Raising intensity of the LHC beam in the SPS - longitudinal plane

Transcription

1 SL-Note-- MD Raising intensity of the LHC beam in the SPS - longitudinal plane Ph. Baudrenghien, T. Bohl, T. Linnecar, E. Shaposhnikova Abstract Different aspects of the LHC type beam capture and acceleration to 5 GeV were studied for an injected beam with intensities up to.6u (in three batches). While the main hardware problems limiting beam intensity in 999 were successfully solved during the last shutdown, new limitations have been reached.

2 SL-Note-- MD Raising intensity of the LHC beam in the SPS - longitudinal plane SPS July, elenas/public/md/may/n.ps Ph. Baudrenghien, T. Bohl, T. Linnecar, E. Shaposhnikova Keywords: Beam dynamics, Longitudinal, SPS machine Run no. Date 7/5/ -5/5/ Summary Different aspects of the LHC type beam capture and acceleration to 5 GeV were studied for an injected beam with intensities up to.6 (in three batches). While the main hardware problems limiting beam intensity in 999 were successfully solved during the last shutdown, new limitations have been reached. Introduction Maximum intensity accelerated in 999 in one batch was protons and 8 protons in three batches []. The main intensity limitations in 999 were: Insufficient feedback around the main TW MHz RF system Electron cloud effect: - this perturbes the damper pick-up signals and prevents proper damper operation - the pressure rise leads to interlock trips on the superconducting cavities During new hardware upgrades will gradually be introduced: New feedforward and feedback electronics to control the impedance in the MHz TWC RF system The final version of the power amplifier for the damper will be introduced, along with electronics giving an increased bandwidth ( MHz) and signal detection at MHz to minimize the electron cloud perturbation Experimental conditions Leptons accelerated to GeV (7.5) and GeV (-5.5) were available for LEP filling during these MDs. The GeV limitation of lepton energy is given by the increased coupling in the 5 MHz which was neccesary to obtain high intensity LHC beam in the SPS. The SPS RF systems had following status:

3 - SWC MHz: with active and with passive damping - SWC MHz: passive damping ( cavities) - SCC 5 MHz: active damping, Qext = 5 ( cavities) - TW 8 MHz: undamped ( cavities) RF-feedback on sum of all cavities fed back to cavities and (usual situation for fixed target operation) Feedforward available individually for cavities, only working at injection. Damper system: with upgraded power amplifiers (horizontal). Old electronics (baseband processing). MD on 7.5. Beam parameters The LHC type beam (CY-59) was operational in the SPS from.5 till.5. During this time a single batch of 8 bunches was captured and accelerated to 5 GeV. The maximum intensity of the injected beam was around protons. The bunch length at injection was around.5 ns (see also the measurements below). According to the PS measurement, longitudinal emittance was. evs. To accelerate the beam we used the voltage programme shown in Fig. as a solid line. Besides the flat bottom and the flat top, all through the cycle this voltage programme was designed for acceleration of a.6 evs bunch with a momentum spread equal to.95 of the bucket height (also shown in Fig. as a dashed line).. Measurements and observations In Fig. the batch envelope is shown as a mountain range display during the cycle. In Fig. we present batch envelopes at different moments during the cycle. Injection corresponds to time, the length of the flat bottom was 6 ms and of the ramp - 77 ms. The initial modulation at approximately 8 MHz is due to the debunching-rebunching process in the PS. The lower frequency modulation at. MHz is absent in the initial batch envelope (also shown as a dashed trace in all figures in Fig. ), but could be seen in Fig. at ms after injection. Later, at about s, its frequency increases to MHz ( oscillations in µs long bunch) and does not change much till the end of the ramp. Direct observation of the peak detected signal (not shown here) and comparison with the one expected for this voltage programme (see Fig.) suggested that the bunch continuously blows up during the cycle. Possible noise coming from the synchro-loop was suspected. Parameters of both, the phase loop and the synchro-loop were varied and the effect of different gain settings on the beam was observed on the peak detected signal. These studies showed that although the source of the noise was not removed (coarse frequency programme was suspected) its effect could be reduced by a significant increase in phase loop gain. With this increase the peak detected signal closely follows the calculated one (Fig. ). MD on Beam parameters We had one, two or three batches, of 8 bunches each, injected into the ring with a time interval. s and separated in the ring in the last two cases by µs (nominal spacing is 5 ns) to prevent the kicker rise and fall times hitting the batch.

