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DESIGN AND APPLICATION OF A DIODE-DIRECTED SOLID-STATE 

MARX MODULATOR* 

G.E. Dale^, H.C. Kirbie, W.B. Haynes, C.E. Heath, 

T.A. Lopez, F.P. Romero, and R.M. Wheat Jr. 

Los Alamos National Laboratory, PO Box 1663, Mail Stop J-570 
Los Alamos, NM, USA 


Abstract 

Researchers at Los Alamos National Laboratory 
(LANL) are developing a new solid-state high-voltage 
Marx modulator for the generation of pulsed power. The 
initial application of the LANL modulator is to provide 
power to a magnetron that requires a 46-kV, 160-A, 5-ps 
rectangular pulse. This modulator technology is also 
being developed for other applications, including portable 
millimeter wave sources, a beam energy corrector for 
induction accelerators, and space-based power systems. 
The LANL solid-state modulator has several benefits, 
including wave shape control, switch protection, 
efficiency, and compactness. The present paper describes 
this source technology and its design. 

I. INTRODUCTION 

The traditional Marx generator, named for its inventor. 
Professor Erwin Marx [1], produces a single high-voltage 
pulse by switching precharged capacitors into a series- 
connected string using gas-insulated spark gaps [2]. The 
Marx generator is a rugged, low-impedance source of 
electrical energy that has served well in a wide variety of 
high-peak-power applications for the past 75 years. Marx 
generators are now undergoing a renaissance due to the 
use of modern solid-state switches [3-8]. The use of 
insulated gate bipolar transistors (IGBT) in place of spark 
gaps, for example, gives simple Marx generators the 
ability to produce square-shaped output pulses at very 
high rates. The on/off switching capability of the IGBT 
also allows the output pulse to change width from one 
pulse to the next, enabling the generator to adapt rapidly 
to changing load requirements. Currently, Marx 
generators using solid-state switches are unable to equal 
the high peak voltage and peak power capacity of 
generators using spark gaps, but the operational 
advantages gained in pulse control and high average 
power have transformed the single-shot Marx generator 
into a versatile modulator. 


A schematic of a typical resistively charged Marx 
generator is shown in Figure 1. Other Marx configurations 
use inductors in place of the charging resistors. Our Marx 
modulator replaces these linear charging elements with 
fast-recovery diodes [9], as shown in Figure 2. The diodes 
provide a low-loss, low-impedance path for the Marx 
charging current between pulses and a high-impedance 
path when the Marx bank is erected. 



Figure 1 . Simplified diagram of a four-stage Marx 
modulator with resistive charging elements. 



Figure 2. Simplified diagram of a four-stage Marx 
modulator with diode charging elements. 


The charging inductor shown in Figure 2 is necessary to 
complete the charging pathway when the Marx bank can 
not be charged through the load (such as when used to 
power a magnetron). Supply current enters the Marx 
circuit through the charging inductor, trickles through the 
Marx assembly via diode routing, and returns to the 
power supply near the grounded end. The charging 


* Work sponsored by Los Alamos National Laboratory under US DOE contract W-7405-ENG-36. 
s email: gedale@lanl.gov 


0-7803-9189-6/05/$20.00 ©2005 IEEE. 


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4. TITLE AND SUBTITLE 

Design And Application Of A Diode-Directed Solid-State Marx 
Modulator 

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Los Alamos National Laboratory, PO Box 1663, Mail Stop J-570 Los report number 

Alamos, NM, USA 

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13. SUPPLEMENTARY NOTES 

See also ADM002371. 2013 IEEE Pulsed Power Conference, Digest of Technical Papers 1976-2013, and 
Abstracts of the 2013 IEEE International Conference on Plasma Science. IEEE International Pulsed Power 
Conference (19th). Held in San Francisco, CA on 16-21 June 2013., The original document contains color 
images. 

