13 th Advanced Accelerator Concepts Workshop.
description
Transcript of 13 th Advanced Accelerator Concepts Workshop.
13th Advanced Accelerator Concepts Workshop. Santa Cruz, July 27-August 2, 2008
Seventh-Harmonic Multi-MW K-band Frequency Multiplier: Rf Source for
for High-Gradient Tests*
N.A. Solyak1,2, V.P. Yakovlev1,2, S.Yu. Kazakov1,3, and J.L. Hirshfield1,4
1Omega-P, Inc., New Haven, CT 06511, USA 2Fermi National Accelerator Laboratory, Batavia, IL, USA
3High Energy Accelerator Research Organization (KEK), Tsukuba, Japan 4Physics Department, Yale University, New Haven, CT 06520, USA
*Work supported by the U.S. Department of Energy
Motivation:
● In order to investigate the frequency scaling of breakdown, high power tests are necessary at frequencies from X-band upwards.
● Key elements for these tests are high power, 0.5-1 sec pulsed RF sources.
● A“ quick-and-cheap” high-power RF source, an Harmonic Multiplier, is proposed for this application and described in this talk.
Seventh-harmonic 20 GHz harmonic multiplier:
● Simple two-cavity structure, with room-temperature solenoid
● Uses hardware available at Yale Beam Lab.
the gun built for the CARA accelerator, the SLAC XK-5 S-band klystron as the RF drivermodulators
driver: 2.856 GHz + Δf into TE111 mode,output: 19.992 GHz + 7Δf out in TE711 mode.
Note that phase stability is comparable to that of the 2.856 GHz klystron driver, and that technology (gun, modulator) is already proven.
XK5 S-band klystron and overhead WR-284 evacuated transmission line with variable amplitude and phase splitters in the Yale Beam Physics Laboratory. Also seen is the 300-kV gun modulator and gun tank.
Gun tank, portion of magnetic system, and quadrature WR-284 feeds into CARA accelerator in the Yale Beam Physics Laboratory that can be available for the proposed harmonic multiplier.
65-MW modulator that powers the S-band XK-5 klystron in the Yale Beam Physics Laboratory. Klystron and gun modulator are visible in the background.
20 GHz, waveguide 7th harmonic multiplier [J.L. Hirshfield, et al, 2000]:
Sketch of the proposed two-cavity 7nd harmonic frequency multiplier based on the S-band SLAC:
Drive frequency: 2.856 GHzOutput frequency: 19.992 GHzDrive power: 8.5-10 MWOutput power: 4-4.7 MWMultiplication efficiency: 47%Beam power: 5 MWBeam voltage: 250 kVBeam current: 20 AOperating mode TE111 (drive cavity)Operating mode TE711 (output cavity)
Two-gap gun:
The electron gun is a 100 kV, 10-6 A–V-3/2, Pierce type diode gun with a 200 kV post-acceleration stage. The ratio of the cathode voltage to the intermediate anode voltage can be varied by approximately 10% to allow flexibility in the voltage/current characteristics of the output beam (i.e. the beam current is not entirely determined by the terminal voltage).
Beam power: 5 MWBeam voltage: 250 kVBeam current: 20 ABeam perveance 0.16 ×10-6 A–V-3/2
Cathode diameter 50.8 mmBeam diameter in the drive section 2.5 mmBeam transverse compression ratio: 400:1
Magnetic circuit configuration, with 5 coils and 4 iron pole pieces. Plot at top shows the magnetic field profile generated by this circuit. Drawing at bottom shows drive cavity (left) and output cavity (right).
Beam particle energies (blue) and radial excursions (red) with the TE711 mode output cavity tuned to 19.992 GHz. Cavity outlines are also shown.
The drive cavity:• Operating frequency - 2.856 GHz• Operating mode - rotating TE111
• Loaded Q - 150• Maximal electric field - 58 kV/cm.
• Rotating TE111 wave is excited by two standard waveguides placed by 90 deg of each other in azimuthal direction.
