LHC Injection Working Group

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Summary notes of the meeting held on 17 November 2004

1. Results from TI8 tests

J. Uythoven briefly summarized the results from the TI 8 tests held on the weekends 23 -25 October and 6 - 8 November (slide). More details, as becoming available, are accessible from http://proj-lti.web.cern.ch/proj-lti/.

There is no final result yet available on the optics parameters at the beginning of the line as the data analysis is still ongoing. The coupling between the two planes was measured by means of screens and trajectory measurements during kicker excitations. J. Wenninger found a coupling of the order of 1 - 2 %, derived from a 1 - 2 % amplitude response in the orthogonal plane for an excitation in the other plane. This number is coherent with the preliminary value of  ~ 3 % resulting from fits of screen measurements. J. Wenninger claimed that the coupling stems from one or two sources at the beginning of the line and does not accumulate along the line. B. Goddard pointed out that it is important to know with which accuracy effects like coupling, which contribute to emittance growth, can be measured. Long term measurements over night showed that on a 6 hour scale the trajectories stayed stable within less than 100µm. J. Uythoven also mentioned that the tests did not cause any radiation problems.

2. TDI simulations

L. Sarchiapone gave a presentation on the latest TDI-TCDD-D1 energy deposition studies (slides). The geometry changes, namely the relocation of the TCDD (further away from the D1 due to the insertion of the BPM) and the new layout of TDI, were taken into account. The results refer to ultimate beam intensity. The "normal" case, where the beam is dumped on the TDI (vertical offset 44.4 mm from the circulating beam), and the grazing case, where the beam touches the edge of the TDI, were investigated. The new design of the TDI gave a factor of 8 less peak energy deposition in the D1 for the normal case with respect to the old design. L. Bruno said this was due the fact that the Al is now closer to the absorbing core which reduces the number of escaping particles. In addition the old layout was based on a graphite core, which has a lower density than hBN, which is now used in the core. B. Goddard remarked that this difference might also be due to a lack of statistics. The simulation will be rerun with higher statistics.

In the case of grazing impact, the energy deposition in D1 stays a factor of 10 below the coil damage limit of 87 J/cm³. B. Goddard proposed to rerun the simulation without the TCDD, as the protection level for D1 did not degrade significantly despite the larger distance of the TCDD from D1 in the new layout and the larger opening. 

There are no new results on  impedance calculations or heating through trapped modes so far. Ti coating is foreseen for the hBN core; everything else such as the Al frame is Cu-coated. The beam screen will be made of Cu.

Anti-collision precautions for the two TDI jaws will be based on software rather than on electrical contacts as discussed some time ago. Interlocks on the TDI should be hardwired. L. Bruno stated that the need for any special end switches for the motors or other protections should be brought forward now as the design is almost complete. 

3. Analysis of TT40 incident with high-intensity beam

B. Goddard showed the results of the post mortem analysis of the accident which occurred during the high intensity extraction on 25 October, where the vacuum chamber of the TT40 magnet QTRF4002 and the magnet itself were damaged (slides). A full batch was extracted consisting of 288 bunches with a total intensity of 3.4e+13 particles.

The shot-to-shot logging of BLM and BPCE data showed no losses on the MSE for the bad extraction and a correct bumped beam position (the BPCE reading changed with intensity; this got cured for the second test 2 weeks later). The history of the MSE current however shows a drop of the current by ~ 2.5 % at the extraction moment. B. Goddard also mentioned that problems had already occurred earlier with the MSE having tripped 8 times during the MD. Due to the RF-noise from the beam on the sensor cables, the MSE had fake temperature problems throughout the whole extraction test. The test was not interrupted to sort out the problem.

A picture of the damaged vacuum chamber showed signs of beam impact along ~ 1.2 m on the right side (seen from upstream). The chamber was cut open by the beam around the energy deposition maximum resulting in a slit of about 20 cm. Molten material was ejected onto the opposite side of the chamber over ~ 1 m. The beam was apparently fully stopped in the chamber as there is no sign of beam impact on the downstream flange of the magnet.

Data from screens and BPMs in TT40 were used to reconstruct the event. A screen shot (BTVI4001) of the mis-steered beam showed a ~ 11 mm horizontal offset of the trajectory to the inside compared to a previous shot. The reading of BPK400099 gave a horizontal offset of 4.3 mm inside the initial trajectory. BLM4002 and 4003 recorded the accident at the magnet (the higher reading on BLM4001 might be due to backscattering or possible losses on QTMD4001).

