INVESTIGATION OF THE SELF MAGNETIC PINCH DIODE ON PIM AS A RADIOGRAPHIC SOURCE S. Cloughξ, K. Thomas, M. Williamson, A. Jones AWE, Aldermaston, Reading, Berkshire, RG7 4PR. United Kingdom
Abstract PIM is a water filled Blumlein connected, in parallel, to one or two 1.5 MV, cavities which inductively add voltage on a high impedance (100 Ohm) vacuum transmission line. A self-magnetic pinch configured diode was tested on this 1-3 MV IVA pulse power machine and an investigation into the performance of this radiographic diode was made using simple hollow cathodes with a plane, grounded anode configuration. Time dependant resolution of the total radiation obtained and a 2.5 mm diameter (AWE defined) source was demonstrated. Closing either one or two of the PFL switches on PIM was used to investigate the effects of rise time and peak voltage on the diode.
PIM’s 10 Ω water filled Blumlein generates a 1.5 MV, 100 ns output pulse and is switched by two Laser Triggered SF6 Gas Switches (LTGS) in parallel. A PrePulse Reduction System (PPRS) [3] was installed on the Blumlein output to reduce the pre-pulse by 100. The Blumlein drives two inductive cells producing a voltage gain by inductive addition with an associated reduction in vacuum stalk current. The tapered, high impedance, vacuum stalk threading the cavities was made from affordable components and not matched to the low impedance of the pinch diodes or the Blumlein driver. The machine was under-run with 1.2MV on the Blumlein, which ensured reliable machine operation throughout the self-magnetic pinch (SMP) diode investigation without pulsed power problems effecting the diode operation.
I. INTRODUCTION II.
The PIM (Prototype IVA Module) was conceived as one Blumlein-inductive cell module of a 10 module, 14 MV, IVA pulsed power machine [1] and is driven by 32, 1.3µF Maxwell capacitor Marx generator. Subsequently a second cell was added to PIM in parallel [2] to gain further understanding of modelling the machine and potentially obtain up to 3 MV at the diode. Figure 1 illustrates PIM in the present configuration.
DIODE CONFIGURATION
Silver coated layer on radiused profile
Target
Marx
Cathode tip
Pre-pulse inductor
Split coaxial oil feeds
Aluminium foil
Electron Beam stopper
Figure 2. Schematic of a self magnetic diode operating above 2.2 MV.
Laser Triggered Blumlein Switches
Water filled Blumlein
E-Beam diode location
Figure 1. Schematic of PIM.
ξ
A number of anode-cathode (AK) configurations were examined to confirm the successful operation of a SMP diode and establish some performance baseline for these diodes on PIM. The initial tests were performed on tantalum targets with an aluminum foil stood off 0.75 mm from the target. A silver painted cathode tip (Figure 2) which was tapered to a radius. This radius on the cathode tip appears necessary on present pulsed power machines
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at AWE due to electric field enhancement during the prepulse causing emission from the low work function silver painted surface. This configuration is typical of SMP diodes operated at above 2 MV [4], where pre-pulse is greater than 0.5%. This tip was used as initial configuration to act as a baseline for characterising the diagnostics. The subsequent investigations looked at the dose increase as Blumlein rise time was decreased,[3] dose scaling with the density of target material and the dependency of cathode geometry on diode operation. The installed PPRS improved the performance of low impedance diodes such as the SMP diode. Prior to its installation pre-pulse was greater than 100kV and only 3R at 1m was achievable. In this configuration PIM was able to fulfil the goal of this study - to demonstrate successful operation of the SMP diode and to complete a baseline series of shots prior to a future optimisation study. The low pre-pulse on PIM permitted a small radiused edge on the cathode tip to be applied without detriment to the diode operation. Conceivably this offers substantial scope for experimental research however due to time constraints only some exploratory tests involving internal bore diameter and its depth were investigated (section 4). The majority of this study (section 5) investigated diameter of the cathode tip and measured the optimum AK gap. Profiling the internal bore of a SMP tip with an internal 45º chamfer was postulated to perform better than the standard tip geometry due to improved uniformity of the electrostatic fields in the axial, tip region. However this tip configuration provided lower measured dose than expected and may be due to the internal void volume being inadequate for expanding plasma during the pulse.