14. ABSTRACT 

Researchers at Los Alamos National Laboratory (LANL) are developing a new solid-state high-voltage 
Marx modulator for the generation of pulsed power. The initial application of the LANL modulator is to 
provide power to a magnetron that requires a 46-kV, 160-A, 5-is rectangular pulse. This modulator 
technology is also being developed for other applications, including portable millimeter wave sources, a 
beam energy corrector for induction accelerators, and space-based power systems. The LANL solid-state 
modulator has several benefits, including wave shape control, switch protection, efficiency, and 
compactness. The present paper describes this source technology and its design. 


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inductor is generally sized to shunt no more than 10% of 
the load current at several times the maximum expected 
pulse width. 

Employing diode charging and solid state switching in a 
Marx architecture produces significant benefits in 
efficiency, wave shape control, switch protection, and 
compactness: 

A. Efficiency 

A Marx bank with resistor charging can be no more 
than 50% efficient because at least half of the charging 
energy is dissipated in the charging resistors. In 
comparison, diode charging of a Marx dissipates very 
little of the charging energy resulting in a very efficient 
charging arrangement. Using inductors for Marx charging 
can also be made to be very efficient, but suffers from 
pulse width limitations due to saturation. Diode charging 
has the added benefit of not limiting the pulse width since 
the diodes can remain reverse biased indefinitely (when 
used, an external charging inductor can limit the pulse 
width and so needs to be designed appropriately). 

Another efficiency benefit of using diode charging is 
that it facilitates the recycling of energy that would 
otherwise be lost. When the switches shown in Figure 2 
are closed, the diodes are reversed-biased. When the 
switches open, energy stored in the charging and load 
inductances causes the diodes to become forward biased, 
directing this inductively stored energy back into the stage 
capacitors. Although the charging inductor siphons off 
some of the Marx output pulse, this inductively stored 
energy is given back (recycled) after each pulse. 

B. Wave-Shape Control 

The combination of solid-state switches and diode 
charging adds the new and powerful dimension of 
tailoring the output pulse for optimal performance at the 
load through the unique ability to operate each stage 
independently. Stages not receiving a switching command 
are bypassed by the charging diodes, an arrangement 
producing an output pulse voltage equal to the charge 
voltage times the number of active stages. Similarly, the 
independent stages can be switched on and off within a 
single pulse envelope to produce a digitally synthesized 
pulse shape. 

C. Switch Protection 

The diode architecture that increases the Marx 
efficiency in our modulator also intercepts destructive 
transient energy and returns it safely to the energy-storage 
capacitors for reuse. The switches used in modern Man- 
type modulators are steadily increasing in peak and 
average power-handling capacity, but the drawback is 
their susceptibility to switch damage from energy 
transients. The traditional approach to addressing this 
problem is to absorb and dissipate the incoming transient 
energy by connecting snubbing circuits in parallel with 
the endangered switches. A higher level of protection is 
provided by our Marx architecture by directing the 


transient energy around the vulnerable switches into the 
stage capacitors through the charging diodes. We believe 
this approach may eliminate the need for snubbing 
circuits entirely in future versions of the modulator. 

D. Size and Weight 

Our fully solid-state modulator is inherently compact 
and lightweight. The basic architecture of the modulator 
will allow it to keep pace with the demands of research, 
the military, and industry for high-peak and high-average- 
power sources in ever-smaller and ever-lighter packages. 
Our Marx modulator does not use a large, heavy 
transformer to produce high voltage. Moreover, since all 
charging, switching, and gate-control components are 
solid-state, we have the option of purchasing their silicon 
interiors (dies) instead of encapsulated components. 
Therefore, each Marx stage can be made very small and 
light, either by combining the dies in a multichip module 
or combining their functions within a single application- 
specific integrated circuit (ASIC). This possibility leaves 
the volume of the energy storage capacitors as the 
dominant factor determining the dimensions. Since the 
history of capacitor technology demonstrates steadily 
increasing energy density, in the future it is likely we will 
be able to reduce the size of our Marx modulators even 
further [10]. Use of all-solid-state construction in a Marx 
generator has the added benefit of minimizing the heat 
generated during operation, a result that allows reduction 
in the size and complexity of the attendant cooling 
system. In our first application, the relatively low output 
voltage has allowed more weight reduction through the 
use of sulfur hexafluoride gas (SF 6 ) for the insulating and 
cooling medium. 

