some notes about liquid hydrogen target 1.how it operates 2.modification 2007 3.target length, empty...

Some notes about liquid Hydrogen target

1.How it operates

2.Modification 2007

3.Target length, empty mode

4.Status

The full liquid H2/D2 target system consist of :1. The H2 /D2 gas storage tank of 1.0 m3 , located in the vicinity of tagger.2. The helium-4 gas compressor, in a blue painted box located close to the liquefier and it is connected to it.3. The liquefier, it is a rectangular aluminium box located about 1.8 meters upstream from the centre of the Crystal Ball. It had the “cold-head” and the storage reservoir (“condenser”) for the liquid H2 .4. The liquid H2 transfer line which connects the condenser with the target cell.5. The target cell, located in the centre of the CB.

Compressor

V18

Cold-head

condenser

Target cell

16 K

P targ =1400 /1080 mbar

heaters

P vac =5x10-7mbar

1

Storage tank

2

3

5

4

The transfer line and the target cell are located in the beam pipe, which must have good vacuum, about 3x10-7 mbar. The five components of the liquid H2 /D2 target form a closed system. At the start, all hydrogen is gaseous, a pressure is typically 1400 mbar. When in operation, about 25% of this is liquefied and the pressure in the system drops to typically 1080 mbar. The temperature (pressure, density) of the liquid H2 /D2 is maintained by the delicate balance of cooling by addition of cold liquid and evaporation by two heaters which are turned on and off by the pressure and temperature control system. One heater is located on the condenser and the other on the target cell.

Monitoring of the temperature and pressure at various places is done by a dedicated computer which is presently located in the tagger vicinity.

Target Modifications in 2007

• After successful operation of our new Helium3/Helium4 target the heat exchangers for the Joule Thompson circuit were removed (see picture below). Due to the reduced heat capacity the time for cooling down the target could be made shorter by a factor of 2 (36hours to 18 hours).The heat contact on between condensor and cold head was improved.

The material length at the beam axis for 21K is: 47.6mm+-0.3mm

Ordinary “empty target” procedure

• Advantage – we don’t interrupt the measurement and can continue it with full target in a few minutes any moment as we need.

• Disadvantage – inside “empty target” hydrogen density in ~50 times less then for “full target”.

Filling process after “empty target”

http://webbook.nist.gov/chemistry/fluid/

T

(K)

P

(mbar)

Density*103

d (g/cm3)

Phase Density ratio

d gas/ d liq Method

20.5 1080 70.70 liquid

20.7 1080 1.401 gas 2*10-2 Ordinary

“Empty target”

20.7

10 0.0118 gas 2*10-4 “real empty

target”

273 1013 0.0899 gas 1.3*10-3

• In order to pump target we should interrupt

measurement for 1 hour , expert should be duty during “empty run” and we can continue to work with full target only in 6 - 8 hours. Therefore this method will be able to use at the end of run if we need “real empty target”.

Status• The liquid H2/D2 target worked for 6068

hours on the MAMI beam with CB and TAPS for A2 experiments during 2004-2008 (2004,1/2 of 2005, 2007).

> 70 cycles “cooling down – warming”,

every cycle is stress for target cell. Now we have “cold leak” and should find and fix

it.