Gridded klystron as hardtube
Power-pulse-modulators are used in a small band of
apparatus like gasplama physics, pulse accelerators (physical and medical), radar
systems, corrosion treatment, etc.
The scope of this paper is modulators used in accelerator for physic research.
These modulators are characterising by short rise-times, stable timing and level,
flat top and high efficient use of the mains-power.
Main values;
The modulator designs can be divided in three types;
The magnetic modulator approach is not interesting for the present case. This type of modulator is used for very high power pulses in gasplasma research, etc. The pulse level is build on several stages who are switched by saturable reactors. The relative low efficiency of a magnetic modulator comes about because the saturable reactor is basically a leaky switch. The pulse has not a real flat top and therefore not useful for accelerator klystrons.
Hardtube or switchtube modulator
In this type of modulator a (large) capacitor is connected by a hardtube switch to the klystron. The hardtube switch acts like a current source, during conducting, what results in a flat klystron pulse.
Full level switchtube
Low level switchtube
Gridded klystron
Gridded klystron as hardtube
In this type of modulator is a pulse forming network load
by resonant charging or DC charging and discharged by a triggered device like SCR or
thyratron.
The charge takes place after a discharge of the pulse-forming-network (PFN). This can
be done in a non command mode (diode) or in command mode (switch like SCR).
In most cases a resonant charge system is used, based on a choke and the capacitors
in the PFN. In the non command charging mode the repetition rate is dictated by the
charge choke and the PFN-capacitors plus the delay to the discharge command. During
that time there is a certain voltage drop by the leakage of the PFN, so the repetition
rate is adjustable only within a small range.
By a command mode charging the time between charging and discharging can be constant,
so less leakage effects and higher accuracy.
To stabilise the PFN unit a charge control system is added. Whenever the correct pfn voltage
has attained, the control system has triggered and the stored energy of the charge choke
has stored in a capacitor and dissipated in the resistor or is delivered back in the
power system (non-dissipating system). Hereby it provokes a voltage drop on the charging
device which blocks the pfn-voltage. To reach a stable voltage on the PFN, the charge
control system can be triggered on the bases of;
power supply level.
minimum and maximum delay after the charge moment.
The pulse forming network acts like a long cable by discharging. Therefore it is
important to match the impedance of the network with the load (i.e. klystron) to
avoid reflections and to obtain maximum energy transfer.
By positive reflections the next charging can not reach the right level in the PFN.
This should be improved by a 'tail biter'. A tail biter short circuit the primary
during the falltime of the pulse. The remaining energy charges the PFN, with reverse
polarity, increased with the magnetising energy of the pulse transformer. So a tail
biter converts positive reflection in negative ones and improves the falltime of the
pulse too.
To adapt negative reflections an 'end of line clipper' (eol) is added to the PFN.
The form of the backswing on the load depends on the components of the end of line
clipper. By a resistive clipper the back swing start on a high level and decreased
till zero by a power of e. In this case all the reflected energy is dissipated
in the resistance. By a constant voltage clipper (e.g. zeners) there is a
constant voltage which can be lower then the peak voltage of a resistive clipper.
For the lifetime of a klystron a low backswing with low dV/dt is the best [3].
The remaining energy in the pfn is reused for the next charge.
The manufacturing of chokes is laborious. Chokes are dissipating elements too but are stable, robust and reliable.
Pulse-form corrections on this stage are made by tuning the PFN. The ripple on the pulse top depends mainly of the number of network sections and the parasitic behavior of the pulse transformer.
Useful discharge switches are thyratrons in the range of 100kV@20kA
[4].When a non-dissipated charging control is used, a non regulated power supply can be used
[5].
Principle of a linetype modulator