Transcript Slides - Indico

Klystrons
C Lingwood, Lancaster University/Cockcroft Institute
on behalf of HEIKA (High Efficiency International Klystron Activity)
Introduction
• Increase in RF generation efficiency is high priority for the
future large accelerators (CLIC, ILC, FCC, ESS)
• “Recent” klystron developments targeted high power
neglecting high efficiency
• Few high power klystrons offer 65%+ efficiency
• Fewer with long pulse length
• Proton machines can piggy back on developments for large
projects
Future Machines…large numbers!
•
linear
•
0.5 TeV
ILC e+e-: Pulsed, 1.3 GHz, PRF total= 88 MW
3.0 TeV
CLIC
circular
e+e-:
FCC
FCC ee: CW, 0.8 GHz, PRF total= 110 MW
Pulsed, 1.0 GHz, PRF total= 180 MW
Achieved efficiency at 10
MW peak RF power level
The existing MBK klystrons
provide efficiency very close
to 70%
Operating Principal
(Velocity modulation)
• DC beam passes through input
• Electrons accelerated or decelerated
according by the gap voltage
• More cavities give better bunching.
– (Very) Roughly ¼ plasma
wavelength apart
– Beam is velocity modulated
• Bunches of electrons are formed
– Beam is spatially modulated
• Output cavity is excited by the bunches
• Power is coupled out to load
Cathode
(DC Beam)
RF input
RF output
Collector
Thanks M Jensen
Traditional Approach
• For high efficiency traditionally we chase
𝐼
low perveance:
𝑝𝑒𝑟𝑣𝑒𝑎𝑛𝑐𝑒 𝐾 =
– High voltages
𝑉 3/2
or
– Low currents (many beams)
• For high power both become “unpleasant”.
• Performance limited by the slowest electrons (must
avoid reflecting electrons)
• Traditional theoretical efficiency limited to 80% @ 0.10.2ish microperv
Klystron Technology Limitations
(Frequency)
• Low frequency klystrons are BIG
• Lower bound is something like
300 MHz
• Depends how many are
needed
• How much space you have
• If something else works…
• ESS have 352 MHz 2.8 MW
klystrons
• Tetrode might be nice…
• Upper bound need not concern
us
Klystron Technology Limitations
(Power)
•
•
Pulsed power can go very high
~3-4 ms is more or less CW (ish)
•
Limited by output window to
around 1 MW average
• (Toshiba sell 1.2 MW CW at
~1.2GHz)
•
More windows = more power =
more complexity = more cost
•
Convert from short pulse tube to
long pulse
• Bigger collector
• Windows
Why not 100% Efficient
• The simple answer is
– Imperfect bunching
– Residual Velocity – energy
still in the beam
Bunching monotonic – electrons move to center of bunch
Significant charge outside
bunch. Velocities aligned…
Many electrons miss bunch.
Significant energy left in
Collector depression
•
Decelerate the electrons into the collector to recover energy
•
Efficiency increases with number of stages
•
Hard to cool electrodes
•
Adds to the complexity and cost of the tube
•
Easier to just to design the previous parts better
The (massive) problem with
protons
• Unlike electron machines, need a ramp in power
– heavy so not relativistic at lower energies
• Range of powers of sources
• End up underrating tubes
• Also control overhead is
Needed so tubes cannot be
run at saturation
ESS Power per Cavity, thanks ESS, M Jensen
Underrating
•
• Constant impedance
• Constant voltage
• Manipulate current with mod
anode
• Constant perveance
• Drop modulator voltage (easy)
• Constant voltage and current
• Manipulate input power
Mod annode
• Allows you to run at a lower output
power
• Adds ceramic to copper brazes
• Risk of arcing + reduced reliability
• “Better” to just reduce operating
voltage.
12
Anything else we can do?
• If you drop the voltage you change the reduced plasma wavelength
• Cavities are no longer well spaced so efficiency reduced
• Change the output Q to extract more power and restore some efficiency
Unstable
Thanks to C Marrelli, ESS
State of the art?
• ESS most recently acquiring long pulse tubes
– 352 MHz
– 704 MHz
• Can argue this is more or less the proton linac klystron state of
the art
ESS 352 MHz Klystron
•
•
•
•
•
•
Frequency: 352MHz
Output power: 2.8 MW
Voltage: 108 kV
Current: 46.5 A
Pluse width (catalogue 1.5ms): 3.5 ms
Efficiency: 53%
ESS 704 MHz Klystron
Nominal output power
1.5 MW
Frequency
704.42 MHz
BW
≥ +/- 1 MHz
Pulse width
3.5 ms
Repetition rate
14 Hz
Perveance
0.6*10-6
Efficiency
>60%
VSWR
Up to 1.2
Power Gain
≥ 40 dB
Group Delay
≤ 250 ns
Harmonic Spectral content
≤ -30 dBc
Spurious Spectral content
≤ -60 dBc
Thanks Chiara Marrelli
•
•
Three prototypes are being procured, from three different manufacturers
(Thales, Toshiba and CPI)
Delivery expected in March (Thales), May (Toshiba) and July (CPI) 2016
Other power requirements
• Klystron focusing magnet
– < 20kW often <10kW
– At 1 MW -> 0.1% (very hard to care)
• Cooling?
• Control and HV a modulator issue.
