Klystrons are essential to nuclear weapons. They are the timer which sends electrical charges to the explosives that combine the nuclear material creating a critical mass. The more accurate the timer, the less nuclear material you need and the smaller you can make the bomb, and the missile that carries it.
Even the Soviet Union could never master creating high grade klystrons, so their nuclear weapons and missiles were perhaps twice the size of ours, and used a lot more nuclear material, yet on a per unit basis produced far less yield. Their gigantic Tsar bomba and others got around this by using a lot of nuclear material.
In any event, in perhaps one of the most brilliant intelligence operation of the 20th Century, Israel sent a team of top notch operatives to the US, who hacked our nuclear logistics computer system, and redirected some 200 high grade klystrons to the east coast, where they were put on board an Israeli aircraft and flown to Israel. Since they had everything else they needed, this meant that overnight they had 200 nuclear missiles for their Jericho missile installation.
As a final note about Pollard, consider the case of the Israeli Mordechai Vanunu, who was kidnapped from Italy, and spent 18 years in prison, is frequently rearrested, and cannot ever leave Israel. Just for talking about their nuclear program.
http://en.wikipedia.org/wiki/Mordechai_Vanunu
Thank you for the tutorial on klystrons, as well as the Israeli gambit to grab some of ours (something of which I have never previously heard).
Also, thank you for the account of the fate of Mordechai Vanunu. Interesting, indeed.