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AN ASSESSMENT ON KLYSTRON MODULATOR TOPOLOGIES FOR
THE ESS PROJECT
C. A. Martins(*), K. Rathsman(**)
(*)
- Laval University, Dept. of Electrical and Computers Engineering, Québec, QC, Canada
- European Spallation Source (ESS), Accelerator Division, PO Box 176, Lund, Sweden
(**)
Abstract
The European Spallation Source (ESS) is a joint
European project. ESS will be a world-leading centre for
materials research and life science with neutrons and will
host the World’s most powerful neutron source. A
superconducting LINAC will be required to accelerate
proton beams at energy levels up-to 2.5 GeV. About 200
klystrons will be required which will be supplied
individually by pulsed modulators of the solid state type.
The nominal parameters for the initial test stand prototype
are -115kV/21A, 3.5ms/14Hz.
The state of the art of solid state pulsed klystron
modulators suited for long pulse applications (>100 µs) is
reported. This will include a descriptive study of the
different solutions envisaged which are based on the
following topologies: a)- Monolithic pulse transformer
based; b)- Direct switch; c)- Marx generator; d)Interleaved multi-level converters association; e)- Multilevel resonant converters association.
These topologies will be qualitatively assessed on the
context of the ESS project taking items like reliability,
maintainability, reparability, efficiency, and safety as
comparative criteria.
LINAC [2] delivers 5 MW of power to the target at 2500
MeV, with a nominal current of 50 mA. The reliability is
one of the most challenging parameters to achieve, with a
target value of at least 95% for the whole physics
complex. This will translate directly on reliability
requirements for the different LINAC equipment, like
klystron modulators. Redundant operation schemes will
be defined at the levels of cavities, klystrons and
modulators. The maximum allowed beam losses (1W/m)
is another demanding parameter, particularly for the low
energy sections.
The LINAC main parameters and layout are resumed in
Table 1 and Figure 1, respectively.
Table 1. Key parameters of the ESS Linac
Particle species
protons
Proton energy range
1 to 2.5 GeV
Pulse frequency range
10 to 20 Hz
Pulse length range
0.8 to 3.5 ms
Beam power, nominal
5 MW
Beam on target availability
> 95 %
Beam loss
~ 1 W/m
I. THE EUROPEAN SPALLATION
SOURCE PROJECT
The European Spallations Source (ESS) [1] is a joint
European project, presently formed by 17 European
member states, intended to be an international facility
dedicated to applied scientific research in several fields
like chemistry, nano and energy technology,
environmental engineering, foodstuff, bioscience,
pharmaceuticals, IT, materials and engineering science,
archaeology where neutron beams will be used to probe
various materials. Due to their unique properties neutrons
are gentle probes that can penetrate deep into materials
without causing damage, providing detailed information
on crystallographic structure, atomic and molecular
dynamics and magnetic properties.
II. THE ESS SUPERCONDUCTING LINAC
A high current superconducting proton LINAC, with an
approximate length of 450 m, will be used to accelerate
beams of protons which will be projected into a fixed
target, generating neutrons from the interaction. The
Figure 1. Sections of the ESS Linac
III.
MAIN PARAMETERS OF THE ESS
KLYSTRON MODULATORS
The main electrical parameters of the ESS klystron
modulators are reported in Table 2. It is estimated that a
total quantity of 200 units will be required for the whole
LINAC.
The rise/fall times will have an impact on the energy
lost before and after the flat-top (cavities are started to be
filled only once the voltage pulse is flat within the
required precision) and shall therefore not exceed 10% of
the pulse length.
Droop and low frequency oscillations can be easily
compensated by the LLRF in closed feedback loop.
However, in order to reduce the amount of energy
dissipated in the klystron collector this value shall be
within 1% of the voltage pulse amplitude.
High Frequency ripple (above 1 kHz) cannot be
compensated by the LLRF due to its relatively low
regulation bandwidth and is therefore specified to be
lower than 0.02% (200 ppm). Feedfoward compensation
techniques at the LLRF are avoided since the
reproducibility of HF ripple is very difficult to quantify in
such high voltage, high power and fast power supply
systems. Indeed, such ripple depends a lot on the topology
itself, on the high voltage measurement noise and on the
jitter introduced by the solid state switches and their
driving electronics.
