The development of RF excited guns for high power electron beam welding

TWI Bulletin, September/October 2003

As Technical chief of the Electron Beam (EB) Group, Allan Sanderson is concerned with the design and application of EB welding equipment. He obtained his doctorate on the generation and control of high power beams. This pioneer work led to the dramatic breakthrough in single pass thick section EB welding which has subsequently been applied to a wide range of industrial applications. This paper was first presented at the International Institute of Welding in Hamburg where he received the 1998 Arata award for his outstanding achievements in fundamental research in welding science and technology.

The mobility of gun columns is becoming increasingly a key issue in the design and construction of major installations. As Allan Sanderson reports, for in-vacuum EBW guns any reduction in size, weight or complexity of the welding head is of great benefit; it saves chamber space, reduces the volume of the chamber necessary to permit the required maximum weld lengths, limits pumping costs and minimises gantry loads. All of these factors reduce the overall cost. The reduction in size, complexity and maintenance down-time brought about by the introduction of this unique RF technology has been quite remarkable.

The generation of electron beams for material processing requires not only a high voltage supply to accelerate the electrons but also auxiliary supplies to provide cathode heating and beam current control. Conventionally, the auxiliary supplies are sited in the main high voltage supply tank and must be insulated to withstand the maximum supply voltage. The supplies are then fed down a multi-core high voltage cable to the electron gun. This generally involves the use of a cumbersome large diameter high voltage cable, which is both stiff and prone to internal breakdown. In addition, the auxiliary supplies, which represent a large proportion of the power generation equipment, are complex and prone to failure, particularly during gun discharge events. To avoid the inherent problems associated with conventional auxiliary power supplies, a special locally coupled RF resonant transformer and gun assembly was designed and constructed. This compact, rugged, low cost auxiliary supply, providing both filament and cathode heating power has been successfully applied to a series of high vacuum, Reduced Pressure and non-vacuum machines for generating high power beams.

This report highlights the key elements of the RF technology and its applications in several major high power installations.

The reliability, reproducibility and longevity of EBW guns and power supplies are of paramount importance particularly when welding high value components. Moreover whether it be low, medium or high power operation the quality and consistency of welding is fundamentally linked to the performance of the gun and power source.

Since the pioneering work on the development of high power EBW in the early 1970s, the EB Group at TWI has operated a wide range of power supply and gun types. This has resulted in a progressive evolution of designs and configuration of equipment to achieve the best performance. The earlier high power work was conducted using fairly conventional power sources and directly heated triode guns, but these were shown to be prone to all manner of shortcomings. Filament life was short (typically three hours), beam voltage and current ripple high, beam reproducibility generally poor, and gun discharge tendency high especially for light alloy welding. Although, for example, filament life could be extended and gun discharging largely suppressed by the introduction of devices such as a magnetic trap (which filters out charged and uncharged particles streaming from the weldpool), ^[1] this led to some loss of beam quality, particularly for long working distance operation. Also, the magnetic trap device, of course, did nothing to prevent gun discharges caused by dust particles present in the gun housing or gun electrode surface roughness.

The earlier high voltage power supplies based on 400Hz motor generator technology used relatively crude current overload protection. Current overload level was adjustable, either to trip on the smallest of gun micro-discharge events or at the other extreme, perpetuate power connection beyond a reasonable time period, causing severe roughening of the gun electrodes and consequently an increased frequency of discharging.

It was recognised during the early development years, that there was a strong need to use an indirectly heated gun to extend cathode life and improve beam consistency. ^[2] However, the addition of a third auxiliary power supply, ie the back bombardment supply for indirect electron beam heating (as well as the filament and grid supplies required for triode grid control) further complicated the design of the high voltage tank, the high voltage cable requirements, the electron gun and also the multi-pin high voltage cable connectors located both in the high voltage tank and the electron gun column. Especially in the electron gun, the design requirements of the multi-pin, high voltage feedthrough insulator, which has to dissipate considerable heat from the electron gun, provide sealing between the gun vacuum and the termination medium (usually oil), and be dimensionally stable, posed formidable design problems. Moreover, high voltage gun insulators which met all these criteria were far too often prone to interpin breakdown during gun discharge events.