II. SYSTEM DESCRIPTION 

We developed our Marx modulator to fill the power- 
source needs of several applications, each possessing its 
own set of parameters for weight, size, output voltage, 
current, pulse width, repetition rate, and average power. 
The first of these applications is a compact, gas-insulated 
power source designed to deliver a 46-kV, 160-A, 5-ps 
rectangular pulse to the cathode of a magnetron. The 
magnetron is an S-band e2v Model 6028 with the 
performance parameters listed in Table 1. 


Table 1 . Magnetron performance parameters. 


Voltage 

46 kV 

Current 

160 A 

Pulse Width 

5 ps 

Energy per pulse 

36.8 J 

RF peak power 

~ 3.5 MW 

Rate of voltage rise 

< 130 kV/ps 

Average power limit 

7 kW (input) 


The magnetron produces a nominal load impedance of 
288 Q and requires a pulse that is reasonably flat (<5%) 


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when averaged over the pulse width. The modulator is 
designed to obtain the pulse flatness by storing about 15 
times the energy needed by a single pulse. The droop can 
be further reduced by either adding more stage 
capacitance, using a passive droop compensation network 
[3], or active compensation by wave-shape control. 

The key parameters of the modulator are shown in 
Table 2. Each stage consists of a single IGBT, two 8-pF 
capacitors connected in parallel, two fast-recovery diodes, 
a single gate driver, and an isolated power supply deriving 
its energy from a ferrite isolation transformer. The IGBT 
and gate-control elements are shown in Figure 3. 


Table 2. Marx modulator parameters. 


Max. stage voltage 

1200 V 

Number of stages 

48 (57.6 kV open circuit) 

Capacitance per stage 

16 pF 

Total Marx capacitance 

786 pF 

Max. stored energy 

553 J 

Erected capacitance 

333 nF 

Uncompensated droop 

~ 5% 


HV 



Figure 3. Schematic illustration of a single Marx stage. 

The IGBTs are triggered independently by fiber optic 
signals received through an optical interface on each 
stage. We selected the IXYS model IXBT42N170 for the 
Marx stage switch because it is a good compromise 
between operating voltage, pulsed current, and 
commutation speed [11]. We chose the IXYS model 
DSDI60 fast-recovery diode for the charging elements 
and a self-healing film capacitor from Aerovox for energy 
storage. 

Four Marx stages are collected on a single circuit board 
measuring 12.7 cm x 30.5 cm. Figures 4 and 5 show two 
views of a single circuit board. Each stage is powered by 
a separate winding on a common ferrite transformer core 
with a single high-voltage wire serving as the primary 
winding for the ferrite transformer. The primary winding 
is powered from an Fl-bridge power converter operating at 
100 kHz. 

The entire modulator assembly consists of twelve of 
these circuit boards. The circuit boards are rack-mounted 
on rails in a folded arrangement that saves space. The 


centers of each ferrite transformer share a common axis 
through which the primary winding is threaded. The 
boards are supported on the assembly rails by modified 
circuit card connectors. These circuit card connectors also 
serve to electrically interconnect the Marx boards in series 
using a copper strip. 



Figure 4. Front view of the circuit board showing four 
optical fiber ports, four switches, and four metal 
enclosures containing the gate drive electronics. 



Figure 5. Rear view of the circuit board showing eight 8- 
pF, 1500-V capacitors and a single ferrite-core 
transformer. 


Two views of the assembled modulator are shown in 
Figures 6 and 7. The assembled modulator weighs 11 kg, 
and measures 76.2 cm x 30.5 cm x 17.8 cm. 



Figure 6. Top view of the assembled modulator showing 
support rails and edge connectors. Board spacing on the 
rails is generous to allow single stage measurements. 


