Measurement of the Z boson cross-section in - Harvard University ...

Chapter 3: Luminosity Measurement at the LHC and in ATLAS 73
transformers.
Fast beam current transformers (FBCTs): The FBCTs are capable of inte-
grating the current of each LHC bunch. Their operating principle is illustrated in
Figure 3.3 [66].
L Cs R
Beam, I B
Rise Time
~
r
LC
S
I B
I S
Droop Time
d ~
Figure 3.3: Operating principle of a fast beam current transformer.
The beam current, IB, acts as the primary winding of the transformer. The
secondary winding has an inductance L, resistance R and (stray) capacitance CS. It
generates an output voltage V = L dIB
dt , with a risetime proportional to √ LCS ≈ 10
ps [55]. The output waveform closely resembles the beam current, with a DC offset
due to the transformer droop which can be corrected via software upon digitization.
Fast integrators, working at repetition rates of up to 40 MHz, enable the estimation
of the total charge in each bunch.
Two FBCTs with separate acquisition chains are used in the LHC in each ring.
Each beam dump line also has two FBCTs to monitor the intensity of ejected beams.
DC beam current transformers (BCTDCs): BCTDCs measure the mean cur-
rent of an entire circulating beam, and can also be used to measure the beam lifetime.
The basic principle is to make use of the hysteresis in a pair of toroidal ferromagnetic
L
R