SUMMARY OF RECENT HIGH LUMINOSITY EXPERIMENTS AFTER THE HERA-II LUMINOSITY UPGRADE AND FUTURE PROSPECTS

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1 DESY HERA SUMMARY OF RECENT HIGH LUMINOSITY EXPERIMENTS AFTER THE HERA-II LUMINOSITY UPGRADE AND FUTURE PROSPECTS M. Minty DESY, Hamburg, Germany The beam parameters of the luminosity upgrade including beam-beam interactions with high single-bunch beam intensities have been recently studied in dedicated experiments. In this report are summarized the anticipated challenges and, in view of these, the different scenarios considered in pursuit of demonstrating highest possible luminosity. Such considerations may be of future relevance. The results of the experiments are presented, in which high total luminosity, exceeding significantly the luminosity achieved prior to the upgrade, was demonstrated. Prospects for future operations are analysed. INTRODUCTION The key elements of the luminosity upgrade [1,2] may be summarized as follows. First the proton beam spot sizes at the interaction points were made smaller by moving the final focussing quadrupoles closer to the interaction point (IP). The proton beam sizes at the IP reduced from design values of σ x =189 um horizontally and σ y =50 um vertically to σ x =112 um and σ y =30 um. Then, to match the spot sizes of the proton and lepton beams at the IPs, the lepton beam emittance was reduced in two ways: (i) the optics was modified from a 60 degree to a 72 degree phase advance per cell in both planes, which reduced the horizontal emittance from 40 π mm-mrad to 30 π mm-mrad, and (ii) a shift of the accelerating frequency by ~300 Hz was applied, which reduced the horizontal emittance further from 30 π mm-mrad to 20 π mm-mrad. Sophisticated new quadrupoles, located closer to the IPs, were also included to further reduce the lepton beam sizes at the IPs. As a result the lepton horizontal beam size at the IPs were correspondingly reduced to match that of the proton beam. The vertical beam size, which is determined additionally by the settings for the coupling, is also manipulated to be equal to those of the proton beam. To realize this design, two major difficulties must be addressed. First, the separation of the two beams takes place much closer to the IPs and the bending strengths of the lepton beam are substantially higher. This leads to increased synchrotron radiation, which may produce backgrounds in the particle detectors if not appropriately collimated. Second, while with the 72 degree optics the dynamic aperture is larger than with other configurations of phase advance per cell [3], the beam energy spread increases with the rf frequency shift. These two changes were considered to be complementary, however, the increased energy spread of the beam remains an issue particularly for luminosity operation with the positron betatron tunes optimised for high polarization [4]. In addition, the luminosity upgrade assumes beam intensities, which are higher than that ever achieved at HERA. Given in Table 1 are the beam energies E, currents I, IP spot sizes σ x and σ y, estimated instantaneous (L) and specific (L sp ) luminosities for HERAI* (pre upgrade), HERAII* (upgrade with the same beam currents as used during HERAI*), and HERAII (nominal upgrade parameters). For the latter two cases, the cited luminosities include a reduction factor of 0.92 to account for the hour glass effect at HERAII [5], which arises when the proton bunch length (σ z ~0.19 m rms) becomes comparable to the vertical β-function (β y =0.18 m) at the IP. In the high luminosity experiments to be described in this report, it was taken as given, that the achievable total beam currents would not exceed that demonstrated during prior to the upgrade; i.e. the column for HERAII* is here relevant. Parameter HERAI* HERAII* HERAII HERA-E HERA-P HERA-E HERA-P HERA-E HERA-P E (GeV) I (ma) σ x / σ y (µm) 187/50 187/50 112/30 112/30 112/30 112/30 L (cm -2 s -1 ) L sp (cm -2 s -1 ma -2 ) Table 1. Selected design parameters for HERAI* (pre-upgrade), HERAII* (post-upgrade with total beam currents as in HERAI*, and HERAII (post-upgrade).

