2nd clic advisory committee (clic-ace), cern january 2008 introduction to the clic power extraction...
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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
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
2nd CLIC Advisory Committee (CLIC-ACE), CERN January 2008
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
2nd CLIC Advisory Committee (CLIC-ACE), CERN January 2008
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
2nd CLIC Advisory Committee (CLIC-ACE), CERN January 2008
0 0.2 0.4 0.650
60
70
80
PETS active length, m
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
PETS active length, m
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
PETS active length, m
0 0.2 0.4 0.60.3
0.4
0.5
0.6
0.7
0.8
PETS active length, m
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.
2nd CLIC Advisory Committee (CLIC-ACE), CERN January 2008
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:
2nd CLIC Advisory Committee (CLIC-ACE), CERN January 2008
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
2nd CLIC Advisory Committee (CLIC-ACE), CERN January 2008
E max (135 MW)=41.7 MV/m H max (135 MW)=0.07 MA/m
PETS RF power extractor
reflection
isolation
HFSS simulations
2nd CLIC Advisory Committee (CLIC-ACE), CERN January 2008
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.
2nd CLIC Advisory Committee (CLIC-ACE), CERN January 2008
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
Wedge radial position, mm
E, M
W/m
H, M
A/m
12 14 160
0.1
0.2
0.3
0.4
Wedge radial position, mm
Power dissipated at the slot
extremity
Pow
er
MW
12 14 16 180.5
0
0.5
1
0
Wedge radial position, mm
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.
2nd CLIC Advisory Committee (CLIC-ACE), CERN January 2008
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.
2nd CLIC Advisory Committee (CLIC-ACE), CERN January 2008
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
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