Alexander A. Kim

Fast primary energy storage based on linear transformer scheme

The main advantage of the primary storage based on linear transformer scheme is the ground potential on the capacitor bodies during the shot, allowing exclusion of the total output voltage insulation of the highest stages, and to trigger all the stages simultaneously by using an external trigger pulse. The problem was to build a fast linear transformer driver (LTD), providing both high efficiency of energy transfer into the secondary turn and the current rise time below /spl sim/1 CLs, because of relatively high inductance of the caps and the switches. The key elements of the LTD described below are the HAEFELY capacitors (75 kV, 5.65 /spl mu/F, 13 nH, 40 mOhm, or 90 kV, 3.95 /spl mu/F, 10 nH, 13 mOhm) and the multi gap, multi channel spark switches developed at HCEI for SYRINX project (90 kV, 0.7 MA, 8.5 nH). This LTD was designed as an alternative to the Marx generator in order to improve the performance of the IES technology in the direct drive approach where the only unit providing power multiplication on the load is the POS. The LTD was designed as a single stage with two HAEFELY caps in parallel. The authors describe a 75 kV LTD stage with oil insulation and a 90 kV LTD stage with SF/sub 6/ insulation.

Super Fast 75 ns LTD Stage

Summary form only given. In the report, we present the new super fast LTD prototype which delivers a 75 ns FWHM voltage pulse into a -0.5-0.6 Ohm matched load at ~20 GW power. The stage is designed without of peaking capacitors, it includes 32 GA35436 (8 nF, 100 kV) storage capacitors, 16 spark gap switches and magnetic core with reduced thickness of the tape (50 mum) to reduce the current losses. This stage prototype was specifically designed with an hemispherical vessel to operate with compressed gas (SF6, SF6/dry air mixtures, and pure dry air) up to 6 ata pressure, as well as with transformer oil. Test results of the stage prototype will be given and compared with numerical simulation.

IDERIX : An 8 MV flash x-rays machine using a LTD design

For future flash radiographic needs, an 8 MV radiographic machine, IDERIX, will be developed for the CEA / PEM. This machine will be composed by ∼80 super fast LTD (Linear Transformer Driver) stages. The output voltage of each of these stages (100 kV − 75 ns) will be inductively added along a ∼20 m stepped magnetically insulated transmission line to deliver the power up to the beam diode. In each stage, 16 bricks, made with two 8 nF capacitors (that can be charged up to +/− 100 kV) and one multi-channels multi-gaps switch, are arranged in parallel (with a star pattern). The number of bricks is chosen to adapt the stage impedance to the diode impedance and operate the LTD generator close to matched mode. Moreover, new magnetic cores using a thinner ferromagnetic tape (50μm) allow reducing the losses and improving the performances of the generator. The insulation inside the stage will be done using dielectric oil.

Recyclable transmission line (RTL) and linear transformer driver (LTD) development for Z-pinch inertial fusion energy (Z-IFE) and high yield.

Z-Pinch Inertial Fusion Energy (Z-IFE) complements and extends the single-shot z-pinch fusion program on Z to a repetitive, high-yield, power plant scenario that can be used for the production of electricity, transmutation of nuclear waste, and hydrogen production, all with no CO{sub 2} production and no long-lived radioactive nuclear waste. The Z-IFE concept uses a Linear Transformer Driver (LTD) accelerator, and a Recyclable Transmission Line (RTL) to connect the LTD driver to a high-yield fusion target inside a thick-liquid-wall power plant chamber. Results of RTL and LTD research are reported here, that include: (1) The key physics issues for RTLs involve the power flow at the high linear current densities that occur near the target (up to 5 MA/cm). These issues include surface heating, melting, ablation, plasma formation, electron flow, magnetic insulation, conductivity changes, magnetic field diffusion changes, possible ion flow, and RTL mass motion. These issues are studied theoretically, computationally (with the ALEGRA and LSP codes), and will work at 5 MA/cm or higher, with anode-cathode gaps as small as 2 mm. (2) An RTL misalignment sensitivity study has been performed using a 3D circuit model. Results show very small load current variations for significant RTL misalignments. (3) The key structural issues for RTLs involve optimizing the RTL strength (varying shape, ribs, etc.) while minimizing the RTL mass. Optimization studies show RTL mass reductions by factors of three or more. (4) Fabrication and pressure testing of Z-PoP (Proof-of-Principle) size RTLs are successfully reported here. (5) Modeling of the effect of initial RTL imperfections on the buckling pressure has been performed. Results show that the curved RTL offers a much greater buckling pressure as well as less sensitivity to imperfections than three other RTL designs. (6) Repetitive operation of a 0.5 MA, 100 kV, 100 ns, LTD cavity with gas purging between shots and automated operation is demonstrated at the SNL Z-IFE LTD laboratory with rep-rates up to 10.3 seconds between shots (this is essentially at the goal of 10 seconds for Z-IFE). (7) A single LTD switch at Tomsk was fired repetitively every 12 seconds for 36,000 shots with no failures. (8) Five 1.0 MA, 100 kV, 100 ns, LTD cavities have been combined into a voltage adder configuration with a test load to successfully study the system operation. (9) The combination of multiple LTD coaxial lines into a tri-plate transmission line is examined. The 3D Quicksilver code is used to study the electron flow losses produced near the magnetic nulls that occur where coax LTD lines are added together. (10) Circuit model codes are used to model the complete power flow circuit with an inductive isolator cavity. (11) LTD architectures are presented for drivers for Z-IFE and high yield. A 60 MA LTD driver and a 90 MA LTD driver are proposed. Present results from all of these power flow studies validate the whole LTD/RTL concept for single-shot ICF high yield, and for repetitive-shot IFE.

