Assessment of dynamical parasitics in the EBS

Abstract

Significant improvements will accrue to the eight-beam system (EBS) by dynamical effects (shooting on the fly) depending on whether or not strong saturable absorbers are used in the system. When strong saturable absorbers are used, there is little reserve in peak gain and the gain rises slowly near extraction; thus, the static threshold is greatly improved, but the dynamical threshold is not. In this case the dynamical or conditional thresholds are at g/sub 0/ values 0.6 to 0.7 higher than static threshold. We have also determined that the method of measuring unconditionally stable values leads to values that are 0.5 to 0.3 above static threshold, giving a net improvement over measured values of 0.2 to 0.4. When saturable absorbers are not used, the dynamical threshold increments above static threshold are 1.7 to 1.4 for the four- and six-pass modes respectively. It thus appears that somewhere between 500J to 900J per beam may be put on target without saturable absorbers,. And serious consideration should be given to postponement of saturable absorber installation. When using saturable absorbers, the 1/e rise time of the parasitic oscillation pulse at extraction is from 40 ns to 60 ns in the dynamical mode; and the fullmore » width at half maximum of the pulse is 390 ns to 415 ns when extraction (or damage) does not terminate its evolution.The values quoted here are based upon a few gain curves from one beam line and may not represent what the system could ultimately achieve. In this study we arrived at a single differential equation that adequately represents more detailed TARDAM calculations. The differential equation comes from both a point amplifier model and a model with gain and absorption uniformly spread between the two ends of the cavity. Using the differential equation, simple analytical expressions or results that are easily obtained on a HP-25 hand calculator compare within a per cent of TARDAM results for excess gain.« less