New developments in pulsed power technology and associated electron beam transport mechanisms at the United Kingdom Atomic Weapons Establishment

Abstract

The Pulsed Power Group at AWE is interested in the generation of s d , intense, multi-megavolt X-ray sources for radiographic applications. For this paper, research has been carried out into electron beam transport and focusing studies on one of our single pulse forming line flash X-ray machines.Experiments were undertaken on E Minor to improve the radiographic spot size generated by our 'Paraxial diode'. This is a novel anode I cathode assembly designed to produce a small diameter electron beam focused onto a high Z target.Electrons are propagated through a gas-filled cone such that their electrostatic repulsive and magnetic attractive forces are almost cancelled. For this paper, research has been undertaken to opt-the paraxial diode to improve its performance in both dose output and radiographic spot size. This was achieved initially by varying the drift cone length and subsequently the pressure of the gas in the drift cone. Specific gas species (dry air, helium and laypton) were then applied to the drift cane to study their effect on the efficiency of the diode. A short aluminium secondary ceU, maintained at vacuum pressure, has now been added between the drift cone and the tantalum target. Experiments have shown that this 'secondary cell enhanced paraxial diode' gives a consistently smaller radiographic spot, an improvement of over 30% with only a small reduction in dose of 10%. Gas has now been introduced at a low pressure to the secondary cell to firther increase the neutralisation of the beam's self-electric forces without increasing the neutralisation of its magnetic forces. This has shown great prornise with a further improvement in radiographic spot size. 0 Crown Applied Physics niv, Ias A l a " Nat Lab, Los A b s , NM 87545 IIigb-DOSe X-my R a d h~~p b y Stability of radiographic spot has been under intensive study in recent years bccausc of its Unpartcmce in high-dose flash x-ray radiography. Wc havc investigated n concept of placing a gas cell in fiont ofthe tuget tu improvc thc stability o f the spot size. The idca is ta use the ihcoming elcclron bcam to ionkc thc gas 1 x 1 to II fhvornble demity (in a time scale ofpulse rise lim). whicli causcs clurge and current neutralization on the electron beam. For an elwtrun bcam m C h as thc DARIIT befun (2 kA and 20 McV), which is well beIow the Mven current limit, wc bclicve t h t there is an intermediate gas prcssurc regime where. the sbhk beam propagation can be a c l h d . h i our study, we first use a simple rate equntion to calculate the ioniaion ofthc gas cc11 by the DAIIRT beam. 'lhc modcl includes avalaiwh? mid recomhinotion of the ionized elwtrons. Assuming the ionized pksma Im 8 Maxwellinn distrjbutioii with a tcmperature spread of'T, thc cvohtion of thc plwtnu is calculnted as P function of bcam parameters, grrs pressuic, ionization E-, orid To. The regimc where stable lxxm propagation can occur is boundcd by two limits. Thc lower .limit is determined by ihc onsct o f transvcrsc two-stream in,stcrhility nl under-dcnsc mgime, and th: uppcr limit ...