2 PRACTICAL CHALLENGES In determining the optimum set of operating parameters for demonstration of high luminosity, numerous challenges were anticipated. These are listed below in the order considered at that time to be of highest priority. In parenthesis are given the measures presently being taken to address these issues. background rates at the experiments [6,7] the maximum current product I p I e had been administratively limited to 1200 ma 2 for luminosity runs. This was based on radiation limits, as measured by the experimental diodes, to (50-250) khz for a period of time not exceeding 30 minutes, and >250 khz for no longer than 5 minutes. (solutions: modifications in the experimental regions [6,7], improved surface treatment of absorbers [8]) maximum current of the positron beam the total current was limited by the rf systems, which had not been extensively conditioned, to no more than 40 ma (to be addressed by continued, high current operation) maximum delivered proton single-bunch current and HERA injection efficiency - potentially limited by efficiency in upstream accelerators and tuning between PETRA and HERA (more effort towards improved reproducibility in the P- weg advisable) maximum lepton beam single-bunch current potentially a source of proton beam emittance growth via the beam-beam interaction (not validated for the beam currents used in HERAI [9]) maximum proton beam single-bunch current potentially a source of positron beam emittance growth and loss via large beam-beam tune shifts [validated and avoidable by appropriate choice of lepton beam betatron tunes ([9,10]) positron beam bunch distribution ( fill pattern ) potentially a source of complications via transient beam loading (was not an issue during these experiments) Unexpectedly (aside from the limitation on total beam current), the most serious issue proved to be accumulation in the lepton accelerator for high total beam current. While remedial efforts (frequency shifting between fills from PETRA [11]) were undertaken, the vacuum pressure increases and correspondingly high radiation levels proved to be most challenging. An example, showing the total positron current and the vacuum as measured at two locations near the H1 IP is shown in Fig. 1. During the first accumulation the vacuum pressure increased by over 3 orders of magnitude to a peak value of mbar. For comparison, under conditions of the highest attained luminosity, the vacuum pressure was only to mbar measured at the same location. The diode rates were correspondingly lower (about 100 khz). Figure 1. Time history of the positron current (black, 35 ma peak) and two measurements of the vacuum near the H1 IP (green and brown curves, logarithmic scale)