Figure 1 illustrates the complexity of a conventional power supply system for an indirectly heated triode gun. Such systems were also very prone to auxiliary power supply failure, cable inter-lead failures and invariably led to a large diameter inflexible cable quite unsuitable for mobile gun systems.

Quite apart from the mentioned shortcomings of these earlier conventional systems, low frequency power supplies, eg 50-400Hz, required high capacitance in order to limit the beam voltage and current ripple levels. Stored energy in the high voltage tank and the cable is very detrimental to the operation of an EBW gun since the associated high discharge currents cause gun electrode roughening, again perpetuating discharge occurrence. An increase in supply frequency was therefore sought, especially for high power operation where the capacitive smoothing requirements would normally be much greater for the same voltage ripple level.

All of these considerations demanded a fundamental re-evaluation of gun design and power source philosophy. This has resulted in a unique indirectly heated gun and switch mode power source requiring no conventional auxiliary power supplies, and providing solutions to almost all of the problems which plagued earlier equipment design. This report summarises the work carried out over the 1988-1998 period on EBW equipment development for high power (up to 150kW) in-chamber, Reduced Pressure and non-vacuum equipment. This report is particularly concerned with the development of RF excited guns. Further information on switch mode power supplies and the application of this new technology was presented at the CISFFEL 6 Conference. ^[3-6]

RF excited gun developments

Figure 3 shows typical filament/cathode temperature plots obtained for the prototype system obtained using optical pyrometers directed at the heated elements. It will be seen that the filament temperature rises rapidly, achieving red heat at a total RF input power of less than 12W for a 0.1mm thickness, 1.5mm wide tungsten alloy filament. For a power input of some 120W the filament temperature reaches a plateau level and further increase in input power results largely in cathode heating. For a lanthanum hexaboride cathode sufficient emission for a 100kW beam is achieved at a cathode temperature of approximately 1600°C with a total RF input power of less than 160W including feed cable, aerial and other losses.

Of course, if the primary beam is allowed to strike the rear of the lanthanum hexaboride button directly, it is possible to derive a reverse electron beam during the positive half cycle which will be directed at the filament causing further temperature rise and enhanced emission. However, it has been found that the increased emission, to a large extent, is self-limiting since it results in a lowering of the resonant series current.

The low level of power input (equivalent to that of a large domestic light bulb) combined with an efficient heat exchanger, ensures that the gun cartridge assembly operates at a very low temperature, ie typically less than 150°C for all parts including the filament holder.

In comparison, an earlier conventional directly heated 100kW triode gun has been found to operate at temperatures in excess of 350°C and a later conventional indirectly heated diode at 445°C. The dramatic reduction in operating temperature minimises differential expansion of gun components and enables the gun housing to be vented for maintenance purposes immediately after shutdown without risk of oxidation of major components. Also, handling of the gun cartridge after long periods of operation is much easier and there is far less risk of threaded components becoming bonded.

In order to stabilise the main beam current, a DC current transformer (DCCT) is used which monitors the electron flow either directly (by placement in the gun column) or by placing it in the high voltage transformer earth return circuit. A feedback loop then compares the beam current obtained with the demand level making dynamic changes to the RF power level to achieve the desired beam current level. Although the beam current versus RF characteristic curve is steep it has been possible to achieve good stability over the entire power range using a proportional-integral-differential control loop.

In more detail the new RF excited gun element is configured as shown in Fig.4 where it will be seen that there is only one connection required to the high voltage supply. This greatly simplifies the design of the high voltage feedthrough insulator and allows single core flexible cable to be used (avoiding the need for heavy primary filament leads and relatively high voltages between the cable inner leads), Fig.5. The aerial consists of a thick single turn loop of metal which is grounded at one end, thus providing excellent protection for the RF supply from high voltage flashovers, Fig.6. In addition, the physical clearance of the aerial from the high voltage gun component can be in excess of 80mm thus greatly reducing the possibility of discharges to the aerial.