III.
TARGET ASSEMBLY
A foil layer is installed on SMP diodes operated above 2.2 MV however there is a transition voltage between 0.8 and 2.2 MV below which the omission of the foil is beneficial. The internally chamfered tip series; cathode tips of 6 mm O.D. is incomplete, in that the optimum diode configuration was not determined, (Figure 3) but the result confirms that the absence of a foil layer at ~1.5 MV increases the dose by as much as 50%. This tip configuration produced to largest improvement in dose whereas all types of cathode tips examined were tested at least once without the foil layer and showed some dose increase without changing the spot size. Generally tips were tested with the foil layer installed because it is was envisaged that PIM will be operated at higher voltage.
IV.
CATHODE TIP SHAPE
The operation of SMP diodes on multi-cavity IVA machines was considered and one problem arises from diode debris especially when this debris is deposited on the X-ray tube section. Sputtered target material deposits a relatively uniform layer requiring an appropriate maintenance schedule. The cathode was found to fragment due to damage caused by the ejected target material. The baseline, brass cathode tips, (Figure 4b) which were considered the optimum geometry for high voltage diode operation but they produced fragments up to 8 mm in length. Such fragments lodged in the X-ray section were within the safe operating envelope of PIM but due to their size, regular cleaning was necessary. However increasing the depth and shape of the internal void removed all tip debris problems without effecting the diode performance.
6
a)
Spot Size /mm
Without Foil 5 4 Foil
3
8
2
Without Foil 4
b)
Dose /R@1m
6
2
foil
0 0
2
4 6 AK Gap /mm
8
10
Figure 3. The effect due to presence of a target foil on dose (blue) and spot size (black) for an internally chamfered cathode tip.
Figure 4. Self-Magnetic pinch tips a) Modified SMP tip with large internal void which does not break up b) least damaged standard SM pinch tip tested.
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Thin walled cathode tips where able to be investigated, (Figure 4a has a cathode wall thickness less than 1 mm), without disintegration of the cathode tip provided there was sufficient depth to the bore. 1.0 Radiographic Figure of Merit
0.8 0.6 0.4 7
0.2
6
Careful monitoring of the impedance of the SMP diode impedance was necessary as mismatched diode impedance on PIM would create reflected pulse problems on the X-ray section. Increasing the diode impedance was achieved by reducting the cathode tip diameter. Five tips sizes were examined with outside tip diameters of 15, 9.5, 8, 6 and 5 mm. The dose increased with diode impedance, with the exception of low dose from the 6 mm diameter, chamfered tip. This was believed to be caused by the low volume of the internal void in the cathode tip. The smallest tip produces a FOM above 2( Figure 6) and this is better than the present radiographic machines operating at 2.2 MV.
0.0 Spot Size / mm
5 4 3
12
2
10 Dose / R @ 1m
1 8 0 6
The first tip series used the “optimum” design for the higher voltage machine. However, it was found that a 3 mm spot size was measured with an optimum FOM of 0.9, figure 5. This relatively high spot size implies that insufficient current was present to pinch the e-beam effectively. Reduction of the AK gap provides a lower impedance but it also reduces the dose and FOM. Several tip diameters were therefore investigated.