The maximum energy that can be deposited in a
klystron arc by the modulator is 10J, considering a typical
arc voltage of 50V. This will allow for a safety margin
with respect to the maximal energy specified by the
klystron manufacturers (20J).
Depending on the topology, some klystron modulators
may generate a temporary reverse (positive) voltage on
the klystron cathode at the end of the pulse (voltage swing
due to demagnetisation of pulse transformers). The
maximum repetitive reverse peak voltage shall not exceed
10% of the nominal pulse voltage amplitude (typical
value allowed by klystron manufacturers is 20%).
Table 2. Klystron modulator’s nominal parameters
Pulse width (50% amplitude)
3.5 ms
Flat-top duration
3.2 ms
Rise/fall times (0..99% / 100..10%)
<250 µs
Droop or oscillations on flat-top (low <1 %
freq.,< 1kHz)
HF ripple at flat-top (> 1 kHz)
<0.02 %
Repetition rate
14 Hz
Pulse voltage
115 kV
Pulse current
21 A
Pulse power
2.42 MW
Average power
120 kW
Efficiency
>90%
Maximum energy in case of arc
10J
Maximum reverse cathode voltage
10%
IV.
SOLID STATE KLYSTRON
MODULATOR TOPOLOGIES
The powering strategy for the ESS LINAC klystrons
was settled on the basis of two main principles: 1)klystrons are of the pulsed type (no mode anode
terminal), therefore less expensive and more reliable; 2)only solid state pulsed modulators, with integrated arc
protection capability excluding HV tube crowbars, were
to be considered.
The first four topologies reported herein were derived
from a literature overview of the state of the art for long
pulse applications. The last one (E) is a new proposal
based on a modified version of the SNS klystron
modulators’ topology [3].
A. Monolithic Pulse Transformer based
The simplified schematic of the monolithic pulse
transformer based topology [3] – Reass, W.A.; Doss, J.D.;
Gribble, R.F.; “A 1 megawatt polyphase boost convertermodulator for klystron pulse application”, IEEE Conf. on
Pulsed Power Plasma Science, 17-22 June 2001;
[4][5][6] is presented in Figure 2.
Figure 2. Monolithic Pulse Transformer based topology
Principle
A medium voltage capacitor bank is charged through a
capacitor charger prior to the pulse forming. The output
HV pulse is formed by switching on the HV solid state
switch assembly which connects the capacitor bank to a
pulse transformer (PT).
The PT rises the voltage up-to the level required by the
klystron and its secondary winding (output) is directly
connected between the klystron cathode and body (ground
potential).
A passive LC resonant bouncer system compensates for
the voltage droop, both due to the main capacitor bank
discharge over time and to the PT internal voltage droop
across the primary winding.
A damping circuit is used to demagnetize the
transformer at the end of each pulse, by dissipating the
magnetization energy into a resistor. A non-linear voltage
clamping system is usually required in order to guarantee
that the maximum reverse voltage remains below a
specified limit even in such cases where the main switch
is open, following a klystron arc event.
Advantages
The power circuit is simple and reliable.
All electronic active devices are at a medium-voltage
level (transformer primary side) and placed inside an air
insulated cabinet, therefore with easy access for
maintenance and repair. Only the pulse transformer, the
output voltage and current sensors (all passive devices)
need to be placed inside an oil tank.
In case of arc, the dI/dt is limited by the leakage
inductance of the pulse transformer, which gives time
enough to open the solid state switch (may be rather slow:
~10 µs total switch off-time) without excessive
overcurrent and arc energy.
Since the droop compensation system is based on a
passive LC resonant circuit, high-frequency (HF) voltage
ripple on the flat-top is inexistent (no HF switch mode
power devices are used within the pulse forming loop).
Hard points / limitations
The design and construction of the pulse transformer
B. Direct Switch
At the end of the pulse, the HV direct switch is opened
again and the charging process restarts. The same switch
also stops the pulse in case of klystron arcing. To that
purpose, a small inductance is connected in series which
limits the dI/dt in case of arcing, therefore limiting the
current increase during the switch-off delay time.
A voltage droop compensation system (not shown) shall
be accounted for. Such system could be based on the
same LC resonant bouncer circuit as in the former
topology or it could be based on a fast switch mode power
supply (active bouncer) with closed loop feedback.
