Transcript High Voltage Laboratory

Kicker-related R&D options
Mike BARNES
Laurent DUCIMETIERE
Thomas KRAMER
Viliam SENAJ
Luc SERMEUS
Wim WETERINGS
Kicker-related R&D options
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Generator topology
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Beam induced heating
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SF6 cable, alternative
HV test Facility
Stripping foil
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Ring gate GTO, SiC, Dynistor, FID, ...
HV cables
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Surface flashover and SEY, High Curie Temp. and new Ferrite
Vacuum pumping aspects
HV switches
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Beam screening
improved cooling
Critical materials
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Adder, CLIC CR, MARX
New material, laser stripping
ANSYS analyses
Wake fields
Very High Stability Pulse Power Modulators
1st protype of (low beam
impedance, extremely high field
uniformity [±0.01%]) striplines
Prototype
inductive adder
A goal, for CLIC (DR), is to develop a kicker
system with ±12.5kV (±250A), 160ns duration
flattop, pulses with a stability of ±0.02%!!
An inductive adder, carefully designed,
constructed and using sophisticated
modulation techniques, gives promising
results:
160 ns
Trigger inputs
Vload 1.2 kV
Zoom of 160 ns flat-top
Vripple+droop 3.8 V, ±0.16 %
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High Burst-rate Power Modulator
a)
b)
c)
d)
Required for CLIC CR;
~700 kHz burst rate (~100 pulses);
10 kV, 200 A;
“Collaboration” started with SLAC
(Mark Kemp).
Q1
DC in
Q2
Initial “low” voltage prototype built to demonstrate
basic concept:
• A variant of the typical induction modulator
topology (magnetic material) and full bridge
inverter;
• Cells produce alternating positive and negative
polarity pulses, but load output is single polarity;
• Flux alternates direction in the core, enabling
pulse train output;
Q3
Cell 1
Q4
To
load
DC in
Cell 2
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High Repetition Rate Marx Generator
CLIC requires a high repetition rate (1 kHz, 12.5 kV) pulse generator for RF
breakdown studies. A system, based on a Behlke switch, delivered by FPS in
2011, generally works well and has provided very useful statistics for breakdown
studies: but the Behlke switch sometimes “dies” (the reason is not fully
understood).
A replacement system, based on Marx Generator technology, is under
discussion with Luis Redondo (Lisbon Superior Engineering Institute, Portugal):
Out+
D1
D2
D3
M1
+
Vdc
-
D4
M3
D5
M5
M7
M9
gate1
C1
C2
M2
C3
M4
C4
M6
VMarx
C5
M8
M10
gate2
The Marx Generator is
potentially a very interesting
technology for CERN – and can
provide a modular, fault tolerant,
design with the ability to “shape”
pulses.
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Shielding of Ferrite Yoke from Beam
Ferrite yoke
Heat induced
by the beam
Bea
m
• The beam coupling impedance is
reduced by providing a path for the beam
image current, which screens the ferrite
yoke from wakefields;
• Such a screen is installed between the
beam and the ferrite yoke.
1) For existing systems, where the ferrite yoke needs to be shielded from the
circulating beam, it is not generally possible to include ceramic tubes to
support conductors (space limitations in the aperture).
Interdigitated
comb structure
20 mm spacing
MKE magnets used an interdigitated
comb structure – but this cannot be
used on transmission line kicker
magnets with “short” cells. In addition
it is not as effective as, for example,
the MKI beam screen.
2) For new systems it is not necessarily desirable to include ceramic tubes to
support conductors, e.g. increased aperture size  increased electrical stress
on the pulse generator.
R&D is required to identify and develop effective beam shielding methods.
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MKI Shielding of Ferrite Yoke
The eight LHC Injection (MKIs) magnets upgraded during LS1 are NOT
expected to suffer from problems related to beam induced power deposition, of
the ferrite yoke, during run 2 of the LHC. However, based on current HL-LHC
beam parameters, additional upgrades are required, for implementation during
LS2, to avoid problems of the ferrite yoke temperature exceeding the Curie
Temperature:
 Study further means of further reducing beam induced power deposition:
SS cylinder
Ceramic
1mm gap between ceramic
tube and conducting cylinder
(return [ground] busbar side).