Potential Performance
• Deeper understanding of the klystron physics, new ideas and
modern computational power will help us towards 90%
efficiency
• HEIKA collaboration of many experts working towards this
• For protons most tubes under active development are either
– Wrong frequency
– Short pulse
• All is not lost technology should transfer
– Frequency scaling is ok (so long as we don’t want Hz or THz)
– Pulse length is down to heat dissipation
Method to get high efficiency
Core Oscillation (space charge debunching)
• Bunching split into two distinct regimes:
– non-monotonic: core of the bunch periodically contract and expand (in time)
around center of the bunch
– outsiders monotonically go to the center of the bunch
• Core experiences higher space charge forces which naturally debunch
• Outsiders have larger phase shift as space charge forces are small
• Very long, very efficient tubes result.
Traditional bunching
Core oscillations
Phase
Core oscillations
Space
Cavity
Cavity
Cavity
Cavity
Cavity
Traditional approach
90% Efficient Klystron
• Efficiency increases with number of core oscillations and reaches 88-90% for 45 oscillations
Methods to get high efficiency
BAC Method (I. Guzilov)
• Again based on core oscillations
• Interaction space is wasted “waiting” for space charge forces to
debunch.
• A cavity can achieve the same thing in a shorter space by aligning
electron velocities
• Structure half the length while maintaining efficiency.
Thanks to SLAC
Electron velocity/density
Process in the high efficiency
klystron (bunch rotation)
The fully saturated (FS) bunch
Final compression and bunch
rotation prepare congregating
FS bunch.
After deceleration all the
electrons have identical
velocities.
Mission accomplished
HEKCW Tube
16 beams MBK cavity
R/Q = 22 Ohm/beam
Tube parameters:
•
•
•
•
•
•
•
1.5MW
Voltage: 46 kV
Total current: 36A
N beams: 16
µK/beamx106 : 0.213
N cavities: 8
Bunching method #1: COM
3.54m
HEKCW
HEIKA/HEKCW working team:
I. I. Syratchev (CERN)
II. C. Lingwood (Lancaster)
III. G. Burt (Lancaster)
IV. D. Constable (Lancaster)
V. V. Hill (Lancaster)
VI. R. Marchesin (Thales)
VII. Q. Vuillemin (Thales/CERN)
VIII. A. Baikov (MUFA)
IX. I. Guzilov (VDBT)
X. C. Marrelli (ESS)
XI. R. Kowalczyk (L-3com)
Magic HEKCW #08-03
• Cavity 1 voltage, 0.83 kV:
• Nice Stable output
• No reflected electrons
• Slightly odd modulation current
• Stable in cavity 7 signal.
• Efficiency…..
Output power
83%
Cavity 8 Volts
Cavity 7 Volts
Magic HEKCW #08-03
Electron Animations
• Bunch ”bounces” in output
gap
• Superficially nightmarish
phase space
• Good chunk decelerated
well though
• Quite a good slab bunch
• but not great in at r=0
Cavity 7
Cavity
8
Onwards!
HEIKA Time Scales
• 0.5-1 years further R&D to get to optimal MBK
• 1.5 years technical design to build
• “Igor’s wish”: Test 2018
• If all goes well and it is funded.
Other prospects
5045 Retrofit
SLAC, A. Jensen
•
•
•
•
•
•
Retrofitting a 5045 S-Band) with BAC
60 -> 80MW
45% -> 55%
4 more cavities
Plug compatible (needs new solenoid)
X Band Short pulse
• Results will be reported at IVEC in April, 2016
S-Band Hardware Development
VDBT I. GUZILOV
The first commercial S‐band MB tube
employs the new bunching technology:
• 40 beams
• Permanent Magnets focusing system
• Low voltage: 52 kV
• Peak power: 7.5 MW
• Efficiency: 77% (in simulations)
• Pulse length: 5 microsecond
• Repetition rate: 300 Hz
• Average power: 30 kW
Kladistron (Adiabatic Bunching)
CEA, A. Mollard
•
•
•
•
•
Bunch slowly to keep velocity spread small
Many cavities
Retrofit a TH2166 (5GHz, 56kW, 26kV) as proof of principal
50% -> 55% in preliminary redesign
Soon ordering some components
Performance Figures
(well it depends)
Health warning: some numbers mutually exclusive
RF source type Gain
[db]
Maximum
output
power pulsed
[kW]
Rise
time
[us]
Pulse
length
range
[ms]
Rep
rate
range
[Hz]
Max.
output
power
CW
[kW]
Efficiency at
working
point
[%]
High
voltage
needs
[kV]
Frequency
range
[MHz]
Single Beam
40-50 1000-3000
<<1
(~300
ns)
-4
<1200
kW
55 (65 max) ~90120kV
.3GHz-1.5GHz
MBK
40-50 10,000-15,000 <<1
(up to 1.5ms at (~300
least)
ns)
-4
<1200k
W
(no
point)
60 (70 max)
~90120kV
.3GHz-1.5GHz
Future Single
Beam
-
-
-
-
-
-
70+
40-60 kV .3GHz-1.5GHz
Future MBK
-
-
-
-
-
-
80+
40-60 kV .3GHz-1.5GHz
Cost: 300k€ - 1M€ depending on complexity, number, novelty
Conclusion and Outlook
• Using new bunching theory 80%+ looks possible for FCC/CLIC/ESS/ILC
klystrons
– No new materials or manufacturing techniques needed
– Little additional complexity
– Simply existing technology reconfigured
– Prototypes for proof of concept in progress
– Lower voltages combined with high efficiency appears achievable
• 83% Achieved in PIC
• 90% and stable in PIC so far elusive, but not ruled out
• Prototypes and further validation required
• Lessons learned directly applicable to proton driver tubes
• International collaboration at work