WO1997007574A1

WO1997007574A1 - Free electron laser with safety circuit

Info

Publication number: WO1997007574A1
Application number: PCT/GB1996/001811
Authority: WO
Inventors: John Raymond Hartley
Original assignee: British Nuclear Fuels Plc
Priority date: 1995-08-19
Filing date: 1996-07-30
Publication date: 1997-02-27
Also published as: GB9517018D0

Abstract

In a free electron laser system with energy recovery, the system having an electron gun (110) to supply a beam of electrons; a wriggler array (114) to supply a spatially alternating magnetic field to the electron beam whereby a microwave beam (118) is generated; separating means (115) to spatially separate the microwave beam and the electron beam emerging from the wriggler array; electron collection means to receive the separated electron beam; and a high voltage, low current supply (122) and a low voltage, high current supply (124) connected in series to supply the gun (110); the high voltage supply also being connected between the electron collection means (120) and the gun (110); any instabilities are sensed by sensing the current through the high voltage supply (122); comparing the sensed current with a preset value; and disconnecting the EHT supply (122) from the gun (110) when the preset value is exceeded.

Description

FREE ELECTRON LASER WITH SAFETY CIRCUIT

This invention relates to a free electron laser (FEL) especially a FEL having an energy recovery system, and provides a method for sensing and controlling conditions which could otherwise lead to lower safety levels than are desirable.

In a FEL, an electron gun directs an electron beam into a "wriggler" comprising two parallel, spaced sets of magnets arranged to provide magnetic ields in spatially alternate directions through which the electron beam passes. The electrons are deflected first in one direction then in the other. When viewed along the beam axis, the electrons appear to oscillate, and therefore they emit electromagnetic radiation. A basic disclosure is given in electrotechnology, December 1991/January 1992 in a paper by R A Stuart and J Lucas.

In FELs a substantial proportion of the kinetic energy in the beam, which may easily be many kilowatts, is wasted as heat and x-ray production caused by electron bombardment of a collector. In an energy recovery system, also ^described in the paper by Stuart and Lucas, the collector is held at approximately the same voltage as the electron gun. The kinetic energy of the electrons is converted to potential energy. The energy dissipated in the collectors may now be only a small percentage of the previous level, and the x-ray emission is enormously reduced.

Stuart and Lucas point out that in such an energy recovery arrangement, it is essential to have both a high voltage, low current supply and a low voltage, high current supply.

Figure 1 illustrates a FEL with energy recovery as described by Stuart and Lucas. An electron gun 10 provides an electron beam 12 which passes through a wriggler 14 from which emerge both the unconverted electron beam 16 and a microwave beam 18. The microwave beam can be used as any conventional laser beam. The electron beam is diverted by magnetic means, shown schematically at 15, to an electron trap 20.

Considering now the electrical supply arrangements, the electron gun 10 is supplied by an EHT supply 22, typically at 150kV and an isolated power supply 24, typically lOkV.

In Figure 1, the current equivalent to the electron beam 12 is represented by I_EB; the electron trap 20 does not collect all of the electrons, some are scattered by residual gas particles and collide with the gun anode; the equivalent current is represented as I_SE, passing to ground. The energy recovery provided by the presence of the source 24 is equivalent to I_EB - I_SE In theory, 100% recovery is possible, but in practice lower recovery rates can be expected. An additional loss may be current leakage in the EHT system, represented by current I_L, passing through resistance R_L.

It has been found that the isolated supply 24 provides all of the power used to make up the loss to the electron beam, I_EB - I_SE. The loss equivalent to current I_SE + l_L is supplied by the EHT supply 22. Overall, an energy-efficient ;*FEL system is provided.

However any breakdown of the system could lead to the emission of x-rays at a dangerous level. It is an object of the present invention rapidly to sense and control any instabilities in an FEL having an energy recovery system.

According to the invention, a method of sensing instability in a free electron laser system with energy recovery, the system having: an electron gun to supply a beam of electrons; a wriggler array comprising two parallel arrays of magnets arranged to apply a spatially alternating magnetic field to the beam whereby a microwave beam is generated; means to separate the beams of electrons and the microwave beam emerging from the wriggler array; electron collection means to receive the separated beam of electrons; and a high voltage, low current supply and a low voltage, high current supply connected in series between ground potential and the electron gun, the low voltage, high current supply also being connected between the electron gun and the electron collection means; comprising sensing the current through the high voltage, low current supply means, comparing the sensed current with a first, preset value, and providing an output signal when the first preset value is exceeded.

