modfig.3 (rotation principle diagram)
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The National Institute for Nuclear and High Physics was using high duty factor modulators in its lineair accelarator , MEA. (Medium Energy Accelarator).
Main characteristics of the old MEA electron linac
These modulators were changed, for the AmPS pulse stretcher project, into high power - short pulse modulators;
The modulator principle is shown in the figure. Pulse generation is based on the line type modulator principle. Each one of the 40 pulse forming network (PFN) -units is a printed circuit board with a 2kV line-type (50µs) modulator. Two units feed a primary of a special multi-core pulse adding transformer.
This layout allows the provision of pulses which amplitude or repetition rate can be changed by the number of the units. Klystron pulse repetition rates up to 2000Hz have been obtained by sequentially switching a number of low repetition rate (500Hz) PFN-units. This is controlled by means of a special digital processor.
modfig.3 (rotation principle diagram)
Primary windings, which are not used, are shorten by a special circuit (TSC).
The klystron is used on 3 discrete rf levels. Each level is optimized low power mismatch by the number of active units between pfn and klystron. The figure shows the different discharge schedules for the original levels.
In this way redundancy has also been obtained.
An unregulated 1000 V DC-power supply feeds 6 modulators with a single busbar.
The klystron is water cooled, the pfn-units original by a freon-system, now by a perchloor system..
RF peak power 1 2 4 MW Cathode voltage 80 102 130 kV Cathode current 44 63 91 A Klystron impediance 1818 1619 1428 Repetition rate 2000 1000 500 pps Duty factor 10 5 2.5 % Video peak power 3.5 6.4 11.8 MW Transformer ratio 1:11 1:8 1:11 Number of pfn/prim. 1 1 2 Number of prim./pulse 9 16 15 Pfn impediance 2.08 2.08 2.08
Related paper; P.J.T.Bruinsma, E.Heine et al., "An all solid state line-type modulator", IEEE trans. on Nucl. Sci, NS-20(1973).
Pulse specifications;modfig.4 (output pulsetop graphic) Flat top ripple 0.2 0.2 0.2 % Droop of flat top +0.5 0 -0.5 % rise time 5 5 5 µs fall time 7 7 7 µs width (75% level) 50 50 50 µs
The unit is the pulse generator in the modulator. The modulator consist of 40 units, two connects per primary winding of the special multi-core pulse adding transformer. By selecting of a nimber of pfn-unts and transformer step ratio, the video power output can be varied according to the requirements of the klystron level. modfig.6 (Pfn-unit picture)
charge system
The charge circuit is conventional. Due to the low repetition rate of each PFN-unit the requirements on the charging SCR's (2 in serie) are very moderate.
stabilisation and leveling system
This system can best be discribed as a non-dissipative 'de-Q-ing' circuit. The pfn is charged resonnantly by means of a charging choke. Whenever the correct line voltage is obtained, the SCR is triggered, thereby transferring the stored energy of the charging choke into the capacitor and back into the main DC power supply. Fine control of the RF output is obtained by small adjustments in the line voltage which are also under control of the modulator processor. modfig.7 (SLS voltage graphic)
pulse forming network
The network consists of two printed circuit boards (copper layer = .5mm), each containing 30 printed inductors and 15 capacitors (.47µF-2200V). Each inductor is designed (with a flat top in mind), incuding effects of the pulse transformer and other parasatic circuit elements. The large number of PFN-units justified this approach. Fine adjustment of the characteristic impedance can be done with 12 presetable alluminium shorting vanes just above the inductors. The large number of pulse line sections will smooth out individual capacitor differences.
discharge system
Protection against forward breakover and excessive reverse power is obtained by carefull selection of the snubber circuitry. With these protections standard unselected SCR's can be used. The transformer step-up ratio is selected by triggering the appropriate pair of discharge SCR's. The electrical behaviour of the special pulse transformer implies the short cicuiting of all unused primaries during a discharge pulse by means of aspecial SCR circuit (TSC). The stored nergy in the pulse transformer is transferred back into the pulse forming network. Excess charge is dissipated in the end of line clipper. This end of line clipper is a circuit of zeners to obtain a constant backswing in the shortest time and with te lowest voltage to extend the klystron lifetime.
Transformer shorting circuit (TSC) modfig.8 (TSC diagram)
Primary windings, which are not used, are shorten by a special circuit (TSC). The primaries behaves like current sources. The slave triggering-module triggers only by a current source an not on the back swing of a generated pulse.
Modulator processor modfig.9 (processor diagram)
The central element in this unit is a memory containing RF pulse power information for 256 pulses. This implies a minimum repetition rate of 10Hz with a pulse-to-pulse interval of a number of 400us. The memory is loaded over a dataline with the central control computer. The memory information is first translated into the number of pfn-units to be triggered and then transferred to a 2*20 shift register. By this register the individula pfn charge and discharge triggeres will be allocated. The out-of-order information will be stored in the control part of the processor. By means of a seperate input/output unit the necessary level conversion and noise immunity is provided.Harware
modfig.A (hardware layout diagram)
modfig.B (hardware layout picture)
modfig.C (oiltank layout picture)
Water cooling modfig.10 (cooler diagram)
both accelerator section set in parallel- the rectangular waveguide networdk - the klystron collector. The cooler has been located outside the building, as close as possible to the rest of the unit. The cooler is provided with continuously adjustable fans and air valves. Servo-motor feedback assures linearity between fan drive and fan speed. The pump has been provided with a double speed motor. Part of the flow is led through the cooler.
Temperature specifications;
-a fixed point on one of both accelaration sections 45±.1¡C
-the inlet water of the RWG network 45±.5¡C
-the outlet water of the cooler 40±1¡Crelated paper; J.G.Noomen et al., "A modular cooling system for the MEA high dutyfactor electron linac", IEEE trans. on Nucl.Sci.,NS-28(1981).
Send remarks to pieth@nikhefk.nikhef.nl
Updated 05/01/95
For the cooling of the MEA electron accelarator a modular setup has been selected. Each klystron station has been proveide with an own pumping system and dry cooler. A simplified layout of a modulator unit is shown.
The three blocks represent respectively: