The influence of the plasma density scale length on the production of MeV protons from thin foil targets irradiated at I lambda(2) = 5 x 10(19) W cm(-2) has been studied. With an unperturbed foil, protons with energy >20 MeV were formed in an exponential energy spectrum with a temperature of 2.5+/-0.3 MeV. When a plasma with a scale length of 100 microm was preformed on the back of the foil, the maximum proton energy was reduced to <5 MeV and the beam was essentially destroyed. The experimental results are consistent with an electrostatic accelerating mechanism that requires an ultrashort scale length at the back of the target.

No abstract

No abstract

In a pulsed arc with mercury electrodes the average column field strength E,, was measured for different currents I, and the relationship E,, a 10.67 was found. T h e variations of arc current and voltage with time indicate that the anode and cathode fall regions have a combined thickness of 3 X 10-G cm, and a n average field strength of 2 X loG V/cm. T h e arc plasma is formed maillly from mercury vapor, rather than from the surrounding gas. I NTRODUCTIOSArcs between mercury electrodes are of interest because the mechanism for releasing electrons from the cathode surface is not completely understood. Thermionic emission is ruled out because of the low boiling point of mercury. Wasserrab (1951) has proposed field emission and pointed out that the high current density of l o 6 amp/cm%bserved by Froome (1949) should be accompanied by a field of lo7 volt/cm a t the cathode. This theory gives a reasonable explanation of the electron emission, but no measurements of the extreinely high fields have been reported (Bauer 1964).This paper describes experiments with a pulsed arc between a plane mercury surface and a suspended mercury drop. T h e electrodes moved together during the discharge and eventually touched. The resulting time variation in the arc current and voltage allowed a n approxin~ate determination of the total thickness of the anode and cathode fall regions, and thus of the average field strength in these regions. APPARATUS A N D MEASURING TECHNIQUEThe arc was formed between a mercury pool (cathode) and a suspended mercury drop (anode) in a discharge vessel filled with argon a t 1 atnlosphere pressure (Fig. 1). The current source was a 30-pF capacitor C charged t o a voltage Vo (in the range 200 to 1 500 volts), in series with a current-limiting resistor R (usually 1 ohm). T h e arc was initiated by raising the level of the mercury pool so that breakdown occurred.The oscilloscope trace in Fig. 2(b) shows the time variation of the potential V across AB. As the arc plasma is formed V drops from Vo to the arc voltage in about 2 psec. Figure 2(c) depicts the time dependence of the current I , as obtained from an oscilloscope trace of the voltage drop across R. After about 2 psec I obeys approximately the relationship expected for a capacitor discharging through R in series with the arc resistance RB (the inductance being negligible), vo exp ( ---) I(t) = -R + RB (R + RB)C .

No abstract