2nd CLIC Advisory Committee (CLIC-ACE), CERN January 2008 Introduction to the CLIC Power Extraction and Transfer Structure (PETS) Design. I. Syratchev

2nd CLIC Advisory Committee (CLIC-ACE), CERN January 2008

Introduction to the CLIC Power Extraction and Transfer Structure (PETS) Design.

I. Syratchev


3 TeV CLIC

Decelerator sector

CLIC module

The CLIC Power Extraction and Transfer Structure (PETS) is a passive microwave device in which bunches of the drive beam interact with the impedance of the periodically loaded waveguide and generate RF power for the main linac accelerating structure.

The power produced by the bunched (0) beam in a constant impedance structure:

4

/0

222

gb V

QRFLIP

Design input parameters PETS design

P – RF power, determined by the accelerating structure needs and the module layout.I – Drive beam currentL – Active length of the PETSFb – single bunch form factor (≈ 1)

Introduction


RFDBDBstruct

cstructstruct

RFDBDBstruct

structLinacDB

cpeffective

cm

EL

cNTP

ELN

PLI

NTcG

EN

sec

secCombination

factor

For the given drive beam initial and final energies, the drive beam current is almost uniquely defined by the accelerating structure performance and DB generation arithmetic:

Drive beam energy

DB extractionefficiency (~0.9)

RF transfer efficiency (~0.94)

Accelerating structure

Number of decelerating sectors in the linac: GeVEDB 4.2 (Limited by radiation

losses in combiner rings)

#1. Drive beam current

#2. PETS Power and length

LT=Quad+BPMLPETS x m

L Acc. Str. x n = L unit

PPETS = PAcc. Str. x n/m (n/m-should be even number)

Drive beam

Main beam

CLIC sub-unit layout:

RFTunitb

struct

gr mLLFI

mnPQR

222 ))((2

)/(4/

The equation for the power production now can be rewritten in terms of the PETS parameters(for simplicity the extraction length is taken = 0):

#3. PETS aperture

In the periodical structure, for the fixed iris thickness and phase advance:

GaQR

gr

2/

where a- radius of the structure and G – geometrical parameter. Now the PETS aperture can expressed as:

2/1

2/1

4

2)(

mC

mnPGFLLIa d

struct

RFbTunit

The CLIC sub-unit general issues:

-The CLIC sub-unit length is defined by the decelerator FODO lattice period. In turn, it is tuned to be equal to the even number of the accelerating structures length.-The space reserved for the PETS should satisfy:

(LPETS x m)+LT ≤ Lunit to ensure the highest effective gradient.- For any sub-unit layout chosen (m), the PETS active length should be maximized to reduce the impedance and thus to provide more stable beam transportation.

Introduction


0 0.2 0.4 0.60

0.5

1

TM01

TM02

#4 RF power extraction (aperture)#3. PETS aperture (continued)

Following the CLIC accelerating structure development (CLIC_G), simulations of the beam dynamics in the DB decelerator and the design considerations for the DB quadrupole and BPM, the following parameters were used to finalize the PETS design:

Accelerating structure length: 0.25 mRF power per structure: 63.8 MWTransfer efficiency: 0.94Sub-unit Length = 4xL structure: 1.0 mQuad. +BPM length: 0.4 mCombination factor (Nc) 12Drive beam current: 100 A

Accordingly, the tree possible sub-unit layouts can be envisaged: m = 1,2 and 4, and the PETS active length:

m=1

m=2m=4

Introduction

PETS active length, m

Extr

act

or

leng

th ~

3λ

a/λ

TE11 TM01 TM02

a/λ

PETS aperturesrange

0.29 0.39 0.88

Extractionmethod Classical Choke-type Quasi-optical

Beam aperture 0.49TE12

b>a

0.75

b>a

Extractionlength ~0.5 λ ~3 λ ~4 λ > 5 λ

Design specifics

two feed

Wrap around Four feed

Four feedDouble choke

The choice of the PETS aperture to a great extent, strictly determine the type of the RF power extraction method. The only constrain should be applied to the extractor aperture: b ≥ a .