DRIVER
KLYSTRON
HEAD
DRIVER
-
KLYSTRON OIL TANK
dI/dt LIMIT
HV CABLE
HV CABLE
CAP. BANK & PULSER OIL TANK
HV DIRECT SWITCH
CAP. BANK
CAP. CHARGER
AIR INSULATED
CABINET
OIL TANK
are complex tasks due to the simultaneous presence of the
following three key factors,:- high voltage insulation
(insulating materials, isolation distances); fast pulses
(core materials, leakage inductances); The correct
management of these aspects requires specific
engineering expertise and a long record of practical
experience. Manufacturing of such devices at an industrial
scale is single source.
The fully controllable HV solid state switch, being a
master piece in the system (their reliability is a major
concern particularly for arc protection), are difficult to
find at an industrial level (association of several power
semiconductors in series is still not a standard technique).
Even so, at least 3 industrial companies specialised in the
field and capable of producing such assemblies exist
worldwide.
As the pulse length and the average power requirements
increase, the pulse transformers will become considerably
or prohibitively bulky and expensive. Furthermore,
rise/fall times will increase and may lead to a
considerable impact on the global modulator + klystron
efficiency.
The LC resonant bouncer volume and cost (particularly
those of the bouncer inductor) will become bulky and
costly as the pulse length and average power increases,
with still poor performance (open loop compensation
system). Note that in an optimal design [5], the oscillation
period of the bouncer shall be around 6 times higher than
the pulse length (=60⁰; : compensation angle) and the
bouncer peak current shall be twice the pulse current
amplitude, taken at the primary side.
Finally, the reverse voltage on the klystron to
demagnetize the pulse transformer may be a strong
limitation to the duty cycle increase. Indeed, for a
complete reset of the core flux, the forward V.s value of a
pulse shall be equal to the reverse V.s. As the forward V.s
increases (either by increasing the pulse voltage
amplitude or the pulse length or both) the reverse V.s has
to increase identically. Hence, the reverse voltage, the
reverse time or both will have to increase, but still the
maximum reverse voltage is limited to 10% and the
maximum reverse time is limited by the pulse repetition
rate (off-time between pulses). An active bouncer system
(fast switch mode power supply) could be an alternative
that would avoid such constraints.
KLYSTRON
BODY
+
Figure 3. Direct Switch topology
Advantages
Since no pulse transformers are needed, very fast
rise/fall times are possible.
It is easily adaptable to a very large range of pulse
lengths and pulse repetition rate requirements.
No reverse voltage is generated on the klystron. The HF
ripple depends solely on the droop compensation
technique adopted (might be zero if a passive resonant
bouncer is used, for instance).
Since the majority of power parts are in oil, a compact
solution can be obtained.
Hard points / limitations
All power components are in oil (longer time access for
repair, large quantities of oil is required).
The reliability in arc protection is dependent on the
reliability of the HV direct switch.
The effectiveness of the droop compensation system
has still to be proven in large average power and
repetition rate applications.
High voltage (up-to ~100kV) IGBT assembly
technology for these levels of power ratings is very
specific at an industrial scale and single source.
C. Marx generator
The Direct switch topology is depicted in Figure 3 [7].
The schematic of the solid state Marx topology [8][9] is
shown in Figure 4.
Principle
A capacitor bank is charged directly at the klystron
voltage level by a HV capacitor charger power supply. To
form the pulse, the HV capacitor bank is connected
directly to the klystron by closing the HV solid state
direct switch. Such switch is formed by a series
connection of medium-voltage power semiconductor
devices like IGBT’s.
Principle
A number of N capacitor banks are pre-charged in
parallel from a single medium voltage capacitor charger
(typically ~10 kV). During this state, all switches T1’..TN’
are closed and all switches T1..TN are open.
In order to generate the HV pulse, switches T1’..TN’ are
open and simultaneously switches T1..TN are closed. All
capacitor banks are thus connected in series during the
pulse forming period; therefore the output voltage is N x
VC; (N: number of marx cells, VC: Voltage across each
capacitor). Inductors for dI/dt limitation in case of arcing
are added within each cell, in series with the discharging
switches T1..TN.