This technology has allowed the design and construction of a series of high power guns. The first prototype using a resonant frequency of 49MHz was used to generate beams in the EU86 Eureka Non-vacuum project. Beam power levels of up to 110kW at 270kV were successfully produced and welds made in steel up to 60mm at atmospheric pressure. Subsequently, a more advanced 84MHz resonant transformer and filament/cathode structure were developed for a 150kV, 100kW in-chamber gun, Fig.7. This innovation allowed the gun including the resonant transformer to be housed in a 0.2m ³ cube with excellent access for gun maintenance. With a side entry high voltage cable termination the overall gun column length was only 760mm for a structure including two focusing lenses, a double high speed deflection system, a comprehensive colour TV camera and coaxial viewing system and a DCCT, Fig.7. 

More recently the 84MHz design has been incorporated into several major installations for thick section copper welding on large scale nuclear fuel waste disposal canisters ^[3] and into prototype EB equipment for offshore J-pipe laying ^[4] . In these two systems the gun housing is 0.3m ³ permitting operation at present up to 220kV with a planned extension to 300kV for non-vacuum equipment with an ultimate power rating of 150kW.

Gun operating modes

In principal, the RF excitation technology can be applied to all manner of gun types including indirectly heated diodes and triodes but as mentioned earlier, the EB Group at TWI has concentrated in recent years on the design of diode guns for the following reasons. The use of a grid electrode in a conventional triode electron gun can be used very effectively to change the beam current rapidly. The negative potential applied to the grid also acts like an iris, limiting the active emission area of the cathode which potentially improves beam brightness particularly at low current level. Unfortunately, the electric field created by the grid produces considerable spherical aberration, Fig.8. The outer electron trajectories are focused more strongly than the inner trajectories resulting in a spread of focus which reduces beam quality and intensity in the beam at the workpiece.

Also, as the grid voltage is altered the beam convergence angle dramatically changes from, at best, a highly convergent beam at low current, to a near parallel beam at high current. This is accompanied by a strong axial displacement of the primary cross-over, Fig.8. It is difficult to avoid these effects particularly for medium and high power guns. Electrode shaping in an attempt to control high levels of space charge at the high current level, in order to achieve modest beam convergence, invariably result in excess convergence at the lower current levels, whereas acceptable convergence at low current levels generally gives rise to a near parallel beam at the high current level, or indeed a divergent beam. In the case of near parallel beams at high power level, subsequent magnetic focusing fields have little effect since the beam envelope shape is largely controlled by positive ion effects; thus beam intensity at the workpiece cannot easily be adjusted.

Clearly, these characteristics of a triode gun are undesirable for systems where axial focus stability and high beam intensity are important. For example, where it is necessary to transmit the beam through small orifices, as in the case of Reduced Pressure or non-vacuum EB machines, the effects are detrimental. Further, changes in axial focus position with beam current invariably entails considerable adjustment of the focusing lens coil current in sympathy with beam current changes to avoid nozzle damage.

For diode guns, where beam power is controlled by cathode temperature, there are no aberrant elements and near theoretical maximum beam intensities for a given cathode diameter can be approached. Moreover, the shift in primary cross-over position with increasing beam current is far less pronounced.

Consequently, it is possible to operate Reduced Pressure equipment with fine bore nozzles separating the various pressure stages without the need to change focus lens settings over the entire power range.

Regarding the equally important comparison of gun discharge performance of triodes versus diodes, it has been observed that most high voltage gun discharge events in conventional triodes are associated with an instantaneous loss of grid potential which unleashes the full power potential of the gun. This is a highly undesirable characteristic which in the case of high vacuum EBW results in excessive component melting and in Reduced Pressure or NVEB equipment nozzle damage, both of which can precipitate further discharges events.

In contrast, the beam current in a diode gun under gun discharge conditions invariably tends to reduce and when used in combination with an intelligent switch mode power supply with flashless control, component damage and defects can be avoided.

Conclusion

RF excited guns are now being used universally in all EB installations at TWI (with the exception of two 150kV, 6kW commercial equipments) and also in strategic projects concerned with nuclear waste burial and offshore pipe welding. ^[3,4] It is believed that this development represents a significant breakthrough in EB gun technology which can now be applied to great advantage in not only high power equipment but also in medium and low power equipment.

Some of the benefits are:

• Reduction in size and complexity of the EB gun column
• The possibility of single core small diameter high voltage cable
• Greatly improved beam axial stability and quality over the entire power range
• Avoidance of severe component damage by gun discharges
• Greatly simplified high voltage power supply

References