot

s

Abs
orbe
r

Wa
ter

Based on the property of the trapped mode we havedesigned a special HOM absorber.
The absorber vacuum box is placed around the beam
pipe connection. Inside the box we have ceramic
absorbing tiles and copper corrugated plates .
The beam pipe in this place have longitudinal slots, whichconnect the beam pipe and the absorber box. Outside thebox we have stainless steel water-cooling tubes, braised tothe copper plates.
The HOM fields, which are generating by the beam in
the Interaction Region pass through the longitudinal slots
into the absorber box.
Inside the absorber box these fields are absorbed byceramic tiles, S.because
theyhave high value35of the loss
Novokhatski
11/15/19
tangent.
The heat from ceramic tiles is transported through thecopper plates to water cooling tubes.

coo
lon
g tu
bes

Common pipe

am
r
e
C

ic

Two beam pipes
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s
it le

To demonstrate how HOM absorber works we show two cases when ceramic tiles do notabsorb the HOM power (loss tangent is zero) and absorb the HOM power.
HOM mode in the IR region when ceramic tiles have vacuum parameters :
e=1, d=0

Field is in the beam pipe

HOM mode in the IR region when ceramic tiles have real parameters : e=22,
d=0.1
S. Novokhatski 11/15/19

36

Field is concentrated in the absorber region and in the ceramic tiles
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A new geometry with a smaller central Be pipe of 20mm in a diameterAs a request for further improvement of the Interaction Region,
we analyze the possibility of modifying the IR beam pipe for asmaller diameter of the central beryllium pipe to 20 mm This
modification gives more freedom for the FCC detector.

Smooth transition fromthe “ellipse” to a roundpipe of 20 mm

30 mm round pipes
IP central
round pipe
A. Novokhatski 2/10/20

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Smooth transition to ahalf of an “ellipse”
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Synchrotron radiation masks

A. Novokhatski 2/10/20

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Wake field and frequency spectrum of a new FCC IR

f1=
f2=
f3=

6.144261
6.323226.442526

A. Novokhatski 2/10/20

39

GHz
GHz
GHz

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Main trapped mode (there are can be several modes)

A. Novokhatski 2/10/20

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A mode trapped between a central small pipe and a SR mask
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Unavoidable modes

It needs more analysis to investigate
whether to A.
use
or not to 2/10/20
use HOM
Novokhatski
absorber

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FCC-ee beam parametersparameterZW

H (ZH)

ttbar

45.6

80

120

182.5

horizontal emittance [nm]

0.27

0.28

0.63

1.45

vertical emittance [pm]

1.0

1.0

1.3

2.7

longitudinal damping time [ms]

414

77

23

6.6

SR energy loss / turn [GeV]

0.036

0.34

1.72

9.21

total RF voltage [GV]

0.10

0.44

2.0

10.93

energy acceptance [%]

1.3

1.3

1.5

2.5

0.038 / 0.132

0.066 / 0.153

0.099 / 0.151

0.15 / 0.20

bunch length (SR / BS) [mm]3.5 / 12.1

3.3 / 7.65

3.15 / 4.9

2.5 / 3.3

bunch intensity [1011]
no. of bunches /beam

1.7
16640

1.5

1.5

2.8

2000

393

39

beam current [mA]

1390

147

29

5.4

beam energy [GeV]

energy spread (SR / BS) [%]A. Novokhatski 5/31/17

Averaged bunch spacing [ns]

42

19.5

luminosity [1034 cm-2s-1]

>200

>30

>7

>1.3

luminosity lifetime [min]

70

50

42

44

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Heat load vs bunch length for central beam pipe of 30 mm, 20 mm and 10 mm
2000

The heat load increases
approximately inversely
proportional to thediameter of the beam pipe

1800

power load [W/m]

1600
1400
1200
1000

ø=10 mm800

ø=20 mm

600
400

ø=30 mm
200
A. Novokhatski 2/10/20

43

0
3

4

5

6

7

8

9

10

11

12

bunch length [mm]43
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eRHIC IR beam chamber with round pipes in the final focus
quads

Synchrotron mask
The wake field almost follows the shape of abunch. That corresponds
to creation of the
S. Novokhatski 11/15/19
additional bunch self electromagnetic field in
the pipe of a larger size.

HO
M
44

ab
so
rbe
r

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Conclusions• Electromagnetic interaction of the beam with the accelerator environment become more
and more important with the progress of the accelerator technology.
• Very careful analyses of this interaction is needed for new accelerator projects andupgrade of the operating machines.
• I’m very glad to be involved in this very interesting activity.

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