Mini-torus update Mark Pitt, Virginia Tech (all simulation work done by Juliette Mammei)

1. Mini-torus update Mark Pitt, Virginia Tech (all simulation work done by Juliette Mammei) • Conclusions first: • In the new primary collimator layout, we prefer to run with the • mini-torus OFF during production running. • We could probably live without the mini-torus for Run I, if we can take • Q2 calibration data with a hydrogen gas target. • The mini-torus could potentially still be an important tool, depending on • how well we need to control the beam properties during Q2 calibration • running. It would potentially allow us to run the full tracking system • (with a hydrogen gas target) up to beam currents of ~ 300 nA, • where the stripline monitors can be used to monitor the beam position.

2. e- Beam Luminosity Monitor Need for Mini-torus: Moller Rates in Region II chambers location: z = 4.5 and 5.0 meters from target center At the location of middle chambers: e-p elastic rate: ~ 4.9 kHz/nA Moller rate (30 - 70 MeV): ~ 2.7 MHz/nA This would limit us to running these chambers at about 0.2 nA, instead of the desired 10 nA.

3. Mini-torus in operation - "bending down"

4. Recent Mini-Torus History • The new primary collimator aperture was designed for mini-torus off. • Mini-torus considerations were not taken into account in its design. • Almost all factors (other than the mini-torus) favored a downstream • collimator solution. With the downstream collimator, the mini-torus • affects the acceptance (and therefore the average Q2 when it is • operating). • In the new design of the upstream region, there is less lever arm for • the mini-torus to operate. • The result is that we have taken a somewhat different point of view of • how we will actually use the mini-torus in the experiment, but we believe • it still can be an important tool.

5. Mini-torus location “large” version shown

6. Mini-torus parameters and resulting Moller rates Note: The “small” version is what the mini-torus budget was based on.

7. Value of Average Q2 in Various Situations

8. Liquid LH2, full radiaton Gaseous Hydrogen Target Focal Plane Profiles for Different Conditons Liquid LH2, no radiaton Liquid LH2, “medium” mini-torus on

9. Comparison of Q2 Distributions – radiated versus unradiated Radiation off <Q2> = 0.0259 GeV2 Radiation on <Q2> = 0.0258 GeV2

10. Scenario 1: Run mini-torus during production running • When the mini-torus was downstream of the primary collimator, the • plan was to have it on during production running. This was acceptable • because it didn’t affect the acceptance and <Q2> of the experiment. • With the mini-torus upstream of the primary collimator, it shifts the • <Q2> of the experiment by ~ 2-6%, thus putting a much greater • burden on reliable operations of the mini-torus. • The mini-torus will operate in a harsh radiation environment, and it • will have much less dollars and manpower devoted to it than the main • torus. • Default choice: do production running with mini-torus OFF • (if mini-torus proves to work reliably, then we always have the • option of choosing to run with it on during production running)

11. Scenario 2: Make primary Q2 calibration with H2 gas target • As shown earlier the difference between <Q2> for gas and liquid target • is ~1%. • Numbers for cold gas hydrogen target • (assuming 25 K, 1 atm. H2 and 3.5 mil Aluminum end windows) • Aluminum quasi-elastics: .054 kHz/nA • Aluminum elastics: .054 KHz/nA • H2 elastics: .083 kHz/nA • Total = .19 kHz/nA (44% is H2 signal) • Moller rate (68 kHz/nA) : so could run at 10 nA beam current with • no mini-torus • Then increase H2 pressure to 2 atm., take difference (44% increase) • Assuming 100 “pixels”, 1% statistics per pixel, each run takes ~ 500 seconds

12. Step-by-Step Procedure for Scenario 2 • Measure the light-weighted Q2 distribution with a cold (25 K) hydrogen gas target at 1 atm. and 2 atm. at 10 nA beam current with no mini-torus. Take the difference to get the light-weighted Q2 • distribution for the gaseus H2 component alone. • The distribution from 1 can be used as input in Monte Carlo, and after corrections for the small external bremsstrahlung in H, internal bremsstrahlung, and external bremsstrahlung in air or He, it is the Q2 measured at the vertex that we want. • Furthermore, we can use the distribution from 1, plus the Monte Carlo, to predict what the focal plane light-weighted spatial distribution should be for the full liquid target. Then we can CHECK that with the region 3 chambers (at 100 nA) or the quartz scanner (at full beam current).

13. Scenario 3: H2 gas target plus mini-torus So why a mini-torus at all? In scenario 2, the maximum beam current we can operate the entire tracking system at is ~ 10 nA. At this beam current, we will have to resort to special techniques to determine the beam position, size, halo AND we won’t be able to monitor beam position continuously during the data-taking. If we combined a mini-torus plus the H2 gas target we could run the Region 2 chambers at currents of up to (Region 1 will be at 20 MHz at 300 nA): ~ 200 nA “small” mini-torus ~ 545 nA “medium” mini-torus (3% shift in Q2) ~ 1100 nA “large” mini-torus (6% shift in Q2) At all these currents, the usual stripline beam position monitors can operate to give us beam position information continuously. The “price” is that the <Q2> shifts when mini-torus is on, but by measuring the shift at various mini-torus currents, the effect can be self-consistently understood in Monte-Carlo.

14. Conclusions • In the new primary collimator layout, we prefer to run with the • mini-torus OFF during production running. • We could probably live without the mini-torus for Run I, if we can take • Q2 calibration data with a hydrogen gas target. • The mini-torus could potentially still be an important tool, depending on • how well we need to control the beam properties during Q2 calibration • running. It would potentially allow us to run the full tracking system • (with a hydrogen gas target) up to beam currents of ~ 300 nA, • where the stripline monitors can be used to monitor the beam position.