4 The intensity per batch of the beam injected into the SPS (BCT measurement) was varied from.7 to 5. protons. According to the PS, the measurement of longitudinal emittance was.5 evs and.65 evs corresponding to low and high intensities. The bunch length at injection was.5 ns for lower intensities and slightly more than 5 ns for higher intensities.. Measurements and observations For longitudinal emittance ε =.5 evs and bunch length τ =.5 ns in the low intensity case the matched voltage is kv. We started with 7 kv as the capture voltage which would be the matched voltage in the absence of intensity effects (beam loading, potential well distortion) for a bunch with an emittance of (.-.5) evs and a bunch length (.5-5.) ns. Later we varied this voltage for the high intensity beam (see below). A voltage of.5 MV at the flat top is defined by transfer to the LHC and was varied during the last part of the MD as well... Single batch In the case of a low intensity single batch (injected intensity of.7 ) typically.5 were accelerated to the flat top. Main losses occured at capture and beginning of the ramp. At the end of the flat bottom, the injected bunch had an emittance of around.5 evs due to filamentation in the mismatched bucket. Lowering the voltage below 7 kv was not possible due to beam loading problems. Quadrupole oscillations creating finally very flat bunches, could be observed at the beginning of flat bottom. Already at this intensity small bunch oscillations were observed on the flat top. The bunch length estimated from bunch profile measurement (photos, not shown here) gives.5 ns (.8 ns at.5 level from data acquired with fast digital scope). For a voltage of.5 MV this corresponds to emittance of.9 evs (. evs for.8 ns). Continuous losses along the flat bottom were detected when the intensity in single batch increased to 5... Two batches For the high intensity case (5 per batch) bunch length estimated from photos for bunches from the first batch at flat top was (.8 ns)... Three batches For.7 injected in each batch,.5 were accelerated to high energy - similar to the one batch case. For increased intensity significant particle losses were observed along the flat bottom and at the beginning of ramp. For example, for. /batch injected, 8. were measured on the flat top. Transmission decreased to 7% for iinjected intensities of 5 /batch. The bunch length at the end of the cycle was about.8 ns (photos). Flat bottom Raising the capture voltage increased the continuous losses on the flat bottom, but decreased the sharp losses at the beginning of the ramp. The effect of lowering the capture voltage was opposite - less losses on the flat bottom and increased losses at the beginning of the ramp (less particles captured). Apart from the fact that for the high intensity beam we had too large a longitudinal emittance, it seems

5 that the beam was suffering from some continuous noise in the system increasing the emittance along the long flat bottom as well. In Fig. 5 we present the mountain range display on the injection plateau for capture voltages (see also Fig. ): 8 kv (left) and 5 kv (right). The bunches seem to be stable. However the peak amplitude is decreasing along all of the flat bottom as can be seen in Fig. 6. One can also see a sharp decrease of the peak amplitude for lower voltages at the moment of the second injection -. s, (beam loading?) and an increase in bunch length for higher voltage just before the moment of the third injection -.8 s, (fluctuation?). Flat top The last experiment performed was to change the voltage on the flat top. Raising the beam intensity, we observed strong dipole oscillations on the flat top. With the voltage programme used, the bucket area is kept constant (.6 evs) over all the cycle and sharply increased during the last.5 s before the flat top. We varied the voltage at the end of the ramp from.5 MV to 6 MV. Generally oscillations persisted in all cases, however bunches were less stable with low voltage, even though the nonlinearity of particle motion was increased. In Fig. 7 a mountain range display is presented for different voltages on the flat top (shown also with dotted line in Fig. ):.9 MV (left) and.6 MV (right). In Fig. 8 the bunch length (top) and peak amplitude (bottom) are shown for the second bunch in Fig. 7. Note, that in mountain range display corresponding to a voltage of.6 MV (right figure) the first bunch seems to be more intense than the central one and is unstable at an earlier time (from the beginning of the acquisition at 5 ms). This fact points out to ae low Q impedance as responsible for this instability. The growth rate of the instability estimated from Fig. 8 seems to be roughly of the order of ms. 5 Conclusions A total intensity of in three batches was accelerated to top energy with a transmission efficiency of 7%. Continuous particle losses are observed on the flat bottom for high intensities. The reason of them is not clear. High intensity beam is unstable on the flat top independent of the final voltage value, however the situation seems to be worse for lower voltage. Limitations to the intensity are - too large a longitudinal emittance sent by the PS for intensities above per batch - acceptance of the SPS in connection with this large injected emittance - possibly noise in RF loops (additional emittance blow-up) Measurements described in this paper were done with help of SL/OP. References [] T.Linnecar, Limitations on LHC beam intensity in the SPS in, Workshop on LEP-SPS Performance, Chamonix X,. Nevertheless in this MD the situation with noise was improved in comparison with previous MD on 7.5.

6 6 5 voltage programme ε=.6 evs qp=.95 V (MV) Time (s) Figure : Voltage programme used for the LHC beam operationally and during the MD (solid line) together with voltage programme designed for ε =.6 evs and filling factor in momentum spread q=.95 (dashed line). Dotted lines - voltages for data from MD on Figure : Batch envelope as a mountain range display from injection till end of the cycle. MD on 7.5, single batch with intensity. 5

7 .8 ms 56.9 ms ms 87.7 ms ms ms Figure : Batch envelope at different moments in the cycle together with batch envelope at injection (dashed trace in all figures). 6

8 .8.6 ε=.6 evs µ=. peak line density Time (s) 5.5 bunch length ε=.6 evs τ (ns) Time (s) Figure : Top: peak line density during the cycle for binomial distribution of line density with µ = (parabolic for short bunches). Bottom: bunch length during the cycle. Solid line - operating voltage, dashed line - voltage programme for q=.95. 7

9 Time ns Time ns Figure 5: Mountain range display on the flat bottom. Left: low voltage (.8 MV). Right: high voltage (.5 MV). 6 6 Bunch Length ns Bunch Length ns Peak Amplitude a.u..5.5 Peak Amplitude a.u Figure 6: Bunch length (FWHM) - (top) and peak amplitude (bottom) for bunches from mountain range display in the previous Figure. Left: low voltage (.8 MV). Right: high voltage (.5 MV). 8

10 Time ns 5 Time ns Figure 7: Mountain range display on the flat top. Left: low voltage (.9 MV). Right: high voltage (.6 MV). Bunch Length ns Bunch Length ns Peak Amplitude a.u..5.5 Peak Amplitude a.u Figure 8: Bunch length measured at.5 level of peak line density (top), and peak amplitude (bottom) for central (second) bunches from mountain range displays in the previous Figure. Left: low voltage (.9 MV). Right: high voltage (.6 MV). 9