3 OPERATIONAL SCENARIOS Given constraints (arising for example from high backgrounds) on the total beam currents, it is well known that decreasing the total number of colliding bunches, while maintaining high total beam currents, can lead to increased total luminosity. This can be seen easily from the simplified formula (assuming head-on collisions between beams with Gaussian transverse intensity profiles disregarding effects related to long bunches) for the luminosity L: N p N e L = 2 π Σ Σ x y f col, Eq. (1) where N p and N e are the single-bunch populations of the protons and leptons respectively, f col is the collision frequency given by the number of colliding bunches times the revolution frequency (N col f rev ), and Σ denotes the convoluted beam sizes at the IPs. The relation between the bunch populations N and the total beam current I is I p,e =N p,e ef rev, with the electron charge e= C. From Eq. (1), then I L = π I p e rev 2 e 2 Σ Σ yn x col f. Eq. (2) Therefore, given a constraint on the total current product I p I p, reducing the number of colliding bunches N col leads to higher total luminosity. For the purposes of optimising the likelihood of success, it was assumed that the total positron current would not exceed 35 ma (as this had been demonstrated only rarely since the upgrade). Furthermore, for operational ease, it was assumed that total bunch numbers of N col N col =60 60, , or were easily realizable (reflecting the proton bunch number as delivered normally by PETRA). With these assumptions the expected luminosity versus total proton current is sketched in Fig.2. The different (colored) curves correspond to the different N col while for each set of colored curves, two different proton emittances are assumed: the higher curve assumes ε p,x /ε p,y =14/16 as measured with the highest luminosity achieved prior to the experiments on , while the lower curve assumes ε p,x /ε p,y =18/21, which is closer to design and was measured on With 35 ma total lepton beam current, a goal of L= cm -2 s -1 would impose challenges on the delivery of high proton beam intensities. A vital consideration concerned the constraint on total beam current product, which is plotted in Fig. 3 versus total luminosity. The administrative current limit of 1200 ma 2 used during luminosity operations is also shown. It is clear that to demonstrate high luminosity, the total current limit is best alleviated using a reduced number of bunches of higher single-bunch beam current. Moreover, to attain L= cm -2 s -1 even with only 120 colliding bunches the administrative current limits would have to be raised. The radiation limits are set using the experimental diodes. During initial commissioning these limits were ~50 khz for up to 30 minutes, and >250 khz for 5 minutes. For the high luminosity experiments, the first of these limits was relaxed to (50-250) khz for up to 30 minutes while the second limit remained unchanged. This translated into a total current limit of about 2000 ma 2. From the considerations just presented and taking into account the achievable single-bunch proton intensities, we concluded operation with bunches would be most suitable for demonstrating high luminosity. Summarized in Table 2 are selected relevant parameters compared to those achieved prior to the high luminosity experiment. In the final column is shown also the goal of these experiments. In Table 2 the symbols used are as follows: N b.p/e = the number of bunches in each beam (protons leptons), N c = the number of colliding bunches, I p = total proton beam current, I e = total positron beam current, ε px,y = proton emittances in the horizontal (x) and vertical (y) planes, ε ex,y = lepton beam emittances (the emittance units are 1 su = 1 mm-mrad and denote the 1-σ normalized emittances), L = instantaneous luminosity (1 su = cm -2 s -1 ), L sp = specific luminosity (1 su = cm -2 ma -2 s -1 ), I p I e = total current product, ξ / IP = the incoherent beam-beam tune shift per IP, and I sb,p/e = single bunch intensity of the protons and positrons respectively. The notation e/m refers to expected and measured quantities. The parameter L sp ratio gives the ratio of the measured to expected specific luminosity.

4 Figure 2. Total luminosity versus total proton beam current for different bunch populations: green (60 60), blue ( ), and red ( ) assuming a total lepton beam current of 35 ma. The different curves with the same color assume optimistic (upper) and more realistic (lower) proton beam emittances as described in the text. Figure 3. Product of the total beam currents as a function of luminosity. The color coding is the same as in Fig. 1

5 Table 2. Summary of design, selected (best performing) measured and achieved luminosity together with goal for these experiments (final column). The symbols and units are given in the text. HIGH LUMINOSITY EXPERIMENTS AT HERAII A typical fill pattern used in the high luminosity experiments is given in Fig. 4. A total of about bunches were used and, to avoid potential complications from heavy beam loading of the rf systems, the current distribution was made as uniform as possible.

6 Figure 4. Typical bunch distribution showing proton (top) and positron (bottom) currents along the accelerator. Time histories of the beam currents and measured luminosity at H1 are shown in Figs. 5 and 6 during the course of the experiment, which consisted of 6 runs. The measurement strategy was based on step-wise increases to the singlebunch beam currents. In Table 3 are summarized the results.. For the calculated luminosities, the design positron emittances and the hour glass factor as derived from the measured proton bunch lengths were taken into account. Run 0 represents the best luminosity achieved prior to the accelerator studies. Runs 1-6 show the results from the high luminosity studies. For comparison, the average peak instantaneous luminosity of year 2000 with HERAI was L= cm -2 s -1. In these experiments with a reduced number of colliding bunches, an instantaneous luminosity of at least L= cm -2 s -1 was achieved. Ip and Ie (ma) Feb 21-23, 2003 Figure 5. Measured total beam currents of the protons (blue) and positrons (green) during the course of the 6 high luminosity experiments. Luminosity (/cm^2 s) 3E+31 3E+31 2E+31 2E+31 1E E Feb 21-23, 2003