The signal may be an alarm signal, but preferably the method further comprises disconnecting the low current high voltage supply from the electron gun when the pre-set value is exceeded.

Preferably the method further comprises the step of comparing the sensed current value with a second, preset value higher than the first value and arranging automatically to switch the EHT supply to a constant current supply mode when the second preset value is exceeded. An indication of operation of the EHT supply in this mode may be provided. f The invention will now be described with reference to Figure 2 in which items of apparatus identical to those in Figure 1 are given the same reference numeral but advanced by 100.

In addition, there is now provided a current sensing resistor 126, connected between ground and the EHT supply 122, and a control and trip unit 128 connected across the resistor 126 and to the EHT supply 122. The resistor 126 may be the current sensing resistor conventionally provided in an EHT supply, or may be an additional resistor in series. The control and trip unit 128 may also be the conventionally provided unit or an additional unit.

The invention is based on the realisation that the current provided by the supply 124 is at least a factor of 10 greater than the current provided by the supply 122. An increase in the current loss to ground, I_SE, results in a much larger percentage increase in the current through the EHT supply 122 than the percentage fall in the current through the floating supply 124. This gives high sensitivity to transients or to long term changes in the efficiency of the energy recovery system.

Typically the current through the EHT supply 122 equivalent to I_L + I_SE is about 150 microamps; if this current increases to, say, 300 microamps, then there is a clear indication of a malfunction in the system.

Control and trip unit 128 senses the current drawn by the electron gun 110 from the EHT supply 122; any loss of control of the direction or focusing of the electron beam 116, or any electrical breakdown of the system results in a rapid increase of I_SE and therefore in current through the resistor 126. Any significant transient or change in the DC operating condition may indicate an increase in x-ray emissions. The control and trip unit 128 is therefore arranged to contain a first, preset value of currents through the resistor 126; if the current exceeds this value, ;fche supply 122 is disconnected from the electron gun 110. Production of electron beam 112 therefore immediately ceases and any x-ray emission is prevented.

In addition, the constant current facility of a conventional EHT power supply 122 can be used to provide a further safety feature. If the current drawn from the supply 122 exceeds a second level, the EHT supply will automatically change to constant current supply mode. If this second current limit is set at, say, 25% above the first current limit, then a large increase in current cannot be supplied through the supply 122, this gives a very sensitive indication of the malfunction of the supply 124.

Overall, the arrangement in Figure 2 provides three safety features:- a. a fast, reliable trip signal indicating the partial or complete failure of the energy recovery system, which could result in the generation of high x-ray emission; b. a stable and reliable signal for monitoring long term changes in the energy recovery system and in the electron beam focusing system of the FEL; c. detection of transient fluctuations arising from electrical breakdown.

As illustrated, the resistor 126 carries current arising from both the electrical breakdown on the EHT supply and from transients in the electron beam control supply.

In a variation, not illustrated, the resistor 126 could be repositioned to measure only the leakage of electrons from the grounded parts of the illustrated system, provided additional safety features were provided to deal with features a and b above.

In a variation, the position in the circuit of supply 124 may be changed to the position of supply 24 in Figure l.

In a further variation, the electron gun 110 may be earthed and the wriggler 114 held at the same voltage as the •EHT supply 122; resistor 126 can operate as before, provided it is sited at a position in the circuit at which it can be earthed.

Claims

1. A method of sensing instability in a free electron laser system with energy recovery, the system having: an electron gun (110) to supply a beam of electrons (112) ; a wriggler array (114) comprising two parallel arrays of magnets arranged to supply a spatially alternating magnetic field to the electron beam whereby a microwave beam (118) is generated; separating means (115) to spatially separate the microwave beam (118) and the electron beam (116) emerging from the wriggler array; electron collection means (120) to receive the separated electron beam; and a high voltage, low current supply (122) and a low voltage, high current supply (124) connected in series so as to supply the electron gun (110) , the high voltage, low current supply (122) also being connected between the electron collection means (120) and the electron gun (110) ; comprising sensing the current through the high voltage, ;low current supply (122), comparing the sensed current with a first, preset value, and providing an output signal when the preset value is exceeded.

2. A method according to claim 1 further comprising disconnecting the EHT power supply (122) from the electron gun (110) when said preset current is exceeded.

3. A method according to claim 1 further comprising the step of comparing the sensed current with a second preset value higher than the first value and switching the EHT supply to a constant current supply mode when the value of the sensed current exceeds the second value.

4. A method according to claim 3 further comprising the step of indicating when the EHT supply (122) operates in constant current mode.