At each cell, DC/DC converters are used to power the
driver electronics. These DC/DC converters are fed from
small capacitors connected in the same way as the main
capacitor banks (therefore forming a mini-marx
generator) and following the same charge/discharge
cycles [9].
A droop compensation system (not shown) may be
either formed by a single fast DC/DC switch mode power
supply (vernier regulator) located in series with cell #1 on
the low potential side; or formed by N fast DC/DC
regulators (PWM choppers) distributed within the power
circuit, one located per cell. In the first case, a global
regulation of the output voltage is performed whereas in
the second case individual regulation of the voltage
within each cell is obtained.
Advanced modelling using Finite Element Analysis
(FEA) and shielding techniques are required.
The long term reliability of the solid state switches
under such circumstances is still to be proven.
In case of arcing, all discharging switches (T1..TN) shall
open. If one fails and is kept closed, the amount of energy
stored in the associated cell capacitor will be dissipated
into the arc. Reliability in protecting the klystron in case
of arcing is therefore divided by the number of cells, N.
The active droop compensation system may generate
significant HF ripple, well above a few hundreds ppm
level. Furthermore, depending on the controls strategy
implemented, the pattern of such ripple may be randomly
variable (reproducibility concern), caused by asymmetries
and bad synchronisation between the different PWM
choppers (one existing per cell).
D. Interleaved Multi–level Sub-converters
A topology based on the association of several
interleaved DC/DC sub-converters in series at a medium
voltage level is shown in Figure 5 [10]. A pulse
transformer is still required.
Figure 4. Marx generator topology
Advantages
Since no pulse transformers are used, very fast rise/fall
times are possible.
A compact mechanical layout can be achieved,
particularly if “field shaping elements” are used [9];
Since the design is oil free, no oil maintenance and
security issues need to be accounted for.
A very high efficiency (>95%) can be achieved.
The topology and the mechanical layout are modular
with easy access to components for repair. A full cell can
be exchanged completely as a single rack. Redundant
spare cells may be added to increase availability.
The energy stored in the system is segmented in N
independent capacitor banks. This will limit the damage
in case of an internal or external (short-circuit) failures in
a given capacitor bank, since its stored energy is divided
by N with respect to the single capacitor bank solutions.
Hard points / limitations
Air insulation at voltage levels of 100kV and above,
with such short insulation distances required for
compactness of the system, constitute an additional
engineering challenge and may have a strong impact on
long term reliability, which will be affected by air and
cleanness conditions inside the cabinets.
Fast and intense electric fields constitute a major
concern in the reliability of sensitive electronic
components, particularly those used in the IGBT drivers.
Figure 5. Multi-level topology based on the association
of interleaved DC/DC sub-converters
Principle
A special 3-phase transformer, with multiple (N)
secondary winding systems, feeds N isolated capacitor
chargers. Each capacitor charger feeds one capacitor
bank, which in turn supplies one DC/DC sub-converter,
either of one quadrant (in black) or two quadrants (in
black & red).
The outputs of the N sub-converters are connected in
series for voltage multiplication. Therefore, by using any
technique of interleaved control (PSM or multi-level
PWM) a variable DC voltage is obtained at the end of the
series connected sub-converters, although with high HF
harmonics. A common passive filter extracts such HF
harmonics.
The obtained filtered voltage is then applied to the
primary winding of a pulse transformer which rises up the
voltage till the value required by the klystron.
Advantages
Active demagnetisation of the pulse transformer is
possible by using some two-quadrant DC/DC subconverters out of the total N (typically 10 to 15% would
be enough), however requiring additional switchnig
devices (in red in Figure 5).
Active droop compensation is intrinsic to the topology,
therefore requiring no additional sub-systems. The
DC/DC sub-converters are regulated through PWM or
PSM in closed loop feedback during the whole duration
of the pulse, including the demagnetisation period at the
end.
Active klystron arc extinction is also possible by
applying a controlled reverse (positive) voltage to the
klystron whenever such an arc is detected.
Partial modularity is obtained which will increase the
availability. Automatic detection of module failures and
subsequent reconfiguration are possible.
All active power electronics (at primary side of the
pulse transformer) is housed in air insulated cabinets,
therefore with easy access for repair and diagnosis. Only
the pulse transformer is within a HV oil tank.