• Two compensating protrusions to restore field symmetry
Drive Cavity Design
Electric field
Magnetic field
Output Cavity
Cavity parameters:
– Operating frequency, GHz – 19.992 (7x2.856 GHz);– Operating mode - rotating TE711 – Loaded Q - 900
– Maximal electric field -175 kV/cm.
Concept of output cavity coupler
At least two waveguide are necessary to load both polarizations. Unfortunately, such a scheme doesn’t work for the modes with high azimuthal index:
•Large perturbation of field due to coupling slots •Coupling to other modes
Problem of power input / output from the cavity with rotating wave is important for many high power devices for accelerator applications
Solution is well-known* – azimuthally distributed coupling•waveguide runs around cavity and coupled through many slots•Wave phase velocity in WG and azimuthal location of coupling
slots have to be chosen accordingly to azimuthal index in cavity to excite travelling wave
*Suggested by N. Solyak at 1985 for 7 GHz , 60 MW gyrotron.
High power RF devices based on such approach:
Output cavity
of the 7GHz, 60 MW Gyrocon (frequency multiplier)
(BINP,1985, N.Solyak)
Pulse compressors
● 11.4 GHz, 135 MW (BINP/KEK, 1991)
● 3 GHz, 80 MW
(CERN,1998,I.Syrachev)
KlystronProposal for CLIC 937 MHz, 50 MW MBK design (CERN, 2005, I.Syrachev et al.,)
1-RF cavity; 2-collector3-collector; 4-coupling holes.
The output cavity with incorporated mini-windows.
Rotating TE711 mode in output cavity is coupled thru the series of coupling slots to waveguide. The amplitude of rotating mode in cavity is equal:
Ncav – cavity azimuthal index - azimuthal coordinate (angle).
Each coupling slot will excite waveguide with the amplitude:
- coupling between cavity and WG i =2i/Nslot - angular position of each slot
In waveguide two waves are excited: direct wave D (running in the same direction as rotating mode in cavity) and reverse wave R (opposite directions). The amplitudes of these modes are as follow:
Nwg - WG azimuthal index
Nslot - number of slots
)(0)( cavNtieAA
)( iw AA
slot
slotwgcav
wg
N
m
N
mNNi
Ntiwg eeAND
1
2)()(
0)(
slot
slotwgcav
wg
N
m
N
mNNi
Ntiwg eeANR
1
2)()(
0)(
Theory of traveling wave coupling
Waveguide dimensions
Waveguide dimensions is defined from equation:
0=c/f – free space wavelength, c- critical wavelength in coaxial waveguide, which is defined from the equation:
Here a,b – waveguide radial dimensions, k=Nwg – azimuthal index.
IndexNwg
WG width b=27mm
WG widthb=28mm
6 9.35451 9.247367 10.45173 10.245868 12.36498 11.919079 16.68916 15.35814
10 57.00037 29.70131
02222
c
kc
kc
kc
k
aY
bJ
bY
aJ
20
0
)/(1
2/
c
wh
WG width for a=21mm (inner radius)
Width of the waveguide vs. b (a=21mm)
810
121416
182022
2426
23 24 25 26 27 28 29 30 31
b, mm
h_w
g,
mm
Nwg=6Nwg=7Nwg=8Nwg=9Nwg=10
wh
a b
(gradient limit) 6 < Nwg < 10 (dispersion)
Number of coupling
slots
Nwgfor Reverse
wave
Nwgfor
Direct wave
Ratio D:R(One slot is
missing)
Ratio D:R(Two slots
are missing)
5 8 7 4:1 2:16 5 7 5:1 4:18 9 7 7:1 6:1.4149 2 7 8:1 7:0.34710 3 7 9:1 8:0.6211 4 7 10:1 9:1.3112 5 7 11:1 10:1.73213 6 7 12:1 11:1.94215 8 7 14:1 13:1.95616 9 7 15:1 14:1.84821 7 20:1
28 7 26:0 D - amplitude of a direct wave
R - amplitude of a reverse wave.