For the range of the specified PETS apertures, the power extraction length should be ~ 3λ. Now the PETS active lengths can be re-iterated:

][/6.0/)( mmmLLL Tunitactive

]m[075.0/6.03/)( mmLLL Tunitactive


0 0.2 0.4 0.650

60

70

80


E s

urf.

MV

/m

1,0)(

)1(1sin2)( )1(2

LzzW

L

ze

c

zKqzW cQ

z

Introduction#5 RF constrains (aperture)

Grudiev A. 09.2007

0 0.2 0.4 0.65

10

15

20

25

W_G


20

% M

arg

in

Pt1/

3 /C

CLIC_G target

PETS target

X-75 klystron

#6 Beam stability (aperture)

The transfers wakes can significantly distort the drive beam and cause heavy beam losses. The wake potential in a finite length structure with a high group velocity is:

The best scenario to suppress wake field effects will be to reduce the wake amplitude and to increase the group velocity. In a periodical structure the wake effect ~ a -3 x L and the group velocity ~ a.

0 0.2 0.4 0.60.1

1

a -3

x L

x m


0 0.2 0.4 0.60.3

0.4

0.5

0.6

0.7

0.8


1-

PETS PETS

Accelerating structure x4

135 MW250 ns

QuadDrive beam

Main beam

100 A2.4 GeV 0.24 GeV (0.87 km)Deceleration: ~6MV/m

1.2 A9 GeV 1500 GeV (21 km)Acceleration: 100 MV/m

Finally, as a result of multiple compromises, the PETS aperture a/λ = 0.46 was chosen.


Introduction

Frequency, GHz

0 0.75 1.5 2.25 31 10

3

0.01

0.1

1

10

Wx_10_t_120_1_7m

Wx_10_t_120_1_2m

Wx_10_t_120m

sm

0 15 30 45 600

100

200

300

400

0 15 30 45 60

100

200

300

400

Radial

Inclined

Flat(actual choice)

#7 Phase advance

#8 Iris thickness

#9 PETS cross-section

900

1200

3.5 mm

2.0 mm

Iris 2.0 mm

Phase/cell 1200

The lower phase advances and thinner irises favor the beam stability

Frequency, GHz

Frequency, GHz

Tra

nsve

rse

Impe

danc

e

Tra

nsve

rse

Impe

danc

eT

rans

vers

e Im

peda

nce

Frequency = 11.9942 GHzAperture = 23 mmActive length = 0.213 (34 cells) Period = 6.253 mm (900/cell) Iris thickness = 2 mmSlot width = 2.2 mm R/Q = 2222 Ω/m V group= 0.459C Q = 7200 Pt1/3/C = 13.4E surf. (135 MW)= 56 MV/m H surf. (135 MW) = 0.08 MA/m (ΔT max (240 ns, Cu) = 1.8 C0)

In its final configuration, PETS comprises eight octants separated by the damping slots. Each slot is equipped with HOM damping loads. This arrangement follows the need to provide strong damping of the transverse modes.

PETS parameters:


E max (135 MW)=56 MV/m

H max (135 MW)=0.08 MA/m

PETS regular iris shape and matching cell

To reduce the surface field concentration in the presence of the damping slot, the special profiling of the iris was introduced. As a result, compared to the circularly symmetric geometry, the 20% field amplification was achieved. The special matching cell was designed to provide the most compact and efficient connection between the regular PETS part and the RF power extractor.

PETS machining test bar

Special matching cell

Electric field

RF power density


E max (135 MW)=41.7 MV/m H max (135 MW)=0.07 MA/m

PETS RF power extractor

reflection

isolation

HFSS simulations


The PETS tolerances issues

100 50 0 50 100

0.996

0.998

0.999

20 20

P

G

t/2

Fabrication errors, micron

Pow

er, n

orm

.