Hard points / limitations
The HF voltage ripple at the pulse flat-top may be
considerably high with respect to the 200 ppm limit,
depending on the interleaving control technique adopted
and on possible dissemetries existing between modules.
Thermal cycling of semiconductors, operating under
hard-switching conditions, may have an impact on their
lifetime. To overcome this effect, either the
semiconductor are oversized or the number of modules is
increased.
Two special transformers are required: the 3-phase
multisecondary-winding & HV pulse transformer.
All other inconveniences related to the usage of a pulse
transformer (stated in point A) apply.
E. Multi-level Resonant Sub-converters
A new topology based on the association of several
resonant sub-converters in parallel/series configuration,
on a multi-level structure, is shown in Figure 6.
Figure 6. Multi-level topology based on the association
of multiple resonant DC/AC/DC sub-converters
Principle
A set of N H-bridges are fed from a common DC-link
bus at medium voltage (~ 1 to 2kV). This DC-link bus is
formed by several capacitor banks connected in parallel.
Several capacitor chargers may be also connected in
parallel on a one to one basis with respect to the capacitor
banks. Although not mandatory on a functional point of
view, this last principle will improve the modularity of
the system.
Each H-Bridge supplies a circuit formed by:- a LC
parallel resonant circuit, a HF step-up transformer, a
diode rectification bridge and an output HF filter.
The outputs of the different circuits herein referred are
connected in series; the total output voltage is therefore
multiplied by N.
The resonant circuits are excited by the H-bridges with
a frequency above the resonance one. A voltage
amplification factor of up-to 3 times, exclusively
procured by the resonant circuits, is easily achievable in
practice. The HF transformers can additionally multiply
the voltage by a factor of 10. Typically, a number of 5 to
6 modules would be enough for a 100kV and above
application.
Soft switching techniques of the H-bridge IGBT’s
should be implemented to reduce switching losses. They
could be considerably high otherwise at switching
frequencies typically in the order of 20kHz, with a
consequent negative impact on thermal cycling fatigue of
IGBT’s.
Advantages
All active power electronic components are located at
the transformer primary side (medium voltage level). Like
the topologies A and D, the majority of power electronics
are installed in standard air insulated cabinets, whereas
only the HF transformers, diodes and other passive
devices are in an oil tank. This will facilitate access for
repair and minimizes the quantity of oil required.
Semiconductor switches and drivers are of standard
commercial types. Several industrial sources exist
worldwide.
The HF transformers operate in AC mode which is the
natural way of operation for such devices. On opposition
to pulse transformers, no intrinsic limitations exist on
their design with regards to the pulse length. No
demagnetisation circuits are needed. The design and the
construction of this type of transformers seem easier to
carry on than pulse transformers.
Like in D, the flat-top voltage (droop) is regulated in
closed loop, by measuring the output voltage and by
adjusting in consequence the H-bridge control signals
dynamically, during the whole pulse duration.
In case of klystron arcing, the resonant circuits will be
“automatically De-Quewed” as their load resistance will
be virtually zero. Therefore, even without disabling the
H-bridge control signals, an intrinsic voltage shut-down
with “self-current” limitation is obtained.
The topology and the mechanical layout are entirely
modular.
Hard points / limitations
The construction of the HF transformers is still a
challenge:- mechanical stress due to pulsed operation;
high frequency (20 kHz) design with high voltage
insulation (100 kV and above at highest point); high
average power (tens kW).
Due to the poor power factor of the resonant circuits,
the H-bridges shall handle a significant amount of
reactive power and must be therefore oversized.
Due to the existence of the intermediate resonant stages,
which operate in AC mode and require an additional
rectification and filtering stages, long rise times (typically
in the order of 100µs) are obtained.
Assuring “soft-switching” of the IGBT’s in all
operating points might be complex, particularly during
pulse “build-up” (low current). Reliable interlocking
circuitry that detects and prevents missing soft switching
conditions is mandatory.
A HF ripple content on the flat-top is to be expected. Its
real value depends not only on the theoretical design
design and on the output filters, but mainly on practical
aspects like control accuracy (jitter) of IGBT’s, symmetry
on assembling and component tolerances between
modules.