Screen
conductors
(graded lengths)
Loverla
p
f res 
1
Loverlap
Metallized
ceramic
End of
metallization
3mm gap between ceramic
tube and conducting
cylinder (HV busbar side)
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Cooling of Ferrite Yoke
 Study further means of heat removal from the ferrite yoke, e.g. liquid cooling of
HV busbar and/or higher emissivity of vacuum tank (see slide by Wim):
MKI vacuum tanks were
electopolished (for septa)
 emissivity of ~0.15.
Note: visible light 400nm
to 700nm wavelength.
TE-VSC have carbon coated
SS316-LN and, with a ~600
nm thick coating, achieved
emissivities of >0.8 (600nm
thick C). A method of applying
this to a “large” thank would
need to be developed (e.g. by
TE-VSC).
Plot courtesy of T.
Bardo & S. Calatroni
(TE-VSC)
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Cooling aspects
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Ansys studies of extracting (beam induced) heat.
• Benchmarking with measurements during bake-out.
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Cooling of HV conductor or other cooling options.
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HV cooling feedthrough, without H2O-Vac interface.
Surface Flashover of Ceramic
It is thought that “high” pressure caused an electrical breakdown of an MKI in the LHC.
Hence surface flashover of the ceramic tube, in a test (“Simi”) tank, is being/will be
studied as a function of:
1) Pressure of injected gas;
2) Ionization of gas;
3) Gas species (e.g. H2, H2O, CO, ….)
Very important… e.g. could result in a “common mode” failure in all 4 MKIs, at
an injection point!
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Surface Flashover of Insulators
aC
1.
4
Cr2O3
 Uncoated ceramic has a maximum Secondary Electron Yield (SEY) of ~10.
 Coating the ceramic with Cr2O3 or Amorphous Carbon (aC) can reduce the SEY
to less than 1.4 (a magic number for the LHC, with 25ns beam).
 In addition Cr2O3 has been shown, by other researchers, to increase the
voltage at which surface flashover occurs.
 An order has been placed in industry to develop a technique to apply Cr2O3 to
the inside of a 3m long (MKI) ceramic tube. A shorter (50cm) long coated tube
will be HV tested in the “Simi” tank.
 Be careful of other potential problems, e.g. dust (UFOs)
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Ferrite
Even with improved cooling of the yoke of a kicker magnet, there are advantages
(and disadvantages, such as outgassing – see slide by Wim) of using a ferrite with
a higher Curie Temperature (e.g. 180˚C, compared to ~120˚C for ferrite presently
used).
In addition future fast kickers (e.g. for the FCC) may need to provide a stronger
field, and hence ferrites with a higher saturation flux-density will be required.
Vacuum compatibility of suitable ferrites needs to be checked (by TE-VSC) and a
prototype kicker magnet built and tested.
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Backup for MKB vacuum pumping –
NEG cartridge
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Improvement of MKB pumping
Backup in case of power cut
Under development - thermal
shield – to protect magnet
during cartridge activation (350
– 400 deg)
Differential pumping
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Kicker magnet design with differential pumping.
• Separation of magnet vacuum from beam vacuum,
relaxing constrains on material for magnet construction.
• Could be an option for MKI heating issues, coating,
use of high temperature ferrites, ….
MSP Magnet (G. Schröder et al., EPAC2000, pp.e-proc.
2370)
Switch technology overview
and kicker applications
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GTO ring gate
LHC-MKD
Peak current (kA)
Thyratron
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PSB-TK
SPS-MKDV
GTO
SPS-MKDH
LHC-MKB
AD-Horn
PSB-EK
LHC-MKI
SPS-MKPi
SPS-MKP
PS-FAK
SPS-MKE
PS-TIK
1
FID
PSB-BI-DIS
IGBT
CLIC-DR
MOS
0.1
0.01
0.1
1
Rise time (us)
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100
HV switches
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Thyratron
Still widely used, large range of application
Effort to reduce misfiring (erratic) in view of high intensity ?