4 2 0 0
1
2
3 4 5 Anode Cathode Gap / mm
6
7
LSF equivalent UFC ESF
8
ESF
Figure 5. Radiographic performance of the baseline SMP tip
LSF
V. CATHODE TIP PERFORMANCE The performance of a radiographic diode is measured in terms of a radiographic figure of merit (FOM) which is 2 Dose/(Spot size) . At the configuration for optimum FOM 2.65 the dose scales as IV . Two machines at AWE routinely operate SMP diodes and were used to provide comparative data points, these are MEVEX, at 0.8 MV, spot size of 2-2.5 mm and Mini-B, operating at 2.2 MV with spot size of 2.5-3 mm. The voltage on PIM is variable in a range over these points and for all diodes should follow the dose scaling law. 3.0 Mini-B at 2.2 MV
2.5
10 2.0 8 1.5 1.0
6 4
Mevex at 0.8 MV
0.5
Dose / R @ 1m
Radiographic Figure of Merit
12
4
5
6 7 8 Position at Source / mm
9
10
Figure 7. Edge Spread Function (ESF) and Line Spread Function (LSF) of the radiographic source from 5 mm OD cathode tip with the best fit Uniformly Filled Circle (UFC) of 2.0 mm diameter. The diode tip which produces optimum FOM, (5 mm OD) had a 2.0 mm spot size (AWE definition) (Figure 7). However the fit to a uniformly filled circle was poor. This may be due to the limited current available from PIM in its present configuration (~40 kA). An improved vacuum stalk design [5] is due to be installed to better match the Blumlein output through to the diode, delivering an increased current and voltage.
2
0.0
VI.
0 0
2
4
6 8 10 12 Cathode Tip Diameter / mm
14
FURTHER WORK
16
Figure 6.Radiographic performance of SMP diodes with various cathode tip diameters.
PIM is an ideal machine in which to investigation the presence of a foil layer on SMP diodes as it offers a voltage range from 1 up to a potential 3 MV and with
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such a low pre-pulse that a large range of cathode tip designs can also be investigated. A matched stalk is due to be added to PIM and by charging the Blumlein to a higher voltage will increase the voltage delivered to the diode to 2-2.5 MV. Improved laser triggering of the Blumlein switches will enhance the diode investigation capabilities, by reducing timing jitter of the diode voltage from ±14 ns to ~2ns. The FOM obtained from these initial experiments using a low prepulse IVA machine was slightly better than current radiographic machines. However radiographic benefit is possible if the FOM could be doubled to ~4. This may be met by operating PIM near to 2.4MV, (scaled voltage ~ 18R at 1m) and maintaining a 2.0 mm spot size or, operating the diode at approximately Mini-B voltage and reducing the spot size by ~10%.
VII.
REFERENCES
[1] K.J. Thomas, M.C. Williamson, S. G. Clough and M. th J. Phillips, “Prototype IVA Module (PIM),” Proc. 13 IEEE Int. Pulsed Power Conf., Las Vegas, U. S. A., p306-309, 2001 [2] S. G. Clough et al., “Experiments on PIM in Support of the Development of IVA Technology for Radiography at AWE,” 14th Int. Conf. on High-Power Particle Beams, Albuquerque, U. S. A., pp 167-170, 2002 [3] K.J. Thomas et al., “Results of the Installation of a Prepulse Reduction System on the PIM,” presented at the 15th IEEE Int. Pulsed Power Conf., Monterey, U. S. A., 2005 [4]I. Crotch, J. Threadgold, M.A. Sinclair and A. Pearce, “Self-Magnetic Pinch Diode Experiments at AWE,” 26th Int. Conf. on Plasma Phys., Albuquerque, U.S.A., pp 507510, 2004 [5] S.G. Clough, S.J. Croxon, A.W.P. Jones, M.J. Philips, K.J. Thomas, M.C. Williamson, B. Oliver and J.E. Maenchen, “Upgrade Designs to PIM Using PIC and Transmission Line Modelling,” 2nd European Pulsed Power Symposium, Hamburg, Germany. 2004
Crown Copyright (2005) “This document is of United Kingdom origin and contains proprietary information which is the property of the Secretary of State for Defence. It is furnished in confidence and may not be copied, used or disclosed in whole or in part without prior written consent of the Director Commercial 2, Defence Procurement Agency, Ash 2b, MailPoint 88, Ministry of Defence, Abbey Wood, Bristol, BS34 8JH, England”.
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