Introduction

When the right tail of the Maxwell’s distribution is popullated by electrons exceedeng certain crtical velocity, this electrons are less affected by Coulomb collision friction, therefore they are accelerated up to relativistic velocities. The upper limit for the velocity if given by the synchrotron radiation friction. In a collisional plasma, Dreicer field is the lower electric \(E\) limit for which REs can be created. It is given by \[E_{D}=\frac{n_{e}e^{3}}{4 \pi \epsilon_{0}^{2} k T_{e}}Ln\Lambda \\\] Where \(Ln\Lambda=Ln\frac{\lambda_{D}}{b_{0}}\) is the Coulomb logarithm, \(\lambda_{D}=\sqrt{\frac{\epsilon_{0}kT_{e}}{n_{e}e^{2}}}\) the Debye length and \(b_{0}=\frac{Ze^{2}}{4\pi\epsilon_{0}}\frac{1}{m_{e}v^{2}}\) the lower limit for Coulomb impact parameter. In general, electrons with velocity greater than \(v_{c}\) along the electric field \(E\), will be accelerated to relativistic velocities. \[v_{c}=\sqrt{\frac{n_{e}e^{2}(1+Z_{eff}/2)}{4\pi\epsilon_{0}^{2}m_{e}E}Ln{\Lambda}}\] Hot tail mechanism occurs when the plasma is rapidly cooled so that the critical velocity is decreased (look at the Coulomb logarithm). Therefore a bigger population of electrons become relativistic. Avalanche production occurs when energetically enough electrons collide with thermal ones accelerating them to the RE region.

In the scope of isotopic studies in H2 and Ar, break dawn voltage is to be considered. In principle, when working with Ar, it should be possible to go to lower pressures than those for H2.

Relativistic runaway electron avalanche
RE Studies
Dreicer field
Coulomb collision
Paschen Law

Aims

Investigation of REs’ on H2 and Ar.

• Characterize RE and background plasma.

• Estimate REs distribution.

• Involve colleagues on experimentation.

Aparatus

Possible diagnostics:

1. Scintillation detector plus photo-multiplier (NaI(Tl), YAP(Ce)). Hits / deposited energy.

2. PH32-based detector (Strip detector).

3. Timepix (SXR + HXR? Timepix(3?) CdTe?)

4. Dosimeters

5. Cherenkov Detector?

6. Spectrometer(s)

Possible parameters:

1. Pressure, keep low (\(5\ mPa < P \leq 10\ mPa\), \(14\ mPa\)).

2. Electric field, keep low for low Plasma current (\(440 \geq E\ V\), \(325\ V\)).

3. B field (\(\sim 1300\ V\), \(1200\ V\)).

4. Working gas (H2, Ar or He).

5. Stabilization System.

Reference shots: 29398 27762 27738 27745 27751 27756 27757 29364 31052

Methodology

Functional areas:

• Background plasma characterization by means of electrostatic probes.

• Plasma column stabilization system.

• RE count and energy distribution measurements.

• Working gas manipulation.

Session schedule:

1. Reference shot on H2.

2. Several shots with different conditions, still on H2.

3. Chamber conditioning from H2 to Ar.

4. Several shots with similar conditions as 2 but for Ar.