Ncav = 7 (Considering 6 < Nwg < 10 only)
Parameters for excitation Direct or Reverse traveling Waves
Two configurations: 7-8-9 and 7-28-7
CASE 1: Configuration 7-8-9
Traveling waves in Cavity and Waveguide are rotating in opposite directions
CASE 2: Configuration 7-28-7
Direction of rotating of TW in cavity and waveguide are the same
Configuration: Ncav-Nslot-Nwg
Inner radius, amm
Outer radius, bmm
Waveguide Widthhwg, mm
21 26 18.479
21 27 16.689
21 28 15.358
21 29 14.316
21 30 13.474
Table: Waveguide dimensions for 7-8-9 configuration
Red line shows dimensions, which are close to standard waveguide - WR51: width x height = 12.954 x 6.477 mm. It will be easy to mach them.
S13 (wg-to-cavity) vs. Frequency
0
0.05
0.1
0.15
0.2
0.25
19.95 20 20.05 20.1 20.15 20.2 20.25
Freq. GHz
S13
Eight slots, size = 15.358 mm x 2 degrees; Frequency = 20 GHz, Q=400 (over-coupling)
Configuration 7-8-9 (HFSS simulations)
• Good directivity (Small amplitude of direct wave)
• Some transformation to quadrupole mode, which is not trapped in cavity
Magnitude of electric field in cavity and waveguide at 0 phase.
Case 7-8-9: HFSS simulations (2)
Slot/2= 6 mm x 3º.
Freq = 20 GHz.
Source : WG port #1.
• Half-cavity with the magnetic boundary conditions on plane of symmetry.• Ports on the waveguide• radiation boundary conditions at the end of beam-pipe
Dipole mode
Configuration 7-28-7
Inner radius, amm
Outer radius, b
mm
Waveguide Widthhwg, mm
21 24 11.217429
21 25 10.931234
21 26 10.678429
21 27 10.451725
21 28 10.245861
21 29 10.057099
21 30 9.882853
Waveguide dimensions for 7-28-7 configuration
WR51: 0.51”x 0.255” = 12.954 mm x 6.477 mm.
WR42: 0.42” x 0.17” = 10.688 mm x 4.318 mm
Configuration 7-28-7: Cavity parametersS11 phase vs. Freq
Case 7-28-7. Slot 4mm x 2 deg
-150
-100
-50
0
50
100
150
19.97 19.98 19.99 20 20.01 20.02
Freq, GHz
Ph
as
e,
de
g
Frequency = 19.995 GHzQ_ext = 909Waveguide dimensions:
•Rmin= 21 mm •Rmax= 27 mm•Width = 10.45 mm •Slot size = 4mm x 2
• Fig. shows half-cavity
• Cavity is exited by port#1.
• Complex amplitude of the electric field
• Good quality of traveling wave
Compensating Coupling protrusions slots
No propagating modes!
Waveguide matching to standard WR51
a = (0.1, 27.762); b = (5.536, 20.284); c = (9.603, 25.228); d = (6.578, 28.953)
Dimensions of matching section in mm:
S11 was matched at the level of 0.03 at the broad band
WR51: 0.51”x 0.255” = 12.954 mm x 6.477 mm
Layout #1 of the output cavity inside magnetic system.
General Layout of the output cavity in 7th harmonic converter
Layout#2 Waveguides are penetrating through the gap between magnetic
(blue-magnetic coil, grey – cavity, red – beampipe, magenta – waveguide).
Conclusions
● Through use of 8.5-10 MW S-band drive power from SLAC XK-5 klystron and the beam from the CARA gun, preliminary simulation results indicate that a simple two-cavity 7th harmonic multiplier can be designed and built to furnish ~4-5 MW of phase-stable RF power at 20 GHz for use in high gradient accelerator R&D.
● Theory is developed of the azimuthally-distributed coupling of the cavity with rotating whispering-gallery mode to the rectangular waveguide. Basing on this theory, the design of the output cavity with 28 coupling slots for the 7th harmonic multiplier is developed.
● The preliminary design of the 7th harmonic multiplier is made.