7 Figure 6. Total instantaneous luminosity measured at the H1 experiment during the course of the 6 high luminosity experiments. For reference, the average peak luminosity before the upgrade was L= cm -2 s -1. Table 3. Summary of the high luminosity experiments. The beam current product I p I e is given at injection. While it is difficult to draw conclusions from the limited statistics of these experiments, a few observations made during the studies warrant discussion: Run 0 ---Reference taken during normal luminosity operations with proton currents a factor of 2 higher than had been used previously. The instantaneous luminosity achieved at H1 was L= cm -2 s -1. The specific luminosity was L sp = cm -2 ma -2 s -1, which exceeded the HERAII design goal by 10% (due to the small proton emittances) and significantly surpassed that achieved in HERAI by more than a factor of 3. The calculated and measured specific luminosities were in agreement. Runs 1,2 --- First two high luminosity experiments performed with only 120 colliding bunches and with modest increases to first the proton, then the positron total beam current. The instantaneous luminosity achieved exceeded the previous HERA record of L= cm -2 s -1. The calculated and measured specific luminosities were in agreement. Run Third high luminosity experiment made with a second refill of the positron beam (that is, reinjection while keeping the stored proton beam). Poor choice of positron betatron tunes led to coherent beam-beam instability [9,10] and subsequent blowup of the proton beam emittances. The betatron tunes used during the 6 high luminosity runs are shown in Fig. 7. It is interesting to note that, as had been observed during the beam-beam experiments [9], the positron beam was observed frequently to lock onto the 3Q x +Q y resonance with collisions at 1 IP only. Runs 4,5 --- These two runs evidenced a significant and surprising (~15 to 20 %) discrepancy between measured and expected specific luminosity. An attempt was made to attribute this discrepancy to dispersion generated by the correction magnets used to center the lepton beam at the IP [12]. The measured horizontal beam positions as inferred from the IP beam position monitors (BPMs) located +/- 2 m from the interaction point are shown in Fig. 8. While significant differences (up to 0.5 mm) in beam positions at the IP were observed, the increase in beam sizes in both planes due to dispersive contributions [13] were found to be negligible. A possible explanation could be increased positron beam emittance due to the beam-beam induced resonance at 3Q x +Q y. Unfortunately, the positron beam emittances could not measured during this experiment (due to the high total beam currents). As reported from the beam-beam studies [9], which were performed with lower total lepton beam currents, the positrons were seen to lock onto this resonance with the consequence of an enlarged (up to a factor of 4) vertical emittance. With regards to the instantaneous luminosity L, the estimate for run 4 was scaled down by 20% to account for the presence of a strong proton satellite bunch.

8 run 6 run 2 run 0 3Q x +Q y run 4 run 5 Q y =1/3 2Q x +2Q y run 3 Q x =1/4 run 1 Figure 7. Tune diagram for the positrons showing the tunes of the colliding (open circles) and noncolliding (closed circles) during the 6 high luminosity runs. Figure 8 Horizontal beam positions at the H1 interaction point as inferred from the two IP beam position monitors. Run This final run demonstrated the highest luminosity achieved to date at HERA. In this case the vertical betatron tune of the noncolliding bunches of the positron beam was set to higher than Q y =1/3. In this way the proximity of the colliding bunches to the dangerous 2Q x +2Q y resonance was smaller. In addition, the beam position as measured by the IP BPMs was comparable to the previous runs for which the measured and calculated specific luminosities were in agreement. While the measured proton beam emittances did not evidence degradation due to beam-beam induced coherent instability at 2Q x +2Q y, the possibility of emittance blowup of the positron beam due to (single-beam) nonlinearities in the lattice can not be excluded. Based on the results from the beam-beam experiments [9, Fig. 5], however, the influence of this single-beam resonance on positron emittance likely can not explain the observed discrepancy between measured and calculated specific luminosity in this experiment. A more likely explanation pertains to the anticipated pile-up effect in the measurement of the luminosity, which arises from saturation of the detectors at high luminosity. At the time of the measurements, this effect was thought to underestimate the measured luminosity by about (10-20)%. However, detailed Monte-Carlo simulations will be required for future experiments. In