P t

G

Fabrication and assembly errors can detune the PETS synchronous frequency and thus affect the power production:

2

0

022 1

2exp

4

/

L

gb dzz

ci

V

QRFIP

2R

100 50 0 50 100

0.996

0.998

0.999

Assembly errors (R), micron

-16

-15.5

-15

-14.5

-14

-13.5

-13

-12.5

-12

-11.5

-11

98 99 100 101 102 103 104 105 106 107 108 109 110 111 112

[mm]

[mm

]

-16

-15.5

-15

-14.5

-14

-13.5

-13

-12.5

-12

-11.5

-11

17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

[mm]

[mm

]

-16

-15.5

-15

-14.5

-14

-13.5

-13

-12.5

-12

-11.5

-11

180 181 182 183 184 185 186 187 188 189 190 191 192 193 194

[mm]

[mm

]

PETS machining test bar fabricated in IMP (Italy)

Metrology results translated into the frequency error:

0 10 20 300

10

20

If the hypothetical PETS will be constructed of such 8 identical bars, the expected power losses will be 0.03%.

The fabrication accuracy of ± 20 μm is sufficient enough and can be achieved with a conventional 3D milling machine.


OFF: 20 msec

ON: 20 sec

RF power contour plot

Coordinate along the PETS

ON

OFF

Wedge radial position, mm

Pow

er

norm

.

Phase

, d

eg

ree

RF phaseOutput power

12 14 160

10

20

0

0.05

0.1

Max .fields amplitudes on the wedge surface


E, M

W/m

H, M

A/m

12 14 160

0.1

0.2

0.3

0.4


Power dissipated at the slot

extremity

Pow

er

MW

12 14 16 180.5

0

0.5

1

0


Main beam acceleration

Gra

die

nt,

norm

.

OFF position

Gradient

(PETSONOV)Local termination of the RF power production in the PETS

The net RF power generated by the beam at the end of the constant impedance structure will be zero if the structure synchronous frequency is detuned by the amount ±2βc/(1-β)L, where β – Vg /c and L - length of the structure. We have found that such a strong detuning can be achieved by inserting four thin wedges through four of the eight damping slots.


0 15 30 45 600.1

1

10

100

1 103

1 104

Distance, m (log)

Wt,

V/p

C/m

/mm

(lo

g)

HFSS

Frequency, GHz

Re

(Zt)

V/A

/m/m

m (

log)

Reflectedwave

Qc

z

2exp

Wake model (single mode from HFSS)

GDFIDL PLACET – the tool to simulate beam dynamics

With slot

No slot

|E|

HE

EH

PETS transverse wake. Introduction.

12

HFSSGDFIDL

1,0)(

)1(1sin2)( )1(2

LzzW

L

ze

c

zKqzW cQ

z

a/λ=0.46

3σ b

eam

env

elop

e

Quadrupole number

Computing tools:

Moderate damping: Q(1-) ~ 20

-To provide the drive beam transportation without looses, heavy damping of the transverse HOW (Q(1-) < 10) is absolutely needed.- In periodical structures with high group velocities, the frequency of a dangerous transverse mode is rather close to the operating one. The only way to damp it is to use its symmetry properties. To do this, only longitudinal slots can be used.

In the presence of the longitudinal slots, the transverse mode field pattern is dramatically distorted so that a considerable amount of energy is now stored in the slots. The new, TEM-like nature of the mode significantly increases the group velocity, in our case from 0.42c to almost 0.7c.


PETS transverse wake damping and beam stability

With introduction of the lossy dielectric material close to the slot opening, the situation improves further. The proper choice of the load configuration with respect to the material properties makes it possible to couple the slot mode to a number of heavily loaded modes in dielectric. This gives a tool to construct the broad wakefields impedance.

The transverse wakepotential simulations in a complete PETS geometry (GDFIDL).

Snapshot of the electric field pattern at the moment when the bunch left the structure

RF load

Dielectric with moderate losses

The computer code PLACET was used to analyze the beam dynamic along the decelerator in the presence of strong deceleration and calculated wakefields. The results of the simulation clearly indicate that the suppression of the transverse wakefields obtained, is strong enough to guarantee the beam transportation without losses

With HOM

Without HOM

Q effective ~ 2

Dielectric with heavy losses

Documents

2nd CLIC Advisory Committee (CLIC-ACE), CERN January 2008 Introduction to the CLIC Power Extraction and Transfer Structure (PETS) Design. I. Syratchev