Rebonjour, BillyO:
1/ Leo Bodnar 40 pS is designed to drive a 50 Ohm termination or 50 Ohm scope input.
There is no spec for other Zo, so current is the same with and without the attenuator, and load not the issue.
See the spec sheet of the LB 40 pS.
2/ A constant impedance atten is a resistive T or Pi network designed to attenuate with same Zo input/output eg 50 Ohms.
See spec sheet of Mini Circuits HAT-20 wideband 50 Ohm attenuator.
See Tee /Pi attenuator
Thus the load on the LeoB pulser is 50 Ohms with or sans the HAT-20 attenuator.
3/ Purpose of the attenuator is to the reduce reflected energy from scope t pulser due to VSWR or Zo mismatch.
Thus pulse aberrations due to reflections from scope to pulser are reduced and the test aberration is reduced.
Pix: Leo B USB utility , allows Vo adjust 50 mV>>1.5 V set at 1.0V P-P into 50 Ohm.
Yokogawa DL7440 500 MHz BW, FS = equv 100 G/sec, sweep at 1 nS/div
Upper trace LeoB40 direct to scope 50 Ohm input
Lower trace LeoB40>>Mini Ckts HAT-20 atten>scope 50 Ohm input
Many previous old EEVBLOG threads on similar like "show us your squarewave" TD pulser, Avalanche pulser, etc.
Hope all is now clear!
Jon