9 applying the results of the high luminosity studies to predict the luminosity realizable at HERA will be assumed a 10% correction factor to account for the possible systematic measurement error of the luminosity monitor. SUMMARY OF HIGH LUMINOSITY STUDIES AT HERAII Plotted in Fig. 9 is the measured specific luminosity versus single-bunch proton current. The light blue dots were obtained under normal luminosity conditions with bunches and low total beam currents (roughly I p = 30 ma and I e = 25 ma at the start of each run). The yellow dots were measured during accelerator studies in year 2002 for which the number of colliding bunches was reduced to 10 or 40 and the total beam currents correspondingly smaller (about I p = 8 to 12 ma and I e = 3 to 6 ma). The dark blue dots show the results of the high luminosity runs presented in this report taken with 120 colliding bunches and I p ~ 45 to 70 ma and I e ~ 25 to 35 ma. For comparison are shown the year 2002 (red dot) and the HERAII (orange dot) goals as predicted for higher single-bunch proton intensities. The downward trend in specific luminosity with increasing proton current was found in separate measurements to be fully explained by changes in the beams transverse emittances [9]. From Fig. 9, with the exception of the high current studies undertaken in year 2002 during early commissioning, most of the measured specific luminosities lie near (or are greater than) the design HERAII specific luminosity L sp = cm -2 ma -2 s -1. While it is anticipated [9] that the effect of the transverse resonances on the beam emittances will be minimized in the future using the betatron tunes required for high polarization, synchro-betatron resonances [4] may prove an issue. Figure 9. Measured specific luminosity versus single-bunch proton current (Oct, 2002 Mar, 2003). The measured instantaneous luminosity is plotted in Fig. 10 versus total current product. Until mid-feb, 2003 the radiation levels at the experiments imposed an administrative limit on the current product of I p I e =1200 ma 2. Prior to the accelerator studies this administrative limit was raised to I p I e =1800 ma 2. The higher-than-design instantaneous luminosities arise from the smaller-than-design beam sizes at the IP.

10 Figure 10. Measured instantaneous luminosity versus total current product (Oct, 2002 Mar, 2003). Fig. 11 compares the accelerator performance with respect to integrated luminosity. Shown are the beam currents (left) and the integrated luminosity (right) under three different conditions. The typical HERAI run had an average peak luminosity of L= cm -2 s -1 with 94 ma protons and 45 ma positrons in ~180 bunches. The best run of HERA I had a peak luminosity of L= cm -2 s -1 with 97 ma protons and 45 ma positrons in ~180 bunches. The best run of HERAII to date, with L= cm -2 s -1 was achieved with only 67 ma protons and 32 ma positrons in ~120 bunches. Since high single-bunch luminosities have been demonstrated, extrapolation to the full bunch population (by the bunch number ratio ~180/120) should be valid. With the current limits lifted, we can with confidence predict instantaneous luminosities of L= cm -2 s -1 *180/120= cm -2 s -1.

11 Figure 11. Proton and positron beam currents (left) and integrated luminosity (right) for 3 different cases: a typical HERAI* run (top), the best run achieved during HERAI* (middle), and the best run achieved to date with HERAII* (bottom). The time scale, chosen to start at the beginning of a luminosity run, is the same in all plots. Note that that HERAII* data presented were obtained under the constraints of significant administrative limits on the allowable total beam currents. IMPORTANT CONSIDERATIONS FOR HIGH INTEGRATED LUMINOSITY While there are many factors (including e.g. conflicting requests from the different experimental groups), which affect the duration of a typical luminosity run T R, optimisation of this time is critical in maximizing the delivered integrated luminosity L int. In the following the luminosity is expressed in a way that shows how it may most easily be maximized. In doing so, the optimum run duration T R will be evaluated and used to determine the average integrated luminosity. The results will be presented in terms of an efficiency, which depends critically on the up-time of HERA. This analysis is an extension of that presented in Ref. [14]. Here the set-up time, T S, required for turning on the experiments after luminosity has been established is also taken into account. The foreseen factors, which can be controlled and used to maximize the integrated luminosity, are given expressly. The average luminosity L avg per run is obtained by dividing the integrated luminosity L int per run by the total time between injection cycles, T F +T R +T S : L avg = T R τ 0 LL + T + T F S e TS τ L e TR TS τ L Eq. (3) T F = T int (1-η eff ) is the fill time which includes the efficiency η eff, which describes the percentage of time that a luminosity run is possible (i.e. it takes into account operational downtime), T int denotes the time period of

