# Presentation: 717958

## Vacuum System Considerations for Ion Source and RFQ of the Proton Driver 

**Outline:**
**             ****1. Analysis of vacuum requirements**
**                 ****for the Ion Source**
**          **
**             ****2. Preliminary look at expectations **
**                 ****for the RFQ vacuum  **
**            ****3. Pumping stations assignment and**
**                ****location (a sketch)**
**                 **

## Vacuum Requirements for Ion Source of the Proton Driver

**Ion Source vacuum system counters two sources of gas:**
**              ****-  Non-ionized H2 gas feeding the Ion Source**
**              ****-  Out-gassing from the surfaces of the Ion Source **
**                 ****system (possible contamination of ion beam)**
**Consequently, there are 2 vacuum states of considerably**
**different strength (by order of magnitudes) to be considered**
**in evaluating the required pumping speed for the Ion Source**

- A. Zlobin

- 2/24/98

## Ion and Current Production from H2 Gas

**    ****- Number of H2 molecules per 1 cm3 @ 760 Torr:**
**           ****1 mol (H2) = 2 g**
**           ****H2 density = 9 E^{-5} g/cm3**
**           ****Number of molecules/cm3 = 1⁄2 x 9 E^{-5} x 6.022 E^{23}**
**                                                     ****= 2.7 E^{19} **
** ****- Theoretical charge/current produced:**
**     ****If all H2 ******** 2 H+ (or 2 H-),**
**    ****Maximum charge => 2.7 E^{19} x 1.6E^{-19} C =  4.3 C/cm3**
**  ****- For 1 cm3/sec gas flow, maximum current => 4.3 A**
**  ****- Duty factor + ion production inefficiency  =>  < 0.001**
**                    ****H+/H- currents => few mA**
**                         **
**  **

- A. Zlobin

- 2/24/98

## Ion Source Vacuum Requirements

**To minimize the scattering of produced ions the vacuum in the ion production mode is typically kept at: **
**                              ****< 10 mTorr**
**A flow of 10 cm3/sec @ 760 Torr  is equivalent of the**
**    ****out-gassing rate of: 7.6 Torr_L/s,**
**The pumping speed of ~1000 L/s will keep vacuum**
**    ****level at 7.6 mTorr **
**Most Ion Sources use ~ 3,000 L/s of total pumping**
**    ****speed which probably gives an effective speed at the source in the range ~ 1000 L/s **

- A. Zlobin

- 2/24/98

## Ion Source Vacuum Requirements

**Sketch of the Duoplasmatron Source     Assembly of  the Accelerating Column      **
**                                                             ****of the Duoplasmatron Source at MS6**

- A. Zlobin

- 2/24/98

## Ion Source Vacuum Requirements

**Typical Ion Source has inner surface area ~ 10,000 cm^2**
**There are multiple large O-rings**
**System is un-bake-able, so H2O is a dominant source of out-gassing with a rate of ~ 3 x 10^{-10} Torr_L/s_cm2**
**    ****giving a total H2O out-gassing of 3 x 10^{-6} Torr_L/s  **
**With an effective pumping speed of 1000 L/s, the**
**    ****projected vacuum level with no H2 gas feed is:**
**                     ****~ 3 x 10^{-9} Torr, or**
**                     ****~ 1 ppm of the fed H2 gas**
** ****As the H2 gas purity is also ~ 1 ppm, we have a match!**

- A. Zlobin

- 2/24/98

## RFQ Vacuum Considerations

**The RFQ is a massive, large object primarily made out of copper, with multiple brazed joints and multiple function ports, and very few (2 ?) vacuum ports **
**There are two options considered for the RFQ vacuum system:**
**         ****- RFQ is embedded inside a large vacuum chamber**
**         ****- The pump-out ports are directly on the RFQ body**
** **
**The required vacuum level is 10^{-8} Torr, or better**

- A. Zlobin

- 2/24/98

## RFQ Vacuum Considerations

- RFQ Outside Look

- RFQ Inside Look

- A. Zlobin

- 2/24/98

## RFQ Vacuum Considerations

**    ****RFQ inner surface area (Cu):         ~ 40,000 cm2**
**    ****RFQ outer surface area (Cu), est.: ~ 35,000 cm2    Vacuum chamber area (SS), est.:  ~ 40,000 cm2**
**For an un-baked system the out-gassing is dominated by H2O, at a rate of ~3 x 10^{-10} Torr_L/s_cm^2. **
**    ****Therefore, the estimated out-gassing rate is:**
**    ****(a) RFQ inside the vacuum vessel:**
**         ****115,000 cm2 x 3  x 10^{-10} Torr_L/s_cm^2 => **
**                         ****3.5 10^{-5} Torr_L/s **
**    ****(b) Pump-out port(s) directly on RFQ:**
**                         ****1.2 10^{-5} Torr_L/s**
**    ****To obtain a vacuum level of ~10^{-8} Torr, the effective pumping speed must be :**
**    ****(a) (3.5 10^{-5} Torr_L/s)/10^{-8} Torr  > 3500 L/s for,**
**    ****(b) (1.2 10^{-5} Torr_L/s/10^{-8} Torr   > 1200 L/s for,**
**    ****(that is when ignoring the pump -> RFQ conductance)**

- A. Zlobin

- 2/24/98

## RFQ Vacuum Considerations

**For a well-baked system: **
**     ****(150 deg. for Cu, 300 deg. for SS @ 50h) **
**    ****the out-gassing is dominated by hydrogen at rate of:**
**                    ****10^{-12} Torr_L/s_cm^2 for Cu, and**
**               ****5 x 10^{-13} Torr_L/s_cm^2 for SS **
**    ****leading to an estimated total out-gassing rate of:**
**      ****(Cu)  75,000 cm^2 x 10^{-12} Torr_L/s_cm^2    +  **
**      ****(SS)  40,000 cm^2 x 5 x 10^{-13} Torr_L/s_cm^2**
**                  ****~ 10^{-6} Torr_L/s**
**So, for the RFQ mounted inside the vacuum chamber, the required pumping speed would be only ~ 100 L/s.**
**    ****This is unlikely, in practice it is not possible to reduce that much the out-gassing of H2O, but it indicates that with a moderate baking (part of brazing process?), and a few 1000 L/s pumps, a 10^{-8} Torr vacuum is feasible**

- A. Zlobin

- 2/24/98

## RFQ Vacuum Considerations

**In summary, we conclude that with the appropriate heat**
**    ****treatment of RFQ assembly to lower the out-gassing rate of H2O, and with the uhv-type cleaning of the RFQ vacuum associated surfaces, the vacuum level of < 10^{-8} Torr is probably feasible for both systems (with or without vacuum vessel) **
**The spill of the residual H2 gas from the Ion Source to the RFQ probably scales down with the Ion Source to RFQ beam pipe conductance, which we estimate as (0.1-1) %.**
**     ****So the spill is (0.1-1)% x 3 x 10^{-6} Torr_L/s, or**
**                                        ****< 3 x 10^{-8} Torr_L/s , **
**    ****and so it can be intercepted at the Ion Source – RFQ beam pipe using a modest (Turbo or Ion) pumping station**

- A. Zlobin

- 2/24/98

## Ion Source and RFQ Vacuum System

- A. Zlobin

- 2/24/98