[en] On the basis of time-integrated emittance measurements, several different types of field emitter diodes were characterized at 1 to 3 kA, 1 MeV. These measurements were part of the design effort for the injector system of a linear induction accelerator, to be used as a flash x-ray source at Lawrence Livermore Laboratory. The experimental parameters were the cathode type, the anode mesh texture, the diode spacing and voltage, and the level of collimation of the emerging beam. Experimental results are presented that show that over a wide range, the emittance was proportional to the level of collimation. For any one diode, with the spacing left fixed, the emittance was found to be essentially independent of the diode voltage and current. Differential focusing of different energy beam components affects most strongly the peripheral components of the beam, and strong collimation was found to minimize the effects of momentum spread on the emittance. The lowest emittances (30 to 40 mr-cm at 400 A) were obtained with a foil-type cathode in a ball-over-plane configuration, using an etched tungsten mesh anode, and collimating the beam to one quarter of the total current

[en] A switching system using low-jitter spark gaps has been designed for a 20 MeV Flash X-Ray (FXR) linear induction accelerator that is being built at the Lawrence Livermore National Laboratory. The accelerator, which is now under construction, will consist of 54 modules connected in tandem. Each module will contribute up to 400 keV of energy to the electron beam traversing it. The experimental work included a test of one accelerator module designed to produce 400 keV. Of particular concern was the thorough characterization of the Blumlein switch in terms of lifetime, voltage hold off and operating pressure range to produce minimal jitter and an acceptably low prefire rate. The data was recorded by means of a computer-based data-acquisition system which was set up for later, automatic data reduction and display fo the results. Optimization of the spark gap operating parameters resulted in a firing jitter of less than one nanosecond rms

[en] The FXR machine is a nominal 4-kA, 20-MeV, linear-induction, electron accelerator for flash radiography at LLNL. The machine met its baseline requirements in March 1982. Since then, the performance has been greatly improved. We have achieved stable and repeatable beam acceleration and transport, with over 80% transmission to the tungsten bremsstrahlung target located some 35 m downstream. For best stability, external-beam steering has been eliminated almost entirely. We regularly produce over 500 Roentgen at 1 m from the target (TLD measurement), with a radiographic spot size of 3 to 5 mm. Present efforts are directed towards the development of a 4-kA tune, working interactively with particle-field and beam transport code models. A remaining uncertainty is the possible onset of RF instabilities at the higher current levels

[en] Measurements have been carried out to characterize a 1 MeV, 1 to 5 kA field emitter diode and to determine the emittance of the electron beam generated. The diode was found to be approximately space charge limited, and the beam emittance at 300 to 500 A of collimated beam was measured at 30 π to 70 π mr-cm, depending on the emitter geometry. Simple confinement of the beam over a one meter distance also was demonstrated, using axial magnetic confining fields of less than one kilogauss

[en] Construction of the new flash x-ray induction LINAC (FXR) at Lawrence Livermore National Laboratory has been completed. Initial tuning of the machine has produced stable current pulses in excess of 2 kA at the design energy of 20 MeV, with an 80 ns FWHM pulse width, producing single-pulse radiation doses near 500 Roentgen at one meter from the target. The electronic spot size on the bremsstrahlung target is estimated at 3 to 5 mm. In this paper we will discuss the basic FXR design; running-in and tuning of the machine; emittance measurements; beam stability; switch gap synchronization; and measurements of the radiation dose and angular distribution

[en] The now-mature FXR (Flash X-Ray) radiographic facility at Lawrence Livermore National Laboratory will be briefly described with emphasis on its pulsed power system. The heart of each accelerating cell's pulse forming Blumlein is it's sulfur hexafluoride-based triggered closing switch. FXR's recent upgrade to a recirculating SF₆ gas reclamation system will be described and the resulting accelerator performance and reliability improvements documented. This was accompanied by a detailed switch breakdown study on FXR's Test Stand and the recent analysis of the resulting statistics will be shown

[en] The new flash x-ray machine (FXR) at Lawrence Livermore National Laboratory is scheduled for completion in late 1981. This is a 54 module, linear induction accelertor, designed to deliver 500 Roentgen at 1 m as bremsstrahlung from a 20 MeV, 4 kA, 60 ns pulsed electron beam. The 9 cm diameter, cold-cathode electron source generates a 15 kA emitted beam at 1.5 MeV, and collimation is being used to reduce the transmitted current to 3.5 kA, with an emittance of 70 mr-cm. The collimated beam diameter is 4 cm. Six ferrite-loaded cavities are used in tandem to energize the injector. The high voltage performance of the injector cavities and other pulsed-power conditioning elements was tested earlier in a series of 10⁵ shots at 400 kV per cavity. An overview of the injector design and of the beam test results is given

[en] The FXR machine is a nominal 4 kA, 20 MeV, linear induction, electron accelerator for flash radiography at LLNL. The machine met its baseline requirements in March 1982. Since then, the performance has been greatly improved. The authors have achieved stable and repeatable beam acceleration and transport, with over 80% transmission to the tungsten bremsstrahlung target located some 35 m downstream. For best stability, external beam steering has been eliminated almost entirely. They regularly produce over 500 Roentgen at 1 m from the target (TLD measurement), with a radiographic spot size of 3-5 mm. Present efforts are directed towards the development of a 4 kA tune, working interactively with particle-field and beam transport code models. A remaining uncertainty is the possible onset of RF instabilities at the higher current levels

[en] At Lawrence Livermore National Laboratory (LLNL), our flash X-ray accelerator (FXR) is used on multi-million dollar hydrodynamic experiments. Because of the importance of the radiographs, FXR must be ultra-reliable. Flash linear accelerators that can generate a 3 kA beam at 18 MeV are very complex. They have thousands, if not millions, of critical components that could prevent the machine from performing correctly. For the last five years, we have quantified and are tracking component failures. From this data, we have determined that the reliability of the high-voltage gas-switches that initiate the pulses, which drive the accelerator cells, dominates the statistics. The failure mode is a single-switch pre-fire that reduces the energy of the beam and degrades the X-ray spot-size. The unfortunate result is a lower resolution radiograph. FXR is a production machine that allows only a modest number of pulses for testing. Therefore, reliability switch testing that requires thousands of shots is performed on our test stand. Study of representative switches has produced pre-fire statistical information and probability distribution curves. This information is applied to FXR to develop test procedures and determine individual switch reliability using a minimal number of accelerator pulses

[en] High-resolution radiography using high-current electron accelerators based on the linear induction accelerator principle requires the linac's final spot on the X-ray target to be millimeter-sized. The requisite final focusing solenoid is adjusted for a specific beam energy at its entrance, hence, temporal variation of entrance beam energy results in a less than optimal time-averaged spot size. The FXR (Flash X-Ray) induction linac facility at Lawrence Livermore National Laboratory will be briefly described with an emphasis on its pulsed power system. In principle, the pulsed Blumleins at the heart of the system output a square pulse when discharged at the peak of their charging waveform so that, with correct cell timing synchronization, the effective beam output into the final focusing solenoid should be optimally flat. We have found that real-life consideration of transmission line and pulse power details in both the injector and accelerator sections of the machine results in significant energy variations in the final beam. We have implemented methods of measurement and analysis that permits this situation to be quantified and improved upon. The improvement will be linked to final beam spot size and enhancement in expected radiographic resolution