12 interest (i.e. 24 hours). For example, during year 2000, η eff =56%; that is, 56% of time was available for luminosity runs; the other 44% comprised the time to fill and ramp the beams, a small percentage of set-up time and accelerator downtime T R = the time in which luminosity is provided (it does not include the time T S ) T S = the time required for setup (e.g. luminosity tuning consisting of minimizing backgrounds, moving in detectors, fine-tuning collimators, etc.). τ L = the luminosity lifetime with L = L 0 exp(-t/τ L ) L 0 = the instantaneous luminosity, the optimisation of which with the luminosity upgrade has been analysed [8] For maximum integrated luminosity, the variable most easily under control is the run duration, T R. The optimum run length is given by dl dt avg R = 0, or equivalently, e TR TS τ L = ( T + T + T ) R F τ L S + τ L e TS τ L. Eq. (4) The solution of this transcendental equation is depicted from numerical evaluation in Fig. 12 for the perfect case of zero set-up time, T S = 0, as a function of operational efficiency η eff for HERAI* (April, 2000) with L 0 = cm -2 s -1. The three curves assume luminosity lifetimes of τ L = 5.5 hours (HERAI* average during the peak performing month of April, 2000), τ L = 7.0 hours, and τ L = 8.5 hours. Figure 12. Optimum run duration T R as a function of efficiency η eff evaluated for HERAI* (April, 2000) for three different luminosity lifetimes: 8.5 hours (green, top), 7.0 hours (red, middle), and 5.5 hours (blue, bottom). The dotted lines show the average efficiency (56%) and optimum run duration (8.2 hours) from HERAI*. Longer setup times T S are taken into account in Fig. 13, which shows the optimum run duration T R versus efficiency, and Fig. 14, which shows the integrated luminosity per day assuming the optimum run duration T R has been adhered to. In brief, the longer the setup time, the longer the optimum run duration, and the lower the resulting integrated luminosity.

13 Figure 13. Optimum run duration T R as a function of efficiency η eff evaluated for HERAI* (April, 2000) for three different setup times T S : 1.0 hour (green, top), 0.5 hours (blue, middle), and 0 hours (red, bottom). The average luminosity lifetime was 5.5 hours. The dotted lines show the average efficiency (56%) of year Figure 14. Average integrated luminosity per day as a function of efficiency η eff evaluated for HERAI* (April, 2000) for three different setup times T S : 1.0 hour (green, bottom), 0.5 hours (blue, middle), and 0 hours (red, top). The average luminosity lifetime was 5.5 hours. The dotted lines show the average efficiency (56%) of year A critical parameter in integrated performance is the luminosity lifetime τ L. Many factors, such as orbit and tune control, may influence τ L. To some degree, τ L scales with the lepton beam lifetime τ e, which in turn depends on the lepton beam current I e [15]. Plotted in Fig. 13 are the luminosity lifetime τ L and the positron beam lifetime τ e versus total lepton current during HERAI* and HERAII*. Better control and understanding of τ L is warranted in the future.