The energy stored in the modulator circuit (HF
transformer leakage inductances/leakage capacitances;
output filter devices, etc.), which potentially can be
dissipated into a klystron arc, may be significant and
considerably higher than the maximum 10J allowed.
Adding a special resistive dump circuitry in series with
the output HV line might be necessary to overcome this
drawback.
A klystron modulator system rated for 100kV, 20A,
1ms/50Hz, composed by 5 modules according to Figure
6, was simulated using SABERDesigner software tool.
The results are reported in Figure 7, Figure 8 and Figure
9.
loop and the configuration of the output filters.
Figure 8 shows the soft-switching conditions obtained
(ZVS for the leading leg and ZCS for the lagging leg).
Finally, Figure 9 shows the condition of a klystron arc
simulation (short-circuit applied at the modulator’ output
at instant t=0). No modifications have been implemented
on the control loop nor on the H-bridge control signals
during the time period shown. Although the converter is
left running “normally” after the occurrence of such arc,
the output current “is self-limited” to about 3 times the
nominal value. This illustrates well the De-Qweing effect
described above.
Figure 8 – Internal waveforms of one resonant circuit
module. Note: ZVS on leading leg (with parallel capacitors
across IGBT’s); ZCS on lagging leg.
a)- voltage (VH) and current (ILR)at the output of one Hbridge, VCR: voltage across the resonant capacitor; b)- Voltage
(VQ1) and current (IQ1) of the same H-bridge’s top/leading leg
IGBT; c)- Voltage (VQ2) and current (IQ2) of the same Hbridge’s bottom/lagging leg IGBT.
Figure 7 – Output voltage pulse shape of a multi-level
resonant klystron modulator.
The rise-time obtained was in the order of 170µs.
Overshoot and voltage droop are less than 1%. The HF
ripple obtained (ideal control of IGBT’s; no unbalances
between modules) was in the order of 800ppm (a factor of
4 higher than the specified limit). The reduction of the HF
ripple under 100ppm and of the rise time down-to 100µs
(0..99%) was recently obtained by modifying the control
Figure 9 – Klystron arc simulation: VLF: Sum of all
voltages across the output filter inductances; ILF:
Klystron arc current.
V. CONCLUSIONS
Five klystron modulator topologies based on solid state
power electronics and covering the current state of the art
in the field were presented and discussed comparatively,
on the context of the ESS project. The main particularities
of such modulators reside on the long pulses required (3.5
ms), considerable average power (120 kW), compactness
and high reliability.
Although the resonant multi-level topology seems
interesting in terms of accessibility, reliability,
modularity, adoption of standard and multi-source
components, self-limitation of arc current, its practical
implementation and validation requires a considerable
amount of effort and time and could hardly be an option.
The Marx generator topology is compact, oil free and
modular. However, due to the large amount of
semiconductors present at HV potential and submitted to
fast pulsed electric fields, its long term reliability and cost
effectiveness is still to be proven on an industrial scale.
All other topologies mentioned in this paper exist on an
industrial scale and have been used in other similar
applications. However, their functional characteristics
(rise/fall times; HF voltage ripple; reverse voltage on the
klystron) and their ease of operation (accessibility;
expected reliability; oil quantity required) differ
considerably between them.
Considering the significant amount of klystron
modulators required (~200 units), costs, accessibility
(Mean Time to Repair) and reliability (Mean Time
Between Failures) are key parameters that shall be
quantified or at least evaluated deeply.
For instance, if the MTBF is 50.000 hours per unit (1
failure in every 5.7 years of continuous operation), and
the MTTR is 2 hours, we obtain a global Machine
Availability of 99.2% due only to klystron modulators.
The construction, testing and evaluation of prototypes,
before series production, are therefore mandatory
intermediate steps, which are required to assess the
compliance of the whole infrastructure with such
challenging operation parameters.
VI.
REFERENCES
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LP ESS best price-performance”, February 2008;
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Conference, IPAC’2010, 23-28 May 2010, Kyoto, Japan;
[3] – Reass, W.A.; Doss, J.D.; Gribble, R.F.; “A 1
megawatt polyphase boost converter-modulator for
klystron pulse application”, IEEE Conf. on Pulsed Power
Plasma Science, 17-22 June 2001;
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Reduced Size And Cost”, 21st Int. Power Modulator
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