mini-torus update mark pitt, virginia tech (all simulation work done by juliette mammei)

M. Pitt, Virginia Tech Qweak meeting,Oct’05

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.


Need for Mini-torus: Moller Rates in Region II chambers

LuminosityMonitor

e Beam

At the location of middle chambers:

e-p elastic rate: ~ 4.9 kHz/nAMoller 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.

location: z = 4.5 and 5.0 meters from target center


Mini-torus in operation - "bending down"


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 ofhow we will actually use the mini-torus in the experiment, but we believeit still can be an important tool.


Mini-torus location “large” version shown


Mini-torus parameters and resulting Moller ratesMini-torus

version

Rmin

(cm)

Rmax

(cm)

Thickness(cm)

Weight per coil

(pounds)

Maximum

Voltage(V)

Maximum

Current(A)

MaximumPower

(kW)

“small” 8.75 26.25 2.8 80 96 208 20

“medium”

8.75 38.75 2.1 141 167 208 35

“large” 8.75 43.75 2.8 241 287 208 60

Mini-torus version

Mini-torus currentdensity (A/cm2)

Moller rate (kHz/nA)

Maximum beam current

(nA)

“small” 470 266 2

“small” 560 136 4

“medium” 470 89 6

“medium” 560 45 11

“large” 470 22 23

“large” 560 22 23Note: The “small” version is what the mini-torus budget was based on.


Value of Average Q2 in Various Situations

Situation Rate(MHz)

<Q2>(GeV2)

% changefrom “no radiation”

No radiation (internal or external) 1169 .0259

Liquid H2 target, no mini-torus 875 .0258 -0.4%

Gas H2 target, 25 K, 1 atm. .0261 +0.8%

Liquid H2 target, “medium minitorus”

819 .0267 +3.1%

Liquid H2 target, “large minitorus” 756 .0274 +5.8%


Focal Plane Profiles for Different Conditons

Liquid LH2, full radiaton

Gaseous Hydrogen Target

Liquid LH2, no radiaton

Liquid LH2, “medium” mini-torus on


Comparison of Q2 Distributions – radiated versusunradiated

Radiation off <Q2> = 0.0259 GeV2

Radiation on <Q2> = 0.0258 GeV2


Scenario 1: Run mini-torus during production running

When the mini-torus was downstream of the primary collimator, theplan was to have it on during production running. This was acceptablebecause 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 greaterburden 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 maintorus.

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)


Scenario 2: Make primary Q2 calibration with H2 gas target• As shown earlier the difference between <Q2> for gas and liquid targetis ~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/nAAluminum elastics: .054 KHz/nAH2 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


Step-by-Step Procedure for Scenario 21. 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.

2. 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.

3. 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).


Scenario 3: H2 gas target plus mini-torusSo 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 canoperate 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.


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.

mini-torus update mark pitt, virginia tech (all simulation work done by juliette mammei)

Documents