BTF Beam Characteristics

The DAΦNE Beam Test Facility (BTF) provides electron and positron beams with adjustable energy, intensity, transverse dimensions, and divergence.

Beam parameters can be optimized according to experimental requirements, covering a wide range of operating conditions, from single-particle beams to high-intensity pulses.

The achievable beam characteristics depend on the particle species, requested energy and multiplicity, experimental hall, and DAΦNE LINAC operating conditions.

BTF consists of two experimental halls, BTFEH1 and BTFEH2, with complementary operating capabilities.

Note: These values describe the overall capabilities of the facility and should not be interpreted as simultaneously achievable operating conditions.

Beam energy, intensity, spot size, divergence, and repetition rate are correlated and depend on the DAΦNE LINAC operating conditions and the selected beam transport configuration.

BTF has historically operated in two main configurations: time-sharing and dedicated mode.

Status: Currently suspended.

Historically, during DAΦNE collider operation, BTF received LINAC pulses not required for injection into the storage rings.

In this configuration, the primary LINAC energy was determined by DAΦNE injection requirements, typically 510 MeV, while beam availability and repetition rate depended on the collider injection cycles.

Time-sharing operation is currently suspended, as injection into the DAΦNE storage rings is no longer performed.

BTF currently operates in dedicated mode, with the LINAC configured according to the requirements of scheduled experimental activities.

This configuration provides flexibility in:

• Electron and positron beam energy
• Beam intensity and particle multiplicity
• Pulse repetition rate
• Pulse duration, within the available LINAC configurations
• Beam transport and focusing conditions

Beam parameters are optimized according to experimental requirements and the operational constraints of the facility.

The facility can deliver both high-intensity primary beams and low-intensity secondary beams.

In primary beam operation, the LINAC beam is transported directly towards the experimental area without interception by the intensity-reduction target.

The primary LINAC beam can reach intensities of approximately 10¹⁰ electrons per pulse or 10⁹ positrons per pulse, depending on the accelerator configuration.

This operating mode is particularly suitable for high-intensity applications and dedicated experimental campaigns.

Secondary beams are produced by inserting a copper target into the primary LINAC beam.

The target thickness can be selected according to the required attenuation and beam conditions.

Interactions in the target produce a broad particle energy spectrum. A momentum-selection system, consisting of dipole magnets and adjustable collimators, selects the required energy and controls the transmitted particle intensity.

The remaining transport line includes quadrupole magnets and steering correctors, which allow the beam trajectory and transverse properties to be adjusted.

This configuration enables the production of low-multiplicity beams, including single-particle operation.

The achievable multiplicity depends on the selected energy, primary beam intensity, target configuration, and beamline acceptance.

In low-intensity secondary beam operation, the number of particles delivered in each pulse can be approximately described by a Poisson distribution:

P(k; λ) = exp(-λ) × λ<sup>k</sup> / k!

where:

• k is the number of particles in a pulse;
• λ is the average number of particles per pulse;
• P(k; λ) is the probability of observing exactly k particles.

For an average multiplicity of λ = 1:

• Approximately 36.8% of pulses contain no particles.
• Approximately 36.8% contain exactly one particle.
• Approximately 18.4% contain exactly two particles.
• Approximately 8.0% contain three or more particles.

The probability of obtaining exactly one particle per pulse is maximized at λ = 1.

The following table illustrates the expected probabilities for different average multiplicities.

These probabilities describe an ideal Poisson process. Experimental distributions may deviate from this model due to beam fluctuations, detector acceptance, and other experimental effects.

The transverse properties of the BTF beams depend on particle energy, multiplicity, focusing conditions, and the selected experimental hall.

The beam transport system includes quadrupole magnets, steering correctors, and adjustable collimators, allowing the beam position, transverse dimensions, and divergence to be optimized.

The achievable beam spot size can vary significantly according to the experimental configuration.

Multiple Coulomb scattering in the target, beamline materials, and exit vacuum window can significantly affect the transverse beam distribution, particularly at low energies.

Beam parameters at the experimental setup must therefore be evaluated for the specific operating configuration.

The following measurements illustrate representative BTF beam characteristics obtained under specific experimental conditions.

These results are examples of measured beam performance and should not be interpreted as guaranteed operating specifications.

Measurement conditions:

• Beam energy: 450 MeV
• Multiplicity: approximately 1.5 particles/pulse in LINAC positron mode and 10 particles/pulse in LINAC electron mode
• Detector: CALOBTF1, BTFDAQ (V965)
• Trigger: LINAC trigger
• Statistics: 3.84 × 10⁷ secondary electrons
• Run leader: BTF Staff

The measurement illustrates the dependence of secondary beam multiplicity on the primary LINAC beam conditions.

The detector resolution was affected by scattering material installed upstream of the calorimeter, including approximately 400 µm of multilayer silicon detector material.

Figure 1: Measured secondary electron multiplicity distributions for different primary LINAC beam conditions.

Measurement conditions:

• Beam energy: 450 MeV
• Average multiplicity: 25 particles/pulse
• Detector: Medipix
• Trigger: LINAC trigger
• Acquisition gate: 200 ns
• Statistics: 3500 pulses

Pixel detector measurements allow the transverse particle distribution to be reconstructed and the beam profile to be characterized.

Figure 2: Transverse beam profile measured using a Medipix detector.

Data courtesy of Gabriele Chiodini.

Beam position stability can be studied by reconstructing the mean transverse position of successive beam pulses.

Measurement conditions:

• Beam energy: 450 MeV
• Average multiplicity: 25 particles/pulse
• Detector: Medipix
• Repetition rate: 50 Hz (historical measurement)
• Acquisition gate: 200 ns
• Statistics: 3500 pulses

Figure 3: Beam centroid position reconstructed over successive LINAC pulses.

Data courtesy of Gabriele Chiodini.

The DAΦNE LINAC is an S-band accelerator operating at an RF frequency of approximately 2.856 GHz.

The beam therefore exhibits a microbunch structure with a temporal separation of approximately 350 ps.

A typical 10 ns macropulse contains approximately 29 microbunches. The actual number depends on the LINAC pulse configuration.

Figure 4: Example of the microbunch structure of a high-intensity LINAC beam.

Data courtesy of the AMY Collaboration.

BTF is equipped with diagnostic devices for measuring and monitoring beam charge, intensity, transverse position, profile, and stability.

Different instruments are employed depending on the beam operating conditions, from single-particle measurements to high-intensity beam diagnostics.

Further information is available in the BTF Beam Diagnostics section.

[1] B. Buonomo, G. Mazzitelli, F. Murtas and L. Quintieri, "A Wide Range Electrons, Photons, Neutrons Beam Facility", EPAC 2008, Genoa, Italy.

[2] B. Buonomo, G. Mazzitelli and P. Valente, "Performance and Upgrade of the DAΦNE Beam Test Facility (BTF)", IEEE Transactions on Nuclear Science, 52, 824–829 (2005).

[3] B. Buonomo, G. Mazzitelli, F. Murtas, L. Quintieri and P. Valente, "Beam Diagnostics for a Wide Range Beam Test Facility (BTF)", Beam Instrumentation Workshop, Lake Tahoe, USA (2008).