The ion beam produced by a plasma focus device will be the primary means of getting electric power out of the device. On February 28, while firing Focus Fusion-1 (FF-1), LPP’s experimental plasma focus device, the team observed a record 380 GW peak power in the ion beam. The previous most powerful beam observed had a peak power of 93 GW, so the new beam is a four-fold improvement. In addition, this was the first beam observed that, at least in part, went all the way down the meter-long drift tube that is attached to the underside of the FF-1 vacuum chamber. It was also the first beam that equaled or exceeded our theoretical predictions. Both the higher peak power and the beam’s more vertical direction are signs of increasing symmetry of the compression that forms the plasmoid, a key goal of LPP’s current efforts.
To give some context for this large power output, the peak input power to FF-1 device from its capacitor bank is currently around 53 GW while the total average electric power used in the United States is 440GW. Indeed, the beam was probably considerably more powerful than the figure we measured, as LPP’s Chief Scientist Eric Lerner calculated that about half the beam spread out beyond the 1-cm wide entrance hole to the drift tube. We believe this is the most powerful beam ever measured from a plasma focus device, although we will have to search the literature more thoroughly to make that claim with certainty.
Of course, the beam only lasted 5-ns, so it and the equally powerful electron beam emitted in the opposite direct carried only about 4 kJ of energy, about 1/15th of the total energy fed into the electrodes during the much longer 2-microsecond rise-time of the current from the capacitors. To get more energy out of the beam than is put in will require much higher fusion yield than is presently obtained in FF-1.
Fig. 1 The Big Beam of shot 4, Feb.28 as recorded by the Upper Rogowski coil. (This is actually an integrated signal, as the coil signal is proportional to the rate of change of the current.)
The LPP team measured the ion beam with two Rogowski coils near the top and bottom of the drift tube. When a beam of ions or electrons passes through these coils, a current is induced in them, creating a signal that is stored on an oscilloscope. Figure 1 shows the signal from the Upper Rogowski coil, close to the plasmoid, with the large beam to the left and a smaller subsequent beam on the right some 35 ns later. The dips following the beams show a reverse current of electrons drawn along behind the ions.
The height of the integrated Rogowski signal gives the peak current in the beam. The difference in timing between the two Rogowski signals gives the velocity of the beam and thus the energy of its ions—in this case 3MeV (million electron volts), again a new record for FF-1. We can check this energy by comparing the timing of the Rogowski coils with the timing for the signal from an x-ray detector, or photomultiplier tube, that detects when the electron beam hits the anode. Again the result is the exact same energy of 3 Mev. By multiplying the average energy by the peak current of 127 kA, we get the peak beam power of 380 GW.