Test of alternative suppliers (USA, Russia) ?
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Dynistor
Almost no dynamic range (+/- fix voltage)
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FID
Tested at CERN, ESRF and BNL
Very limited range (50 W, short pulses)
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GTO
New trigger transformers
Potential progress with ring gates
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Power MOS / IGBT arrays
Commercially available, single sources, mostly custom developments
Should be developed here ?
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SiC technology (for MOS and IGBT)
Recent progress in industry for substrate of about 1 cm2
Higher voltage and lower leakage current, than Si
Pushed by automotive market
New switch technologies – Reversed SwitchOn Dynistor, Ligth Triggered Thyristors…
New trigger transformer development
• Common development for MKD and MKDV upgrade
• Compared to present MKD transformer – double the
gate peak current and triple dI/dt; ABB
recommendation 2 kA peak, 5 kA/us; presently
(Uptu=3500V) we have 500 A peak and 400 A/us
• Allow to gain ~ 200 ns in T_rise on MKD with the
same PTU voltage
• Independent magnetic circuits of individual
secondary windings allowing decoupling of the
GTOs – potentially avoiding conduction of the stack
in case of individual GTO failure
• The only solution sufficiently fast for future ring gate
MKDV switch
HV switches
Ring gate GTO
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Under evaluation
Technology for MKDV new
« quad stack » switches.
New power trigger developed
by EC
SPS MKDV switch
Stacked switches
• MOS-FET switches are faster (ns range) compare to other semiconductors
but can’t carry huge currents nor hold off high voltages.
• Simple stacking is used in industry to produce fast HV-switches.
• Limits of commercially available stacked HV-switches currently around
150kV, 200A , 300ns.
• “Unmoulded” switch shows simple schematics.
• Currently industry seems to mainly “enjoy” their past developments.
• To be improved and adapted to our applications?
HV-Cables
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Sounds straight forward but HV-cables
have proven to be an issue in the past.
Currently trying to keep voltages below 40
kV for fast pulsed systems.
Very special general CERN requirements
(IS23, radiation hard etc.)
Special applications (pulsed, EHV etc.)
Industry has certain concepts in the
drawer but adaptions always need R&D
as well.
Even concepts available in the past need
reengineering and R&D (see SF6 cables).
High risk for industry to engage in a small
and specialized market -> R&D to be
done in collaboration.
Sometimes difficult to find
facilities/equipment for tests -> HV-Lab.
Stripping Foil
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Stripping Foil Exchange unit and Foil Types.
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We will be in for a surprise !
L4 stripping foil test stand to test different holders, foil types, gluing, …
High Voltage Laboratory
Not directly R&D but an essential tool for future R&D performance.
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N. Tesla, Colorado Spring, 1899, monitoring 20 MV discharge.
867 is controlled area and
mostly dedicated to used
Systems
Need for dedicated R&D lab in
non controlled area is evident
(e.g. R&D on high gradient
insulators, surface flash over,
benchmark of simulations, etc.).
Space anyway needed (also for
acceptance tests).
Centralize and manage existing
equipment.
Upgrade equipment for
particular measurements.
“Chief of lab” needed (safety).
Dignified infrastructure could be
made available for outside ABT
and even outside CERN.
Use of Wake Fields for kicker
applications?
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Maybe far fetched from today’s point of view.
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However achieved results in this new acceleration
technology are tempting concerning gradients and
pulse duration.
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Could it be used for e.g. an extremely fast chopper
or dump kicker in “low” energy machines?
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First ideas for kickers using the Dielectric Wakefield
Accelerator principle were already presented at
IPAC (30 cm, 1 GeV e- , 1 mrad, 1 ns).