A composite POS: first proof-of-principle results from GIT-12

Experiments with the microsecond plasma opening switch were carried out on the GIT-12 installation at the conduction current level of 3 MA. In this series an attempt to increase both the critical charge responsible for the conduction phase and the POS voltage was undertaken. The necessity of such improvement is conditioned by the difficulties of application of existing POS technology for construction of a new, multi-MA class IES driver. The experiments were inspired by several phenomenological ideas and theoretical results described as the main background philosophy of this work. Possible improvement of the POS operation is predicted if using the following technique: 1) optimization of the plasma injection length in order to increase the critical charge by keeping the initial plasma density at minimum possible level; 2) use of the initial plasma density gradient in the direction from the load to the generator in order to decouple the plasma densities responsible for the conduction and opening times; 3) use of the controlled plasma motion during the conduction phase for abrupt enhancement of the magnetic field prior to the opening phase. A composite POS satisfying these conditions and consisting of initially prepared MHD and Hall parts was applied. The first, mainly proof-of-principle results from C-POS operation on GIT-12 are reported.

Lifetime of the HCEI spark gap switch for linear transformer drivers

Spark gap switches, storage capacitors, and ferromagnetic cores are the most important components of the Linear Transformer Driver (LTD) cavities. Depending on their output voltage and current, LTD-based accelerators may need several thousands switches, hence the switch lifetime is of crucial importance. Unlike the capacitors and the cores, the LTD switches are still in the developmental stage and are designed and built mainly by various laboratories; thus the lifetime of different switches may vary. In this paper we present recent results of the first full-scale experiments that were aimed to evaluate the life-time of the LTD switches developed by HCEI.

A 1-MA LTD Cavities Building Blocks for Next Generation ICF/IFE

We are actively pursuing the development of new accelerators based on the novice technology of Linear Transformer Driver (LTD). LTD based drivers are considered for many applications including future very high current Z-pinch drivers like ZX and Z-pinch IFE (Inertial Fusion Energy). The salient feature of the approach is switching and inductively adding the pulses at low voltage straight out of the capacitors through low inductance transfer and soft iron core isolation. Utilizing the presently available capacitors and switches we can envision building the next generation of fast z-pinch drivers without the usage of large deionized-water and oil tanks as it is the case with the present technology drivers. The most significant advantage of all is that the LTD drivers can be rep-rated. The later makes LTD the driver of choice for z-pinch IFE where the required repetition rate is of the order of 0.1 Hz. Presently we have in rep rated operation in Sandia a one 500-kA, 100-kV LTD cavity. The compact fast (≪100 ns) LTD was suggested and its development is funded by Sandia at the High Current Electronic Institute (HCEI) in Tomsk, Russia, where a number of larger and stackable 1-MA cavities are under construction.

Experiments on GIT4 with the load upstream from the POS

In experiments on GIT4, a scheme with the load connected upstream from the plasma opening switch (POS) was investigated. The load was connected to the generator output through the surface self-breaking switch. The current switching into the different inductive loads was tested. It was found that the time the POS is open depends on the value of the load inductance. It was demonstrated also that surface self-breaking switch can be applied for switching currents rising at /spl sim/2/spl middot/10/sup 13/ A/s.

Square pulse LTD with 5 th harmonic bricks

Up to now, the Square Pulse LTDs consisting of only 1st and 3rd harmonic bricks were developed and tested [1]. At the same time, discussions with pulsed power scientists indicated that the Square Pulse LTDs with additional 5th harmonic bricks might be even more interesting for some applications. Such LTDs could produce output pulses with a faster rise time and most importantly a flatter and longer length pulse top. In the present report we describe the Square Pulse LTDs, including 5th harmonics, and analyze their benefits and possible drawbacks.

Z driver post-hole convolute studies utilizing MYKONOS-V voltage adder

The modern high current, high voltage pulsed accelerators utilize vacuum-post-hole convolutes to add the current of a number of parallel self Magnetic Insulated Transmission Lines (MITL) to a single one located very close to the centrally located load. The reason of course of using several parallel MITLs to transfer the current pulse from large, ~1.5 m, radii to the 1-2 cm load is to reduce the transfer inductance. For example, the vacuum chamber of the 24-26-MA Z machine has a 1.45-m radius vacuum section containing four parallel conical MITLs merging into one 6cm radial disc MITL adjacent to the centrally located load via a double post-hole convolute array located at 7.62 cm from the axis. Although special care has been taken to reduce the electrical stresses on the cathode hole surfaces in order to avoid electron emission, substantial current losses, 4-6 MA, are observed most probably due to plasma formation and the unavoidable magnetic nulls. In the proposed experiments we will study the behavior of only one convolute using the MYKONOS-V driver. MYKONOS-V is a Linear Transformer Driver (LTD) voltage adder composed of 5 nominally 1-MA cavities connected in series. The voltage adder radial A-K cavity is deionized water insulated. The experimental set-up is designed in such a way to reach conditions on the convolute very similar to those existing on Z. Most importantly, in contrast to Z, it provides full view of the convolute for optical and spectroscopic imaging and gives the flexibility and freedom to study various options in an effort to reduce the convolute losses without affecting the day-to-day Z experiments. This is going to be a dedicated convolute study experiment. The hardware design, numerical simulations and proposed diagnostics will be presented and discussed.