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The modulator has been extensively tested into a 300 O 
resistive load. Detailed performance results are contained 
in a companion paper [12]. 



Figure 7. Top view of assembled modulator showing 
fiber-optic trigger cables. 


III. CONCLUSIONS 

We have produced an efficient power converter that 
takes in low-voltage dc power and puts out a steady 
stream of high voltage pulses in a package with an ultra¬ 
compact future [13]. In addition to building this 
modulator to power a magnetron, we are also developing 
this modulator technology for other applications. These 
future applications include modulators for portable 
millimeter wave sources, an active tuning system for 
pulse forming networks, a beam energy corrector for 
induction accelerators, and spaced-based power systems. 


The key element of the modulator is its unique Marx 
circuit architecture that manages the outflow of power to 
the load with advanced solid-state switches and uses 
diodes to direct the flow of all incoming power, transient 
or otherwise, to the capacitors. The charging diodes 
provide high shunt impedance to the outflow of power 
(when the diodes are off) and converts to a very low shunt 
impedance to the inflow of power (when the diodes are 
on), regardless of whether that power comes from a dc 
power supply or a high-power transient. 

IV. REFERENCES 

[1] J. Salge, "Professor Erwin Marx," in Proc. 4th IEEE 
International Pulsed Power Conference, 1983, pp. 51. 

[2] E. Marx, "Verfahren zur Schlagpmfung von Isolatoren 
und anderen elektrischen Vorrichtungen," Deutsches 
Patent No. 455933, 1928. 

[3] A. Krasnykh, R. Akre, S. Gold and R. Koontz, "A 
solid state Marx type modulator for driving a TWT," in 
Proc. Power Modulator Symposium, 2000, pp. 209-211. 

[4] J. Baek, D. Voo and G. Rim, "Solid-State Marx 
Generator Using Series Connected IGBTs," in Proc. 
Power Modulator Symposium, 2004, pp. 383-386. 

[5] S.G.E. Pronko, M.T. Ngo and R.K.F. Germer, "A 
solid-state Marx-type trigger generator," in Proc. Power 
Modulator Symposium, 1988, pp. 211-214. 

[6] R.L. Cassel, "A solid state high voltage pulse 
modulator which is compact and without oil or a pulse 
transformer," in Proc. Power Modulator Conference, 
2004, pp. 72-74. 

[7] K. Okamura, S. Kuroda and M. Maeyama, 
"Development of the high repetitive impulse voltage 
generator using semiconductor switches," in Proc. Pulsed 
Power Conference, 1999, pp. 807-10. 

[8] J.A. Casey, F.O. Arntz, M.P.J. Gaudreau and M.A. 
Kempkes, "Solid-state Marx bank modulator for the next 
linear collider," in Proc. Power Modulator Symposium, 
2004, pp. 257-260. 

[9] G.E. Dale, H.C. Kirbie, J.D. Doss and M.A. Serrano, 
"Solid-State Marx Modulator Development," in Proc. 
Power Modulator Conference, 2004, pp. 497-500. 

[10] F.W. MacDougall, J.B. Ennis, R.A. Cooper, J. Bates 
and K. Seal, "Fligh energy density pulsed power 
capacitors," in Proc. Pulsed Power Conference, 2003, pp. 
513-517. 

[11] B. Flickman and E. Cook, "Evaluation of MOSFETs 
and IGBTs for pulsed power applications," in Proc. 
Pulsed Power Plasma Science, 2001, pp. 1047-1050. 

[12] G.E. Dale, H.C. Kirbie, W.B. Haynes, C.E. Heath, 
T.A. Lopez, F.P. Romero, and R.M. Wheat Jr., 
“Performance of a Diode-Directed Solid-State Marx- 
Modulator,” presented at the 15th IEEE International 
Pulsed Power Conference, Monterey, CA, 2005. 

[13] M.V. Fazio and H.C. Kirbie, "Ultracompact pulsed 
power," Proc IEEE, vol. 92, pp. 1197-204, 2004. 


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