14 Figure 13. Fitted luminosity lifetime (red dots) and lepton beam lifetime (blue diamonds) as a function of total lepton beam current during a month of peak performance at HERAI* (top) and during the past few months at HERAII*. High luminosity lifetime is achievable in HERAII as demonstrated in Fig. 14 from January, 2003 measured with 30 ma protons and 25 ma positrons in the standard fill pattern with bunches. Here the luminosity at the start of this long run was L 0 = cm -2 s -1 and the fitted luminosity lifetime was τ L = 12 hours.

15 τ L =12 hours Figure 14. Demonstrated high luminosity lifetime from January, 2003 over the course of a long run. PROSPECTS FOR FUTURE HIGH LUMINOSITY OPERATIONS AT HERA In the following, various scenarios are presented for achieving the HERAII design goal of 1 fb -1. We assume: 3 years of operation with 300 days/year dedicated to luminosity running - this implies a goal of 1.11 pb -1 per day, or in the units presently used, 1110 nb -1 per day the single-bunch luminosity which was demonstrated during the high luminosity studies (run #6 of Table 3) the administrative current limits (given by background conditions) are relaxed and the number of colliding bunches can be raised from 120 (run #6) to 174 as was used in HERAI with 180 proton bunches and 189 lepton bunches - from run #6 the total beam currents required are I p =100 ma and I e =48 ma based on the last two items, the instantaneous luminosity is L 0 = cm -2 s -1 and the specific luminosity is L sp = cm -2 ma -2 s -1 the optimum length of a luminosity run is matched given the overall accelerator efficiency (see Eq. 4) Shown in Fig. 15 are the optimised run length T R (left column) and the integrated total luminosity per day (right column) as a function of operational efficiency η eff. For reference, the dotted vertical line at 56% shows the average efficiency attained during HERAI* in year The dotted horizontal line shows the goal of 1.11 pb -1 per day, which would be needed to achieve a total integrated luminosity of 1 fb -1 for the assumptions itemized above. The three rows correspond to three different times required to setup a luminosity run T S, which denotes the time required for the detectors to begin logging data after luminosity has been established. Within each plot are 4 curves corresponding to different luminosity lifetimes ranging from τ L = 5.5 hours (average from year 2000) to 10 hours in 1.5 hour increments. Recall from Fig. 14 that a luminosity lifetime of 12 hours has been demonstrated at HERAII albeit with significantly lower total beam currents. The trade-offs that can be made to achieve the goal are easily inferred from Fig. 15. In order of importance are: maximizing (and maintaining) the luminosity lifetime τ L, maximizing the efficiency η eff, and minimizing the setup time T S. Notice that by maximizing the luminosity lifetime, accelerator operations become easier as the optimum run length T R increases correspondingly.

16 Figure 15. Optimization of integrated luminosity at HERAII. Here T R refers to the optimum duration of a luminosity run, T S refers to the time required for the detectors to begin logging data after luminosity has been established, τ L is the luminosity lifetime, and L int is the integrated luminosity per day. The dashed vertical line shows the average efficiency η eff of luminosity operation in HERAI (year 2000). The dotted horizontal line shows daily luminosity required to achieve 1 fb -1 assuming 3 years of operation with 300 days per year of luminosity running. SUMMARY In the introduction, the key features of the luminosity upgrade were briefly summarized and a representative parameter comparison was presented. Then, the anticipated challenges for the demonstration of high luminosity were reviewed. As expected, the most critical issue concerned the allowable total beam currents, which were limited by background in the experimental regions. Unexpectedly the accumulation of lepton beam current proved difficult due to high radiation levels arising from higher-order-mode driven vacuum pressure increases in the interaction regions. These challenges were taken into account as various operational scenarios were considered. The results of the high luminosity experiments at HERAII were then summarized. Given the administrative constraint on total beam current I p *I e, the studies were performed with a reduced number of colliding bunches N col. For these studies, consisting of 6 runs, the single-bunch (sb) beam intensities were steadily increased until they were equal

