Our Device and Technology
The pulsed power electron beam generator “Thunderbird” (classification name IVR-3) is located in our Urbana facility. The primary function of the pulsed power device is to shape and compress the voltage pulse from the initial discharge of the main capacitor and to deliver this compressed pulse to the relativistic vacuum diode (RVD), which forms a relativistic electron beam narrowly focused into a microscopic volume of the target achieving extreme power density.
Temporal pulse compression:
In our experiments, the main 3 µF capacitor is charged to a voltage in 50–65 kV range accumulating 3.8–6.3 kJ of electromagnetic energy in the capacitor storage. Pulse compression is performed by a set of plasma erosion opening switches (PEOS) fed from a separate capacitor bank. The main capacitor discharges on a typical timescale of ~1.4 µs. This initial pulse is compressed by PEOS to produce >1 MV and <25 ns voltage spike. At the load region this spike is additionally compressed to a width <12 ns, signifying a very efficient, more than two orders of magnitude compression of the initial voltage pulse.
Relativistic Electron Beam Pulse:
The Thunderbird generator produces relativistic (β=v/c=0.85, γ=1.9) electron beam. At 50 kV charging voltage of the main capacitor the maximum energy of electrons is 0.6 MeV, the maximum beam current is 55 kA, the maximum power of the beam is 12 GW. At charging voltage 65 kV, these parameters increase to 0.7 MeV (β=0.9, γ=2.4), 75 kA and 20 GW respectively.
Spatial compression: electron beam self-focusing effect
After temporal compression of the voltage pulse, the RVD provides spatial compression of the electron beam benefiting from the effect of beam self-focusing in plasma of virtual electrodes. In our experiments, this manifests in the so called “hole-boring” effect forming the narrow channel along the axis of the target.
Visual estimations by target destruction provides us the value of electron beam diameter on a scale of 100 µm. Particle-in-cell (PIC) modeling suggests an even smaller diameter of a focused beam <50 µm.
Extreme conditions produced by our device:
Efficient temporal and spatial compression of the electromagnetic pulse allows the extremely high power, up to 20 GW, which is comparable to the average power consumption by an entire industrial country like Norway or United Kingdom, to be delivered all into the size of a tip of a pin, the microscopic volume of the solid target. When such a huge power focuses into a micron size area, its density reaches the level of 10 TW/mm2. This power density level has been targeted since the establishment of the electron beam fusion concept by Gerald Yonas in the early 1970’s [see, for example, G. Yonas, et al., Nuclear Fusion 14, 731 (1974)].
Z-pinch-type loads at the Thunderbird device
Besides the RVD setup, our device can operate in the “z-pinch” mode, exploding a single wire load mounted between the electrodes. Efficient pulse compression technology in our device provides extreme current rise time accelerating from zero to more than 100 kA in less than 15 ns. The rate of current rise is approaching the level 1013 A/s, a figure comparable to that provided by large z-pinch generators.
DEVELOPMENT OF FUSION TECHNOLOGY
A pulsed-power-generated electron beam has been considered as a driver for thermonuclear fusion for more than four decades. Since the dawn of this concept in the early 1970’s the main research efforts have been focused on direct drive Inertial Confinement Fusion (ICF). Two major technological challenges included: (1) meeting high energy and power requirements for the target compression, and (2) minimizing the growth rates of the instability modes during the target compression.
The demand for power density was evaluated in simulations. Depending on the particular scheme, this number varied from 10 to 1000 TW/mm2. For a long time, there was a belief that only a large pulsed power machine can achieve this goal. Yet, it is our mid-scale Thunderbird device that brings the power density up to these extreme levels.
High uniformity of the electron beam had to be maintained in the original direct beam driver fusion scheme in order to inhibit the growth of hydrodynamic instability modes during the target implosion. This problem was aggravated by an intrinsic feature of a high-current electron beam – its filamentation in dense plasma of virtual electrodes. This effect manifests in electron beam splitting into multiple filaments, each filament carrying current up to Alfven limiting value (at our parameter range this value is about 30 kA).
The best efforts to address these challenges were implemented in the Particle Beam Fusion Accelerator-II (PBFA-II) project at Sandia National Laboratories. The light ion beam, which was used as a fusion driver in this project, had much higher Alfven limiting value. Projected characteristics of the particle beam were 3 MA beam current, 30 MV load voltage and 0.5–1 TW/mm2 power density.
Our technological advantage
We see the effect of self-pinching of the electron beam as an advantage that allows achieving extreme power densities necessary for fusion in the small volume of a target. This region of extreme energy deposition under certain conditions can spark the thermonuclear burn to sustain it through the whole volume of the target. Such an approach is similar to the fast ignition concept set forth in the laser driven ICF research.
Our present target design has demonstrated its ability to harness the electron beam self-focusing creating extreme matter conditions.
Early experiments in the field of fusion research were implementing schemes that utilized deuterium gas-puff or frozen deuterium fiber targets. The location of an extreme power deposition region, or the “hot spot”, was randomly changing from one experiment to another.
In our design the location of the hot spot is not random, but it is rather predefined by the target geometry. This is another critical advantage that we are using in the target optimization.
These advantages provide us the opportunity to progress rapidly toward the design of a scaled-up prototype achieving breakeven energy output – the most important milestone toward a fusion power plant.