Preface

Powerpulse modulators are used to make an efficient use of the grid power.

The modulator types can be devided in three kinds;

Magnetic modulator
Hard tube modulator
Line type modulator

Each modulator consists of a number of conversion stages;

3 Phase mains transm. -to- 3 Phase consumer level
AC mains in building -to- AC usable voltage level
AC -to- DC
DC -to- first modulator stage
first stage -to- first pulse level
pulse -to- klystron pulse level

One or two conversion stages depends on the modulator type, but still there is a lot of stages each with its own local structure, savety precautions and driver components.

One of the ways to make an economical design is to minimize the number of conversion stages, to minimize the losses by energy storage in isolation and cores;

3 Phase mains transm. -to- DC
DC -to- controlled AC
AC -to- modulator stage
modulator stage -to- klystron

At the mainspower-end the most profit is expected.

Klystron specifications

pulse voltage 600kV
pulse current 700A
pulsewidth video 5µs
cathode power av. 68kW
repetition rate 50Hz
droop & ripple <.5%
RF frequency 3000MHz
RF pulsewidth 3µs

Conversion structure

Fig.1 gives an impression of the conversion structure with its various stages.
The incoming mainslevel is high as possible and is dictated by;
Mains supplier
manageable conversion to a DC storage

Besides this part of the infrastructure there is need for;
220/380V
Data I/O, all analogue values are digitized to simplifing transmission problems.
timing system
rf-drive system

fig.1 functional modulator set-up

Mainswitch

fig.2 Main switching

The mainswitch between infrastructure highpower lines and local system is by a classic approach (fig.2).
It provide a powerless switched manual switch as savety device for working on the station, a electromagnetic switch for remote on/off switching or savety interference. There is a facility to limit inrush currents. Shorting currents and overload are protected by fuses and thermal switches.
Possible supplier; HOLEC
Thermal switch manual resetable
@phase: V?, I?, Rrun in?, Ifuse?, Itherm?

Power conversion stage 1

fig.3 AC/DC conversion

Classic approach (fig.3), no polyphase rectifing is necessary owing to the seperation by power conversion stage 2.
A crowbar driven fuse is usefull to limit the energy by shorted circuits in the load.
Rectifier to 8kV-dc
Idiode?,Vdiode?,L,C?,Icfuse?,Ifuse?,Icrowbar?,Vcrowb.?
If necessary one diode (3*) stack replaced with SCR's to DC-voltage controle.

Power conversion stage2

fig.4 Chopper device up to modulator voltage dc

Power transport is leveled, regulated and adjusted by a converter and step-up transformer. This is a fair new technic on this power level.

Power conversion stage 3

On this stage the actual pulse forming takes place. It depends on the chosen modulator type which circuit specifications are legitim. There are three modulator main types;
Magnetic modulator
Switchtube modulator;
Full level switchtube
Low level switchtube
Gridded klystron
Linetype modulator

Magnetic modulator

The magnetic modulator approach is not interesting for the present case. This type of modulator is used for very high power pulses for p.e. gasplasma research. The pulse level is build on several stages who are switched by saturable reactors. The pulse has not a real flat top and therefor not usable for accelerator klystrons.

Hardtube modulator

In this type of modulator is a capacitor connected by a hardtube switch with the klystron. The hardtube switch act as a current source, during conducting, what results in a flat klystron pulse.

Full level switchtube

This means a hardtube switch which can hold >600kV and conduct >700A. Such a tube is unknown for this moment.

Low level switchtube

When this system is extended by a pulstransformer, more realistic switchtubes are in the picture for the voltage hold-off (fig.5). One of the most powerfull tubes now a day available is the L-5097 of Litton (170kV-60A@20us), which means that 50 tubes in parallel has to be placed. Beside this impossible number gives the transformer, not easy corrected, side effects like droop and ripple on the pulse top. A 250kV-250A switch tube is under development. What 4 tubes in parallel means by a tranformer ratio of 1:2.5.

fig.5 Hardtube modulator with pulse transformer.

Gridded klystron

A hardtube related solution is a gridded klystron like the tubes in TV-transmitters. This kind of tubes are not available in this power range now. The supplier has to start new developments for the 2500 tubes if he is intersted. This would probable the most efficient, reliable and cheap solution (fig.6).

fig.6 Hardtube modulator with gridded klystron.

Line type modulator

In this type of modulator is an pulse forming network load by resonant charching or DC charging and discharged by a triggered divice like SCR or thyratron.
In this case is choosen for a DC charging direct from the converter output (fig.7).

fig.7 Principle of the linetype modulator

The capacity of the network is exactly loaded by the converter of the previous stage.
Pulseform corrections can be made by tuning the PFN and by placing of a RC filter on a tap (ca. 1: 10) of the secondary. The ripple on the pulse top depends mainly of the number of network sections.
Usable discharge switches are thyratrons in the range of 100kV-20kA( EEV )what means a pulse transformer ratio of 1:12.

Conclusion

There are two possible modulator choices;

Hardtube/gridded klystron;

+ one transformer
+fewest conversion stages
-high transformer ratio
-high stored energy

Linetype modulator;

+easy pulse correction
+well known technics
-two transformers
-+10% power
-more pulse top ripple

Principle of both possibility's are given in fig.8

(a) concept lay-out with linetype modulator stage

(b) concept lay-out with gridded klystron

fig.8 (a) and (b) concept lay-outs for further investigations

Modulator maintenance remarks.

Beside a gridded klystorn all modulator types has a tube, switched or triggered. A tube has a fair short lifetime of 5000-20000 hours.