1.2 LHC Run 3 Upgrades
7
1.2 LHC Run 3 Upgrades
In November 2018, the LHC finished its second running period (Run 2), which started
in September 2015. In this period, a peak interaction rate of 7.5 kHz was reached for
Pb–Pb collisions with centre-of-mass energy of 6.3 TeV per nucleon pair [8]. As a
comparison, in the heavy ion run in 2011 the interaction rate reached was 3 kHz [9].
There is a two year planned shutdown to prepare for Run 3 starting in 2021.
Many of the planned measurements for Run 3 will involve complex probes at low
transverse momentum, where traditional methods of triggering will not be applicable.
Therefore, the ALICE collaboration is planning to upgrade the current detector by
enhancing its low-momentum vertexing and tracking capability, and allowing data
taking at substantially higher interaction rates.
Run 3 has an expected peak luminosity of 6 × 10
27 cm
−2 s
−1 and interaction rates
of about 50 kHz for Pb–Pb collisions [10]. In the proposed plan, the ALICE detector
is modified such that all interactions will be inspected. This implies a major upgrade
of the TPC detector and a modification of the readout electronics of other detectors
to comply with the high readout rate.
1.2.1 Motivation for TPC Upgrade
The drift time of electrons, from centre of the detector to the end plates, in the
currently used Ne-CO 2 (90-10) gas in the TPC is ∼100 µs. The drift time of positive
ions from the amplification region around the MWPC anode wires back to the central
electrode is ∼180 µs. To avoid any ions drifting back from the amplification region
to the drift region, a gating grid is in place, which is enabled after the initial electron
drift time, preventing any back drift, but also preventing electrons to pass into the
amplification region. This leads to an intrinsic dead time of ∼280 µs and a limitation
in the maximum interaction rate of 3.5 kHz.
If the gating grid structure were to be permanently disabled and the TPC ran at the
targeted interaction rate of 50 kHz (20 µs) for Run 3, then space charge distortions
would occur due to the accumulated ions in the drift region, which would render track
reconstructions useless. Operation of the TPC at 50 kHz can thus not be accomplished
with the current gating scheme [11]. It will therefore be replaced by a multi-stage
Gas Electron Multiplier (GEM) system [12]. GEMs have been proven to operate
reliably in high-rate applications and provide intrinsic ion blocking capabilities,
therefore enabling the TPC to operate in a continuous, ungated readout mode where
the readout is dead-time free [13].
7
1.2 LHC Run 3 Upgrades
In November 2018, the LHC finished its second running period (Run 2), which started
in September 2015. In this period, a peak interaction rate of 7.5 kHz was reached for
Pb–Pb collisions with centre-of-mass energy of 6.3 TeV per nucleon pair [8]. As a
comparison, in the heavy ion run in 2011 the interaction rate reached was 3 kHz [9].
There is a two year planned shutdown to prepare for Run 3 starting in 2021.
Many of the planned measurements for Run 3 will involve complex probes at low
transverse momentum, where traditional methods of triggering will not be applicable.
Therefore, the ALICE collaboration is planning to upgrade the current detector by
enhancing its low-momentum vertexing and tracking capability, and allowing data
taking at substantially higher interaction rates.
Run 3 has an expected peak luminosity of 6 × 10
27 cm
−2 s
−1 and interaction rates
of about 50 kHz for Pb–Pb collisions [10]. In the proposed plan, the ALICE detector
is modified such that all interactions will be inspected. This implies a major upgrade
of the TPC detector and a modification of the readout electronics of other detectors
to comply with the high readout rate.
1.2.1 Motivation for TPC Upgrade
The drift time of electrons, from centre of the detector to the end plates, in the
currently used Ne-CO 2 (90-10) gas in the TPC is ∼100 µs. The drift time of positive
ions from the amplification region around the MWPC anode wires back to the central
electrode is ∼180 µs. To avoid any ions drifting back from the amplification region
to the drift region, a gating grid is in place, which is enabled after the initial electron
drift time, preventing any back drift, but also preventing electrons to pass into the
amplification region. This leads to an intrinsic dead time of ∼280 µs and a limitation
in the maximum interaction rate of 3.5 kHz.
If the gating grid structure were to be permanently disabled and the TPC ran at the
targeted interaction rate of 50 kHz (20 µs) for Run 3, then space charge distortions
would occur due to the accumulated ions in the drift region, which would render track
reconstructions useless. Operation of the TPC at 50 kHz can thus not be accomplished
with the current gating scheme [11]. It will therefore be replaced by a multi-stage
Gas Electron Multiplier (GEM) system [12]. GEMs have been proven to operate
reliably in high-rate applications and provide intrinsic ion blocking capabilities,
therefore enabling the TPC to operate in a continuous, ungated readout mode where
the readout is dead-time free [13].