The analytical reconstruction of the trajectory with an aperture model and taking the readings of the BTV and the BPM into account showed that the current of the MSE must have been off by ~ 5.1 %, resulting in an impact angle of 0.59 mrad. The beam size at the impact location was 0.7 mm in both planes. This information was used in a FLUKA simulation. The first simulation seemed to underestimate the energy deposition, as the melting point is not reached (melting point of 314L stainless steel is about 1400°C). The simulation results are very sensitive to the input parameters and conditions. The analysis of the event with FLUKA is still ongoing.  

B. Goddard also mentioned the discrepancy between the nominal point of extraction and the real extraction. The surveillance windows only worked to about 7 ms before extraction. The current was measured at the nominal extraction point where the current was still inside tolerance (~ -0.5 %); in reality the beam was extracted 3 ms later. With the MSE time constant for the current decay of 23 ms, ΔI/I is ~ -5 % over 11ms. An improved strategy is required with the surveillance window tied more closely to the extraction point.

Reconstructed accident scenario: The noise from the LHC beam on the temperature sensor cables of the MSE was transmitted to the PLC, which generated a fake "water flow" interlock and turned off the MSE power supply about 11 ms before extraction. At the moment of surveillance (7 ms before extraction) the current was still within tolerance (taking into account the the surveillance averaged over the last 10 samples). At the moment of extraction the current was down by 5.1 %, and the beam was sent into the QTRF4002 vacuum chamber.

The BIC interlock controller did not have any direct connection to the MSE PLC (only to the supply current surveillance). This was already changed for the second test on 8 November. Prior to this the MSE noise problems were properly studied by just bumping the beam but not extracting it. In the end almost all temperature gauges were disconnected. It turned out to be important to have a carefully worked out setting-up procedure for the high intensity extraction (separated from the TT40 material tests). Further improvements for the second test included: the ROCS current surveillance window was shortened from 10 ms to 1 ms and shifted as close as possible to the real extraction; a bug was also found in the ROCS surveillance, 2 cycles old data were used. Attenuators were added to get a smoother signal from the BPCE.  B.Goddard remarked that there is still no interlock on the MSE girder position and the MKE PFN voltage.

B. Goddard summarised essential lessons learnt for future high intensity MDs: proper procedures are crucial; each problem must be taken seriously and completely solved before carrying on with commissioning  with high intensity beam; safety interlocks and software must be tested before; fast current surveillance for circuits with small time constants must be implemented; passive protection should be used wherever possible. 

4. Transfer Line Collimation

TCDI energy deposition studies, open issues (slides): V. Kain showed the results of studies for different materials for the mask in front of the MSI. In the previous meeting she had stated that a Fe mask (as used for all other  transfer line collimators) would not survive in the event of beam loss on the TCDIVMSI. The materials AlN and hBN showed promising results in dilution for the downstream MSI and survival in case of beam load on the vertical MSI collimator. For a 10 σ impact on the TCDIVMSI an AlN mask would reach 380°C, which is largely within its limits (melting point: 3000°C). The hBN mask would reach 175°C for the same scenario. The mask at the special location of the MSI with its tight aperture will be in vacuum. Because of hBN's less favourable vacuum properties and higher cost, it was decided to foresee AlN as mask material.

All the simulations were done for the initially proposed 4-phase collimation system (collimators at 0-45-90-135° from the MSI). For various reasons (see below) the now preferred solution is a 3-phase collimations system (collimators at 0-60-120° from the MSI). The new TCDI locations are either close to the original ones or further upstream from the equipment and hence less critical. No major problem in terms of energy deposition in downstream equipment in case of accidental beam loss is expected for any of these locations. Rerunning the simulations does not seem to be necessary for all cases.

TCDI settings - Angle control: A method to measure angular misalignment of transfer line collimator jaws was proposed in the last meeting (see link). Preliminary simulation results had shown that co linearity can be measured and corrected to ± 100 µm (95 % confidence), resulting in an aperture gain for the setting of the collimators of 0.5 σ.

The method is based on a transmission measurement and a shot-to-shot beam position measurement at the collimator location. One of its key assumptions is a large beam jitter. In the simulations a beam jitter of σ = 0.2 mm was assumed.  

V.Kain presented the final simulation results, where a tolerance of σ = 25 µm on the inter-jaw parallelism was taken into account. A calibration curve to correlate measured transmission with angular misalignment had been established. A measurement accuracy of ±100 µm R.M.S, corresponding to an aperture gain of ~ 0.3 σ,  can be achieved with the proposed method and based on the used assumptions. 

