Longitudinal Bunch Shape Monitor

U. Raich

Detector developped in Russia

• Detector mainly developped by Institute for Nuclear Research (INR) Moscow

A. Feschenko and collaborators

INR is willing to build detectors for us

Methods and Instrumentation for Bunch Shape Measurements

A.V. Feschenko INR Moscow, proceedings of PAC Chicago 2001

The principle

• Coherent transformation of longitudinal bunch structure into a transverse distribution of secondary electrons

• Consists of – A wire in which secondary electrons are created– An electric field to accelerate the electrons– Deflecting plates with a deflecting field synchronous

with the accelerator RF– An electron detector

The principle (2)

Secondary electrons created in the wire

Only the electrons created by particles in the primary beam with a fixed phase are measured

The deflection corresponds to a phase sweep

Typical phase resolution: 1° at several 100 MHz

Time resolution

350 MHz accelerating frequency

=> 3ns cycle time

Micro bunches are much smaller

Typical phase resolution is 1°

=> Time resolution of some tens of ps

Different types of detectors

• BLVD: Bunch Length and Velocity Detector– The wire can be displaced longitudinally– Displacement by one full RF period (bunch

shape will be the same as the one before displacement

– Measure wire displacement

3D-BSM

Wire moves horizontally

A slit moves vertically

For each point in transverse phase space the longitudinal distribution can be measured

Detector use

Bunch shapes for different RF amplitudes

Bunch shape with a tank on a wrong phase

Longitudinal Emittance

Modify particle energy by changing the accelerating amplitude

Measure the bunch size for each energy

Knowing the optics, calculate back to the last accelerating element

Detached electrons (H- beams)

Detached electrons can be seen in the bunch spectrumThis is a problem if longitudinal beam halo is to be measured

The problem can be solved by analyzing the electron energy

Similar detector developed at GSI

No wireThe secondary electronics are createdin the rest gas (non intercepting)Electrons accelerated in an electric fieldSource region is restricted by a slit

Experience at CERN

• BLVD at Linac-3 (Bunch Length and Velocity Detector)

• 3-BSM at Linac-2– Were used intensively for beam studies with good

results– INR crew has left– CERN linkman in retirement– System was not integrated into the Linac control

system

Result: The detectors are currently not in use

Problems to be solved

• Thermal resistance of the wire (same problem as any intercepting device)

• RF deflector must be adapted to the accelerator’s RF frequency

• The wire works like a stripper and the detached electrons may disturb the measurements