17 to those used prior to the upgrade. At low sb currents, specific luminosities of L sp ~2.5 su (1 su= /su cm 2 ma 2 s) were achieved which are significantly greater than design (1.8 su). At higher sb (nominal positron and up to 80% of the proton) currents, the expected and measured L sp were in good agreement thereby validating the assumed optical functions and the design beam sizes at the interaction point. With nominal sb intensities for both beams a discrepancy, between expected and measured L sp was observed. This may be a result of increased lepton beam size (which for technical reasons can not be measured with total currents exceeding 6 ma) or may be due to pile-up in the luminosity detector. For future operations, Monte-Carlo simulations of the latter effect will be required. A comparison of achieved instantaneous luminosities are given below: HERA-I* <L peak > = /cm 2 s I p *I e = 94*45 ma 2 N col =174 HERA-I* L peak = /cm 2 s I p *I e = 97*45 ma 2 N col =174 HERA-II* L peak > /cm 2 s I p *I e = 67*32 ma 2 N col =120 This demonstrates a substantial luminosity increase over HERAI even with a reduced number of colliding bunches N col and with substantially reduced total currents, I p *I e. Extrapolating the achieved single-bunch luminosities to N col =174, we expect L extr =4.44E /cm 2 s with 100*48 ma 2. Graphical summaries of the high luminosity studies at HERAII during 2002 and 2003 were also presented. Regarding future operations, important considerations for high integrated luminosity were reviewed. A convenient expression for the integrated luminosity was given, which highlights in a relatively independent way the dependencies on various control parameters. These include operational efficiency, the fill time, the run duration, the time required to set-up the experiments (time between establishing luminosity and data logging), and the luminosity lifetime τ L. The luminosity lifetime is as yet not well understood and warrants further study. For the year 2000, for example, the average luminosity lifetime <τ L > was only 5.5 hours. With HERAII <τ L > = 12 hours has been demonstrated. In the final section, prospects for future high luminosity operations at HERA were reviewed. Under the assumptions of accelerator availability (number of days devoted to luminosity running) presented in that section and using the experimentally demonstrated achievable luminosity (run 6 of the above studies), it was found that achieving a total integrated luminosity of 1 fb -1 would be difficult but not impossible. REFERENCES [1] U. Schneekloth (ed.), The HERA Luminosity Upgrade, DESY HERA (1998) [2] G. H. Hoffstaetter, Future Possibilities for HERA, Proc. EPAC 2000, Vienna (2000) [3] G. H. Hoffstaetter and F. Willeke, Electron Dynamics in the HERA Luminosity Upgrade Lattice of the Year 2000, Proc. PAC 1999, New York (1999) [4] F. Willeke, Synchrobetatron Resonances in HERA-e After the Luminosity Upgrade, DESY HERA (2003) [5] G.H. Hoffstaetter and F. Willeke, Beam-Beam Limit with Hourglass Effect in HERA, Proc. EPAC 2002, Paris (2000) [6] U. Schneekloth, ZEUS Background Conditions and Improvements, DESY HERA (2003) [7] D. Pitzl, Backgrounds at H1, DESY HERA (2003) [8] M. Seidel, Vacuum in the HERA Interaction Regions: Recent Experiences and Outlook, DESY HERA (2003) [9] M. Minty et al, Recent Beam-Beam Experiments After the Luminosity Upgrade, DESY HERA (2003) [10] J. Shi, private communication (April, 2003) [11] W. Kriens and U. Hurdelbrink made possible the ability to simulateneously shift the rf frequency of the proton and lepton beams in HERA [12] That significant dispersive contributions to the beam size can be generated using the usual corrector combinations used for centering the beams at the IP was first pointed out by E. Gianfelice [13] The values used for the dispersion generated by the IP bumps as compared with experiment were provided by J. Keil [14] M. Bieler, HERA Betrieb 1999/2000, DESY Beschleuniger-Betriebsseminar, Groemitz (Sept, 2000) [15] M. Hoffmann, Untersuchungen zur Strahlebensdauer in HERA-e, DESY HERA (2003)