Towards the end of the second TI 8 test, the method was tested with beam using the horizontal TCS installed in TT40. The transmission was measured with the BCTs at the end of the line and in the SPS. The beam position measurement was based on the readings of the horizontal BPKs right upstream and downstream (4003, 4004) of the TCS location. According to the proposed procedure, the beam position at the TCS was measured using linear interpolation between the two BPKs with the jaws open. The next step of the proposed method would have been to measure transmission for several shots with one of the two jaws closed and to record the beam position for every shot. This procedure would have been repeated for the other jaw. However, the showers generated in the collimator, as soon as the jaw was moved in, rendered the BPK readings useless. Thus not all of the required information for the proposed method could be acquired. Furthermore one of the jaws could not be moved properly into the beam, possibly due to some calibration error of the motors or misalignment. Only transmission data for one jaw were taken.

Preliminary analysis of the data showed that the transmission was sufficiently sensitive to any adjustment of the jaw. The upstream or alternatively downstream end was moved further into the beam by 2 σ (= one nominal σ beam intensity: 3·10¹⁰ p+, which had a clear effect on the transmission. A surprising result was that the transmission did not change significantly from shot to shot, indicating a much smaller beam jitter than initially assumed. The measurement time per jaw position was relatively short (~ 20 shots; 100 shots were originally proposed). 

Using BCTs for aligning collimator jaws seems to be an option. The proposed method will have to be modified according to real tolerances and errors, e.g. a smaller beam jitter. The test with beam was also used to define some of these tolerances (the analysis is still ongoing). An alternative method (appropriate for small beam jitter) would be scanning the transmission for different jaw tilts (moving either the upstream end or the downstream end further into the beam) and looking for the maximum transmission to align the jaw.

3-phase collimation: The transfer line collimators must provide sufficient phase space coverage to protect the LHC injection aperture of 7.5 σ and the MSI aperture (every proposed TCDI scheme has collimators directly at the MSI as local protection). Several schemes were investigated: 2-phase (0-90° from the MSI), 3-phase (0-60-120° from the MSI) and 4-phase (0-45-90-135° from the MSI). The decision for a scheme is a trade-off between mismatch to the LHC and protection level of the collimators.  

A 2-phase collimation system with a setting of 4.5 σ and including tolerances (1.4 σ with one motor, 1.1 σ with two motors) would not provide enough protection. Simulations taking beta-beating due to power converter ripples of the line quadrupoles and mismatch from the SPS into account gave maximum possible amplitudes passing through the collimation area of 7.9 σ for two motors per jaw and 8.4 σ for 1 motor per jaw. A 3-phase system would give 6.5 σ for 2 motors and 6.8 σ for 1 motor, and a 4-phase system protects to 6.1 σ with 2 motors and 6.4 σ with 1 motor.

The fewer matching constraints one has, the more likely it is to find a satisfying matching solution. The protection level of the TCDI system can only be guaranteed, if the required phase advance conditions between the collimators can be established. Since there are only 3 collimators per plane (2 collimators less than with 4-phase), a 3-phase system with 2 motors gives the same protection level as 4-phase with 1 motor. Due to the significantly lower cost, the new TCDI baseline proposal is a 3-phase system with 2 motors per jaw (if it is not possible to adjust both motors, the maximum amplitude getting through the system is 6.8 σ, which is still below 7 σ = 7.5 σ - 0.5 σ (½ σ margin for jitter downstream of the collimators)).

It was remarked that the 3-phase collimation system fits quite nicely into the space and phase constraints prevailing in TI 8. The line was fully rematched to the new LHC conditions (the Dy bug is fixed now). No special efforts were required to integrate the new 3-phase TCDI system and almost perfect phase advance conditions between the collimators could be achieved (only one collimator is off by 2° from the ideal phase advance).

The tolerance of the new scheme towards future SPS or LHC optics changes needs still to be investigated.

5. TI 2 optics and TCDI locations

H. Burkhardt presented the status of the optics for TI 2 (slides). The changes from V6.4 to V6.5 were not as significant as for TI 8. In principle there is more space for TCDIs. A 4-phase collimation scheme seems to be feasible, however for cost reasons, again a 3-phase system like in TI 8 is being proposed. In terms of integration there is not such a clear advantage for the new system as in TI 8.

The solution so far foresees collimators partly at very small beta-functions (e.g. TCDIH240 < 10 m). A rematch of the line might improve the situation. A sequence for SPS extraction  as available for TI 8 is also still missing.

6. Progress on open actions

The list of open actions was briefly reviewed and has been updated accordingly.

7. A.O.B.

In the framework of the Injection Working Group a Functional Specification will be provided on interlocks related to the LHC injection process.

Next meeting

The next meeting is tentatively foreseen for 12 January 2005.

V. Kain, 1/12/04.