Focusing with quadrupoles?

Hello,

I read that quadrupole magnets focus in one plane and defocus in the
other. I understand that in a doublet configuration (FODO) one can get
net focusing in both planes. But in this case the beta function is big
in one tranverse plane and small in the other for a given position
along the beamline.

How can it be that at a collision point of a particle accelerator one
can squeeze the beam in both planes? I heard that triplets can do
that. Where can I read more about this?

Thanks,
Natalia Sanchez

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